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Inner Filter Effect for Environmental Monitoring

Yong Xie , Mingjie Han , Yuhao Xu , Chenyu Xiong , Ri Wang , Shanhong Xia

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1450 -1460.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1450-1460 DOI: 10.7536/PC200748
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Inner Filter Effect for Environmental Monitoring
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Abstract

Inner filter effect(IFE) refers to the phenomenon that the absorber absorbs the excitation and/or emission light of the fluorophore, resulting in the fluorescence quenching of the fluorophore. Compared with fluorescence resonance energy transfer(FRET) or other techniques, IFE avoids cumbersome labeling processes, and has the advantages of high sensitivity, strong selectivity, simple and flexible operation. The IFE-based fluorescent approach has broad application foreground in the field of environmental monitoring. The absorber and the fluorophore are the two main components of the IFE-based sensing system. The optical properties and the spectral overlap of the two directly affect the quenching efficiency of the IFE-based sensing system. There are relatively limited choices of materials for the absorber and the fluorophore. Discovering new nanomaterials and exploring suitable absorber/fluorophore pair are very helpful to improve the quenching efficiency of IFE and enhance the detection performance of the IFE-based fluorescent approach. In this review, we mainly focus on the recent progress of IFE researches for environmental monitoring, including the detection of heavy metal ions, anions and small molecular environmental pollutants. The important effects of nanomaterials in the IFE-based sensing system are analyzed. Finally, the challenges and future developments of the IFE-based fluorescent approach are discussed.

Contents

1 Introduction

2 IFE applied to environmental monitoring

2.1 Detection of heavy metal ions

2.2 Detection of anions

2.3 Detection of small molecule environmental pollutants

3 Conclusion and outlook

Graphical abstract

Keywords

inner filter effect(IFE) / environmental monitoring / nanomaterials / heavy metal ions / anions / pesticides / small molecule environmental pollutants

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Yong Xie, Mingjie Han, Yuhao Xu, Chenyu Xiong, Ri Wang, Shanhong Xia. Inner Filter Effect for Environmental Monitoring. Progress in Chemistry, 2021, 33(8): 1450-1460 DOI:10.7536/PC200748

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1 Introduction

With the rapid development of society and economy, human production and living standards have been greatly improved, but at the same time, it has also brought serious environmental pollution problems. A large number of residual chemical pollutants in industry, agriculture and daily life enter the environment, most of which will exist in the environment for a long time due to their difficult degradation, or pose a great threat to the ecological environment and human health through bioaccumulation, so it is extremely important to study the analysis and detection technology of environmental pollutants. Fluorescence analysis method has been widely used in the field of environmental monitoring because of its high sensitivity, good selectivity, real-time and portable detection. Traditional fluorescence probe sensing mechanisms, such as static quenching effect (SQE), dynamic quenching effect (DQE), photoinduced electron transfer (PET), intramolecular charge transfer (ICT) and fluorescence resonance energy transfer (FRET), require high fluorescence characteristics and selectivity of the fluorescence probe, which usually makes the preparation and detection process of the probe complex and time-consuming, and limits their practical application[1⇓~3].
Fluorescence inner filter effect (IFE) refers to the phenomenon that an absorber absorbs either the excitation light or the emission light (or both) of a phosphor, resulting in a decrease in the fluorescence intensity of the phosphor[2]. Compared with FRET and other techniques, IFE does not need to consider the specific interaction distance and covalent connection between the fluorophore and the absorber, and avoids many tedious surface modification or labeling processes, so IFE as a simple, effective, flexible and convenient fluorescence quenching mechanism has gradually attracted people's interest. The generation of IFE requires an effective overlap between the absorption spectrum of the absorber and the excitation or emission spectrum of the fluorophore, because the absorbance change of the absorber can be converted into an exponential change in the fluorescence signal of the fluorophore, thus enhancing the sensitivity of analytical detection[2,3]. The higher the extinction coefficient of the absorber, the larger the spectral overlap region between the absorber and the phosphor, and the higher the quenching efficiency of the IFE. Due to the small extinction coefficient of traditional absorbers, it is difficult for the excitation or emission spectra of fluorescent substances to match the absorption spectra of absorbers completely, which limits the development of IFE-based fluorescence analysis methods. Nowadays, with the rapid development of nanotechnology, a large number of new luminescent nanomaterials with unique optical properties have emerged, such as semiconductor quantum dots (QDs), two-dimensional quantum dots (2D QDs), carbon dots (CDs), metal-organic frameworks (MOFs), upconversion luminescent nanoparticles (UCNPs) and metal nanoclusters (MNCs).They can adjust the spectral overlap between the absorbers and the phosphors by changing the particle size or doping elements, so they are widely used in IFE-based sensing systems. In addition, nano-materials such as noble metal nanoparticles and manganese dioxide nanosheets usually have higher extinction coefficients than traditional chromophores and catalytic products, so they have great potential in the field of IFE sensing as fluorescent absorbers with tunable optical properties.
In this paper, the research progress of IFE in environmental detection in the past three years is summarized, including the detection of heavy metal ions, anions and small molecular environmental pollutants, and the important role of nanomaterials in IFE sensing systems is analyzed.Finally, the challenges and development directions of IFE-based fluorescence analysis methods are discussed.

2 Application of IFE to environmental monitoring

The fluorescence analysis method based on IFE has the advantages of simple and flexible operation and high sensitivity, and has a wide application prospect in the fields of environmental analysis and biochemical analysis. Since the fluorophore and the absorber are the two main units of the IFE sensing system, their optical properties and band overlap directly affect the quenching efficiency of the IFE, thus indirectly affecting the detection sensitivity. Therefore, the continuous discovery of new nano-materials and the exploration of suitable absorber-fluorophore combinations can not only effectively enhance the detection effect of IFE sensing system, but also help to open up new application fields and promote the development of IFE-based fluorescence analysis methods. This paper aims to introduce the latest research progress of IFE in environmental monitoring and analyze the important role of nanomaterials in it.

2.1 Heavy metal ion detection

The toxicity of heavy metal ions is persistent for a long time, which can seriously damage human organs or cause chronic poisoning after entering the human body through biomagnification. The problem of heavy metal pollution in water has attracted great attention and in-depth research. In recent years, many fluorescence analysis methods based on IFE have been developed to detect heavy metal ions, including Cr6+, Fe3+, Hg2+ and Cu2+.
Due to its broad and strong absorption band, Cr6+ can be directly used as an absorber to quench the fluorescence of fluorescent probes, which has attracted the most attention in the field of heavy metal ion detection based on IFE. As a common fluorescence donor in IFE sensing system, carbon dots have been widely studied in the detection of heavy metal ions due to their high fluorescence intensity, adjustable excitation and emission wavelengths, photobleaching resistance, good water solubility and good biocompatibility, which are conducive to meet the spectral overlap conditions of IFE[4⇓⇓~7]. When the absorption peak of the Cr6+ overlaps with the excitation peak of the carbon dots, it will induce the IFE,Cr6+ between them, and as an absorber, it will compete to absorb the excitation light of the carbon dots, which will lead to the quenching of the fluorescence of carbon dots. Based on the above fluorescence quenching principle, Yang et al. Prepared carbon dots with high quantum yield by one-step hydrothermal method using natural seaweed as carbon source, using Cr6+ as the fluorescence absorber of IFE, and the synthesized carbon dots as the fluorescence donor of IFE.Due to the overlap of the absorption peak of Cr6+ at 371 nm with the excitation peak of carbon dots, the fluorescence of carbon dots will be significantly quenched by Cr6+ in the presence of IFE, and the change of fluorescence intensity of carbon dots is linearly related to the concentration of Cr6+, so IFE can be used to achieve sensitive and selective detection of Cr6+[8]. However, Cr3+ can be oxidized to Cr6+ by potassium permanganate (KMnO4), so the above method can be used to detect Cr3+ and Cr6+, which provides a new choice for the valence analysis of chromium ions. Many studies have shown that doping nitrogen (N), silicon (Si), phosphorus (P), sulfur (S) and other heteroatoms into carbon dots can change their electronic structure, improve their optical properties, and greatly improve their quantum yield and fluorescence lifetime[9⇓⇓⇓⇓⇓~15]. Nitrogen-doped graphene quantum dots (N-GQDs), Si-CDs, N-CDs, S, N-CDs and other doped carbon dots are used as suitable fluorescence donors to participate in the construction of IFE-based sensing systems, which is helpful to improve the sensitivity and stability of fluorescence detection[10][11][16⇓~18][19]. For the first time, Li et al. Used polyvinylpyrrolidone (PVP) as passivator and carbon and nitrogen source to realize surface defect passivation and nitrogen doping of N-GQDs simultaneously by one-step hydrothermal method, and the prepared N-GQDs had a high fluorescence quantum yield of 64. 2%[10]. The fluorescence of N-GQDs will be significantly quenched under the influence of IFE due to the complete overlap of the absorption spectrum of Cr6+ and the excitation spectrum of N-GQDs, thus achieving a highly sensitive detection of Cr6+ with a detection limit as low as 91 nmol/L. However, the relatively small Stokes shift of the carbon point may limit its application,Therefore, fluorescent materials with large Stokes shift or anti-Stokes shift and high quantum yield, such as UCNPs, silicon quantum dots (SiQDs), polymer quantum dots (PDs), fluorescent polyhydroquinoline derivatives (PHQ) and copper nanoclusters (CuNCs), have been studied as strong competitors for fluorescent donors in IFE sensing systems to achieve highly sensitive detection of Cr6+ or Cr3+[20][21][22][23][24].
Iron is a trace element needed for life activities, but excessive intake will have adverse effects on human health. The detection of Fe3+ is of great significance in the field of environmental monitoring. Fluorescence analysis is favored because of its simple operation, fast response and high sensitivity. A large number of fluorescence sensors have been used in the detection of Fe3+. For example, there are many reports about carbon dot-based fluorescent probes in the field of fluorescence detection, most of which are based on the principle of fluorescence quenching to achieve highly sensitive detection of Fe3+[25⇓~27]. In recent years, many new fluorescent nanoprobes have emerged in the research reports, such as graphite-phase carbon nitride nanosheets (g-C3N4NNs), titanium carbide quantum dots (Ti3C2MXene QDs), UCNPs, AuCu bi-MNCs, RhB @ MOF, and hafnium-based metal-organic frameworks (HfIV-based MOF).They are fluorescent donors with excellent optical properties, which contribute to the highly sensitive and selective detection of Fe3+[28][29][30][31][32][33]. Li et al. prepared graphite carbon nitride quantum dots (g-CNQDs) and applied them to the heavy metal ion detection process based on IFE for the first time. The surface of the synthesized g-CNQDs contains abundant hydroxyl and carboxyl functional groups, and the Fe3+ is first adsorbed on its surface through electrostatic interaction.The fluorescence of g-CNQDs can be rapidly quenched due to the overlap of the excitation and emission spectra of Fe3+ with those of g-CNQDs, thus realizing the highly sensitive detection of Fe3+ with a detection limit of 23. 5 nmol/L, which is much lower than the maximum allowable concentration (5. 357 μmol/L) specified by the US Environmental Protection Agency (EPA)[34]. Quantum dots derived from two-dimensional materials such as graphite carbon nitride, titanium carbide and molybdenum disulfide have been developed continuously, which usually have the advantages of low cost, large specific surface area, good chemical stability and biocompatibility, but their quantum yield still needs to be improved compared with semiconductor quantum dots or carbon dots. In addition, most of the reports about the detection of Fe3+ are based on the fluorescence turn-off mode, and the ratiometric fluorescence detection mode is less. It is reported that the turn-off or turn-on mode may lead to false negative or false positive detection results, while the ratiometric fluorescence mode can minimize the impact of environmental and background signals and improve the reliability of detection results[1]. Yan et al designed a ratio fluorescence mode based on IFE to detect the Fe3+. When the Fe3+ did not exist, the maximum fluorescence emission peak of curcumin was located at 525 nm, and its absorption peak at 420 nm basically overlapped with the emission peak of carbon dots, so the fluorescence of carbon dots was significantly quenched by IFE[27]. When the Fe3+ is added, the curcumin can form a complex, the absorption peak of the curcumin at 420 nm is weakened and the fluorescence intensity at 525 nm is reduced, and the overlapping area of the absorption spectrum of the curcumine and the emission spectrum of the carbon point is reduced,IFE is suppressed, so the fluorescence intensity of the carbon dots at 420 nm is recovered, thus realizing the ratio fluorescence detection of the Fe3+, and the detection limit is 18.11 nmol/L. This method is beneficial to reducing the interference of environmental and background factors.
Hg2+ is a typical highly toxic heavy metal ion, and trace amounts of Hg2+ can produce strong toxicity. Due to the weak absorption properties of Hg2+, they can not be directly used as absorbers in IFE, but usually need to use nanomaterials with high extinction coefficient, chromophores or catalytic products as efficient fluorescent absorbers. Due to the high extinction coefficient and unique localized surface plasmon resonance (LSPR) properties of noble metal nanoparticles, the UV-Vis absorption peak of noble metal nanoparticles will change when the morphology, surface properties, reaction environment or the distance between particles change, and the color of the solution will change accordingly[35,36]. Therefore, the noble metal nanoparticles are used as the fluorescence absorber in the IFE, and the change of the absorption spectrum can be adjusted by changing the morphology or aggregation state of the noble metal nanoparticles, thereby affecting the fluorescence quenching efficiency. Using the strategy that Hg2+ can inhibit the formation of core-satellite nanostructures, Zhao et al. Used core-satellite structures composed of gold nanorods (AuNRs) and gold nanoparticles (AuNPs) as fluorescent absorbers in IFE system, and Rhodamine 6G as fluorescent donors.Compared with the unassembled mixture of AuNRs and AuNPs, the fluorescence quenching efficiency of the core-satellite structure is increased by about 18% due to the red shift and broadening of the plasmon resonance peak of the core-satellite structure, which enlarges the overlap region of spectral bands and strengthens the IFE and plasmon resonance coupling effect[37]. The specific experimental principle is that p-thiophenol aniline (p-ATP) and cetyltrimethylammonium bromide (CTAB) are used as modifiers, and the surface functionalized p-ATP @ AuNRs and CTAB @ AuNPs can form a core-satellite nanostructure by a two-step ligand exchange method, and the transverse absorption peak of the structure overlaps with the emission peak of rhodamine 6G, so that IFE can be induced and the fluorescence of rhodamine 6G can be quenched. When the Hg2+ is added, the p-ATP on the surface of the AuNRs is competitively bound, so that the assembly of the nucleus-satellite structure is inhibited, the overlapping area of spectral bands is reduced, the IFE is weakened, and the fluorescence of rhodamine 6G is restored, so that the quantitative detection of the Hg2+ is realized; and the detection limit is as low as 0.18 nmol/L, and the linear range is 0.6 to 800 nmol/L. In addition to noble metal nanomaterials, in order to explore new absorber-fluorophore combinations, tetraphenylporphyrin tetrasulfonic acid (TPPS), 2,3-diaminophenazine (DAP), porphyrins and other chromophores or catalytic products with high extinction coefficients have been used as fluorescent absorbers in IFE sensing systems, which is conducive to the indirect detection of Hg2+[38][39][40].
The detection of other metal ions based on IFE has also been reported, such as Cu2+, Mn7+, Pb2+, As3+, Co2+, etc. Among them, fluorescent nanomaterials such as upconversion nanoparticles (UCNPs) and persistent luminescent nanoparticles (PLNPs) are used as fluorescent donors to achieve quantitative detection of metal ions[41~44][45,46][47][48][49]. UCNPs can convert long-wavelength radiation into short-wavelength radiation, and have the characteristics of large anti-Stokes shift, narrow emission spectrum and effective reduction of background fluorescence interference. Based on branched polyethyleneimine (BPEI) functionalized UCNPs, Hong et al. designed a colorimetric and fluorescent dual-signal detection method for Cu2+. First, BPEI was used to modify the surface of UCNPs. When Cu2+ was added, BPEI on the surface of UCNPs would form a complex with Cu2+. Because the broad absorption spectrum of the BPEI-Cu2+ complex overlapped with the emission spectrum of UCN,Therefore, the BPEI-Cu2+ can effectively quench the fluorescence of UCNPs, and the color of the solution changes from colorless to blue with the increase of the concentration of the Cu2+, and the concentration of the Cu2+ is linearly related to the absorption intensity of the BPEI-Cu2+ complex at 635 nm. Finally, the detection of Cu2+ in tap water samples is successfully realized, with a detection limit of 69 μmol/L by colorimetry and a detection limit of 45 nmol/L by fluorimetry[50]. In this report, the synergistic effect of colorimetric and fluorescence dual-mode detection is conducive to improving sensitivity, reducing detection limit and expanding detection range. In addition, persistent luminescent nanoparticles (PLNPs) have the advantages of long afterglow, low toxicity, high signal-to-noise ratio, elimination of autofluorescence interference, light stability and strong tissue penetration, which are suitable for biological imaging and detection in complex environments. Wang et al. Developed a fluorescent probe for detecting As3+ based on PLNPs with near-infrared emission. First, polyethyleneimine (PEI) and dithiothreitol (DTT) were modified onto the surfaces of PLNPs and gold nanorod particles (AuNPs), respectively. The negatively charged DTT-AuNPs could be adsorbed around the positively charged PLNPs-PEI through electrostatic interaction[48]. The fluorescence of PLNPs-PEI can be quenched by DTT-AuNPs due to the overlap of the longitudinal absorption peak of DTT-AuNPs at 700 nm with the emission peak of PLNPs-PEI. When the As3+ is added, it will combine with the thiol group of DTT-AuNPs, so that the DTT-AuNPs will be separated from the surface of PLPPs-PEI and the quenching efficiency of IFE will be reduced, thus the fluorescence of PLPPs-PEI can be recovered, and the highly sensitive detection of As3+ can. In recent years, our group has been focusing on the electrochemical and optical detection of heavy metal ions, and has realized the preparation of ultrasensitive localized surface plasmon resonance (LSPR) sensors and fluorescence detection for Hg2+ based on gold nanomaterials and fluorescent dyes, respectively[51⇓~53]. At present, our group is trying to combine metal nanomaterials with core-shell quantum dots or carbon dots with high fluorescence quantum yield to construct simple and portable IFE sensors for heavy metal ions.

2.2 Anion detection

Inorganic anions in water or soil environment are various and widely distributed, and some of them are highly toxic, which can not be ignored in environmental pollution. There are few reports on the detection of anions based on IFE sensing system. Cyanide ion (CN-) is a highly toxic and extremely dangerous ion, which has received more attention in previous studies[54⇓⇓~57]. In recent years, more and more attention has been paid to the detection of nitrite (NO-2) and S2-.
Nitrite is widely used in food additives, preservatives and fertilizers, and is a dangerous carcinogen, which can easily induce diseases related to the blood and nervous system. Song et al. prepared graphite-phase carbon nitride quantum dots (g-CNQDs) as the fluorescence donor in the IFE sensing system, and used the red complex (BPS-Fe2+) formed by the specific binding of Fe2+ with bathophenanthroline (BPS) as the fluorescence absorber. The maximum absorption peak of the BPS-Fe2+ was at 535 nm, which was basically overlapped with the emission peak of g-CNQDs, so the fluorescence of g-CNQDs was quenched under the action of IFE[58]. In the presence of NO-2, the red complex was oxidized to a colorless BPS-Fe3+ with different absorption properties BPS-Fe2+, and the fluorescence of g-CNQDs was recovered.Therefore, a fluorescence and colorimetric dual-mode method for the detection of NO-2 was successfully constructed, which realized the simple and sensitive fluorescence and visual detection of NO-2 in water and sausage samples. In addition, coumarin derivatives and nitrogen-doped carbon dots can also be used as fluorescence donors in IFE to achieve the detection of NO-2 in food samples, which provides a new choice for the highly sensitive and selective detection of NO-2[59][60].
S2- is a common environmental pollutant, which can easily cause damage to human blood, mucosa, digestive system and other organs and tissues, and even cause some high-risk diseases, such as Alzheimer's disease and cardiovascular disease. Manganese dioxide (MnO2) nanosheets can be used as a fluorescence quencher in FRET and IFE due to their high extinction coefficient and significant fluorescence quenching effect. Liu et al. designed a method based on IFE and "turn-on" sensing strategy to detect S2-, using MnO2 nanosheets and carbon dots as efficient absorbers and fluorophores, respectively. Because the absorption spectra of MnO2 overlap with the excitation and emission spectra of carbon dots,Therefore, the MnO2 can quench the fluorescence of the carbon dots, and when the S2- is added, the MnO2 is reduced to Mn2+, so that the absorption intensity of the MnO2 nanosheet is reduced,The IFE is weakened and the fluorescence of the carbon dots is recovered, so that the rapid, sensitive and selective detection of the S2- in the waste water sample is successfully realized[61]. Copper nanoclusters and carbon dots are also used as fluorescent donors in IFE, which effectively realizes low-cost and highly sensitive detection of S2-[62][63,64].

2.3 Detection of small molecular environmental pollutants

2.3.1 Pesticide residue detection

Pesticide is an important guarantee for increasing crop production and income, but pesticide residues have serious food safety risks and threaten human life and health, so rapid and accurate detection and analysis technology for pesticide residues has attracted great attention. Organophosphorus pesticides (OPPs) have the advantages of environmental persistence and high efficiency, but they can significantly inhibit the activity of cholinesterase in animals, especially acetylcholinesterase (AChE), which will cause the accumulation of neurotransmitter acetylcholine in human body and cause neurological disorders[1]. Therefore, organophosphorus pesticides can effectively inhibit the activity of enzymes or participate in enzymatic reactions as important substrates, thus indirectly affecting the change of fluorescence intensity. Based on the above principle and the excellent fluorescence characteristics of carbon dots, He et al. Constructed an enzyme inhibitory carbon dot fluorescence probe based on IFE, and successfully applied it to the detection of paraoxon in soil samples[65]. In the absence of paraoxon, AChE can catalyze the hydrolysis of acetylcholine ester (ATCh) to thiocholine (TCh), which can react specifically with 5,5 '-dithiobis (2-nitrobenzoic acid) (DTNB) to form 5-mercapto-2-nitrobenzoic acid (TNB).The fluorescence of N, Cl-CDs is significantly quenched due to the overlap of the absorption spectrum of TNB and the excitation spectrum of N, Cl-CDs. On the contrary, in the presence of paraoxon, the activity of AChE is inhibited, the amount of TNB produced is reduced, and the fluorescence of N, Cl-CDs is restored, so that the quantitative detection of paraoxon can be realized. However, biological enzymes such as AChE are easily affected by external conditions in actual detection, and are easily degraded or denatured in storage and transportation. In order to avoid these problems, inorganic catalysts may be an ideal substitute for enzymes. Li et al. Used CeO2 with phosphatase-like properties as a nano-enzyme to catalyze the hydrolysis of methyl paraoxon to p-nitrophenol (p-NP) and phosphate groups. Because the absorption spectrum of p-NP overlapped with the excitation spectrum of carbon dots, the fluorescence of carbon dots was quenched, thus realizing the quantitative detection of methyl paraoxon. The linear range was 1.125 ~ 26.25 μmol/L, and the detection limit was 0.375 μmol/L[66]. Later, based on the same catalytic reaction, they designed a fast and simple fluorescence and visual dual-signal strategy for the detection of methyl paraoxon, which has potential for portable detection[67].
In recent years, in order to enhance the quenching efficiency of IFE and improve the reliability and anti-interference ability of IFE sensing systems, persistent luminescent nanorods (PLNRs), gold nanoclusters (AuNCs), luminescent metal-organic framework (RhB @ LMOF-1) and other nanomaterials have been used as fluorescent donors in IFE to achieve highly sensitive and reliable detection of pesticide residues[68][69][70]. For example, PLNRs can reduce or eliminate background signals compared with general fluorophores due to their special phosphorescent properties. Zhang et al. designed an isocarbophos aptamer sensor with dual colorimetric and phosphorescent signals[68]. The aptamer sensor is based on the principle that the aptamer-modified AuNPs can maintain a stable dispersion state in a sodium chloride solution, the maximum absorption peak of the aptamer-modified AuNPs is located at 520 nm and the solution is red, and the fluorescence of PLNRs is significantly quenched due to a large overlap between the absorption peak of the AuNPs and the emission peak of PLNRs. When isocarbophos is added, the isocarbophos is specifically combined with the aptamer, and the AuNPs lose the protection of the stabilizer and are induced to aggregate by high concentration of sodium chloride, so that the absorption intensity of the AuNPs at 520 nm is reduced, the spectral overlap area is reduced, the fluorescence of PLNRs is recovered, and the solution turns from red to blue, thereby realizing the colorimetric-phosphorescence double-signal detection of isocarb ophos. Compared with the traditional colorimetric or phosphorescent analysis, the combination of the two methods can obtain a larger detection range and more reliable detection results. The metal nanoclusters have low toxicity, good photostability, large Stokes shift and wide excitation spectrum, and are suitable to be used as a fluorescence donor in an IFE sensing system, thereby being beneficial to improving the quenching efficiency of the IFE and the stability of a detection signal. Wang et al. Prepared monoclonal antibodies against Bromadiolone for the first time, and introduced AuNCs on the basis of conventional enzyme-linked immunosorbent assay (ELISA) to achieve highly sensitive fluorescence detection of Bromadiolone in serum[69]. Alkaline Phosphatase (ALP) was used to catalyze the hydrolysis of p-nitrophenyl phosphate (pNPP) to p-nitrophenol (p-NP) in conventional ELISA.The fluorescence of the AuNCs is quenched, and the fluorescence quenching degree of the AuNCs is related to the ALP activity (namely, the Bromadiolone concentration); and the detection result of the method is more than one order of magnitude lower than the LOD of a conventional ELISA, and the sensitivity and reliability of the method are enough to meet the requirements of clinical diagnosis. However, the low quantum yield of AuNCs may be a limitation for its wide application in the future. Compared with gold and silver nanoclusters, copper nanoclusters or bimetallic nanoclusters have recently attracted more attention because copper is an abundant and cheap metal on the earth. In addition, the ratio fluorescence probe (RF-QDs) based on cadmium telluride (CdTe QDs) effectively improves the reliability and anti-interference ability of the IFE detection system, and has advantages in reducing system errors and improving sensitivity and accuracy. Dai et al. Constructed a ratiometric fluorescence sensor to detect the organochlorine pesticide chlorothalonil (CHL), and designed RF-QDs by hybridizing two different colors of CdTe QDs, in which the red-emitting QDs act as the reference signal in the silicon sphere with an emission peak at 642 nm, while the green-emitting QDs are attached to the surface of the silicon sphere as the response signal with an emission peak at 542 nm[71]. The fluorescence of RF-QDs at 542 nm was quenched by AuNPs because the emission peak of RF-QDs at 542 nm partly overlapped with the absorption peak of AuNPs. Chlorothalonil (CHL) is an inhibitor of papain (PAP), and PAP can catalyze the hydrolysis of protamine (PRO). In the presence of CHL, the activity of PAP is inhibited, and the positively charged PRO on the surface induces the aggregation of negatively charged AuNPs through electrostatic interaction, and the fluorescence of RF-QDs at 542 nm is restored. In the absence of CHL, PAP hydrolyzes and cleaves PRO, resulting in the deaggregation of AuNPs and the quenching of RF-QDs fluorescence at 542 nm. Therefore, a highly sensitive detection of CHL in agricultural and food samples is realized by the method, and the RF-QDs have a built-in calibration function, so that the environmental influence is eliminated and the detection accuracy is improved. Wang et al. Used sulfur-doped carbon dots and copper nanoclusters as dual-emission ratiometric fluorescence probes, and used a similar ratiometric fluorescence detection strategy to achieve highly sensitive and selective detection of dinotefuran, a nicotinic insecticide[72]. At present, in addition to the representative organophosphorus pesticides, organosulfur pesticides such as ferrous thiram, zinc thiram, organochlorine pesticides such as imidacloprid and other pesticide residues have also been detected based on IFE sensing platform[73][74][75].
The abuse of antibiotics is becoming more and more serious, and the residues of antibiotics exist widely in the environment, which can easily lead to the emergence of drug-resistant bacteria and bring various toxic side effects, and will pose a serious threat to the ecological environment and human health. Some kinds of antibiotics, such as tetracyclines, are often used as fluorescent absorbers of IFE because of their strong absorption coefficients. Excellent fluorescent nanomaterials, such as CDs, MOFs, ZIF-8, Al-MOF, SiNPs, MoS2NPs and CuNCs, have been used as fluorescence donors, providing new options for the highly sensitive detection of antibiotic residues[76⇓⇓~79][80⇓⇓~83][84][85,86][87][88]. Taking carbon dots as an example, some carbon dots with unique functions, such as ultra-long room temperature phosphorescent carbon dots and near-infrared fluorescent carbon dots, have been used as new fluorescent donors to achieve sensitive and reliable detection of tetracycline antibiotics[89,90]. Lin et al. prepared a near-infrared fluorescent carbon dot. Because the broad absorption spectrum of oxytetracycline or norfloxacin overlaps with the excitation spectrum of the near-infrared fluorescent carbon dot, the degree of fluorescence quenching of the carbon dot is related to its concentration under the influence of IFE, thus realizing the quantitative detection of these two antibiotics in milk samples[90]. The near-infrared fluorescent carbon dots used in the report have high quantum yield and good photostability, and can effectively avoid interference signals such as autofluorescence and scattered light in complex matrices, so that the detection results have higher sensitivity and reliability, but the disadvantage is that the selectivity needs to be improved. In addition to carbon dots, luminescent metal-organic frameworks (LMOFs) have attracted more and more attention in the field of fluorescence detection due to their large specific surface area, high porosity, high stability, easy functionalization and high fluorescence intensity, which are conducive to the efficient enrichment of analyte and the realization of reversible adsorption function, thus improving the detection sensitivity and realizing the recycling of materials. Li et Al. prepared a heterostructure MOFs composite (ZIF-8/NH2-MIL-53(Al) by immobilizing ZIF-8 on Al-MOF. Because the size of ZIF-8 synthesized on the surface of NH2-MIL-53(Al) is small, it will be conducive to increasing its specific surface area and target interaction sites.The selectivity and adsorption capacity for tetracycline are greatly improved (the specific mechanisms are chemisorption, hydrogen bonding and π-π stacking interaction), and the excellent adsorption capacity and preconcentration effect also help to reduce the detection limit and improve the detection sensitivity[84]. Since the absorption spectrum of tetracycline overlaps with the excitation spectrum of ZIF-8/NH2-MIL-53(Al), the fluorescence of ZIF-8/NH2-MIL-53(Al) will be strongly quenched by tetracycline under the synergistic effect of IFE, PET and preconcentration effect, thus realizing the adsorption removal and highly sensitive fluorescence detection of tetracyline simultaneously. The traditional removal and detection technologies of tetracycline pollutants mainly rely on expensive chemical reagents or large-scale instruments, which are complex and time-consuming to operate, and it is difficult to achieve simultaneous detection and removal of pollutants. The fluorescent probe prepared based on the IFE principle and the MOFs composite material can capture and detect tetracycline at the same time, has the remarkable advantages of simple operation, rapid response, high sensitivity and the like, and can realize material recycling by using acetone as an eluent,This method provides a new scheme for the development of sensors that can capture and detect target analytes simultaneously, but the sensitivity, adsorption capacity and adsorption selectivity of such sensors still need to be improved. In addition to tetracycline antibiotics, some common antibiotic residues, including nitrofuran nitrofurantoin, aminoglycoside kanamycin, nitroimidazole metronidazole, cephalosporin cefixime and so on, have achieved reliable quantitative detection based on IFE sensing system[91][92][93,94][95].

2.3.2 Nitroaromatic compound detection

Nitroaromatic compounds, including nitrobenzene, nitrophenol and nitroaniline, are widely used in the production of pharmaceuticals, pesticides, dyes, explosives and other chemical products. They are chemically stable, highly toxic and easy to bioaccumulate. They are easily absorbed by the skin and lung, and act on the blood, liver and central nervous system, seriously threatening human health. In the process of detecting nitrobenzene compounds based on IFE sensing system, the target analyte is usually used as an absorber, and fluorescent nanomaterials such as CDs, AuNCs, CuNCs, silicon nanoparticles (SiNPs), molybdenum disulfide quantum dots (MoS2QDs), g-C3N4, CdTe QDs and boron nitride quantum dots (BNQDs) are used as suitable fluorescent donors.Thus realizing the high-sensitivity and selective detection of nitrophenol and isomers thereof, 2,4-dinitrophenol, 2,4,6-trinitrophenol and other compounds[96,97][98,99][100][101][102][103][104][105]. For example, Chen et al. prepared a kind of SiNPs with tunable emission wavelength by one-step hydrothermal method. Due to the large overlap between the absorption spectrum of 2-nitrophenol and the excitation spectrum of SiNPs, based on the synergistic effect of IFE and static quenching effect (SQE), the fluorescence of SiNPs was significantly quenched, thus realizing the highly sensitive and selective detection of 2-nitrophenol[101]. Although SiNPs have the advantages of good fluorescence properties, easy functionalization and low toxicity, the quantum yield of SiNPs needs to be improved, and the application research is still relatively lagging.These methods not only provide a new way for the detection of 2-nitrophenol, but also expand the potential applications of SiNPs. In addition, carbon dots are often used as the fluorescence donor of IFE due to their excellent fluorescence characteristics, and there are many reports on the detection of nitrophenols based on carbon dots. Luo et al prepared a fluorescent nanoswitch based on pH regulation by using the characteristic that the maximum absorption wavelength of 4-nitrophenol (4-NP) changes under different acid and alkali conditions.In alkaline solution, the absorption spectra of 4-NP and water-soluble fluorescent polymer carbon dots (PCDs) were well overlapped, and the fluorescence of PCDs was quenched when IFE was effective.However, in acidic solution, the absorption wavelength of 4-NP was blue-shifted, the bands were no longer overlapped, and the fluorescence of PCDs recovered when IFE disappeared[106]. They also successfully realized the rapid, sensitive and portable fluorescence and visual detection of 4-NP by using polyamide film as the immobilization carrier of PCDs. The traditional doping strategies of carbon dots are mainly based on non-metallic elements, while the doping strategies based on metallic elements are attracting more and more attention, because they can greatly improve the optical properties of carbon dots[107,108]. Wang et al. Synthesized Cr (Ⅲ) -doped carbon dots (Cr-CDs) by hydrothermal method, which has excitation-independent fluorescence characteristics and quantum yield of 20%[108]. The fluorescence of Cr-CDs was quenched by 4-NP due to the overlap of the excitation spectrum of Cr-CDs and the absorption spectrum of 4-NP.Therefore, the highly sensitive detection of 4-NP was achieved based on IFE with a detection limit of 0. 27 μmol/L. As we all know, 2,4,6-trinitrophenol (TNP) is not only a toxic pollutant in the environment, but also a dangerous explosive. It is of great significance to realize its rapid detection with high sensitivity and reliability. Lee et al. prepared four kinds of CdTe QDs wrapped by polythiophene derivatives, which significantly enhanced the quantum yield and stability of CdTe QDs. Because the absorption spectrum of TNP was highly overlapped with the emission spectrum of conjugated polythiophene modified CdTe-QDs (TCPQDs),Therefore, based on the synergistic effect of IFE and intermolecular interactions (including electrostatic, π-π and hydrogen bonding), the fluorescence of TCPQDs is strongly quenched by TNP, thus successfully realizing the highly sensitive detection of TNP with a detection limit as low as 0.56×10-9mol/L[104]. They prepared it into fluorescent test paper and used smart phones as imaging and analysis platform to achieve rapid quantitative and visual detection of TNP on the spot, which provided a new and convenient way to prevent environmental pollution and terrorist threats.
Nowadays, some portable fluorescence sensors have been developed for the rapid detection of organic pollutants in the field, showing great potential in the field of environmental detection[109⇓~111]. For example, Iyer et al. First prepared a naphthalimide derivative (NMI) with the characteristic of aggregation-induced luminescence enhancement as a fluorescence donor in IFE. Because the absorption spectrum of 4-nitroaniline (4-NA) overlaps with the excitation and emission spectra of NMI probe, the fluorescence of NMI probe is quenched under the influence of IFE, so the quantitative detection of 4-NA can be realized[112]. After they prepared the fluorescent probe into fluorescent test paper, its detection limit for 4-NA was 10-4mol/L, which had the advantages of good portability, low cost and rapid response. In addition, the rapid, reliable and on-site detection of trinitrotoluene (TNT) is related to homeland security. George et al. Used polyethyleneimine-coated copper nanoclusters (PEI-CuNC) as the fluorophore of IFE. When TNT was added, the absorption spectrum of TNT overlapped with the excitation spectrum of PEI-CuNC, so IFE was one of the mechanisms leading to the fluorescence quenching of PEI-GuNC[113]. In addition, TNT can combine with PEI (containing amino group) on the surface of PEI-CuNC to form a Meisenheimer complex, and the fluorescence of PEI-CuNC can be significantly quenched by TNT based on the synergistic effect of FRET, PET and IFE, so that a fluorescence and colorimetric dual-signal detection method for TNT is realized. After the PEI-CuNC is prepared into the fluorescent test paper, the detection limit can still reach 10 nmol/L, which makes an important contribution to the field of public safety. In addition to the above-mentioned fluorescent test paper method, Vigalok et al. Also proposed a novel and practical short-wave filtering technology (SWIFT) based on the IFE principle, the specific principle of which is that the excitation wavelength (λexc) of the fluorophore in the actual detection process should be shorter than the maximum absorption wavelength (λmax) (usually more than 100 nm apart).Therefore, when the analyte or its related reaction product exists as an absorber, it will cause a change in the absorption peak at the λexc, and the fluorescence signal intensity of the fluorophore will be quenched or enhanced due to the influence of the IFE.O as to realize the detection of the analyte, but the intensity of the fluorescent signal excited by the same fluorophore at the λmax remains unchanged all the time, so that the integrity and stability of the fluorophore can be monitored in the whole process[114]. According to the technology, the fluorescent emission unit and the analyte response unit can be designed in a separated manner, so that the fluorescent substance can be recycled,Furthermore, commercial fluorescent dyes (such as BODIPY) and common analytical reagents (such as Ellman reagent) can be used to directly detect mercaptans and other substances, which is conducive to the rapid on-site detection or continuous online monitoring of pollutants. Therefore, in the field of fluorescence detection based on IFE, using fluorescent film, test paper and microfluidic chip as portable sensing units, combined with smart phones or other micro-analyzers (integrated optical components) as image data acquisition and processing platform, is conducive to the realization of rapid quantitative and visual detection of pollutants in the field.

2.3.3 Detection of other common small molecular compounds

In recent research reports, IFE-based sensing platforms have been mostly used to detect small molecular chemicals, including ascorbic acid, dopamine, urea, curcumin, methotrexate and other organic small molecular compounds, which exist in the human body or in various drugs.Once excessive or deficient, it may cause harm to the human body, or represent that the human body is in an unhealthy state, so it is of great significance to achieve simple, sensitive and portable fluorescence detection through IFE-based sensing platform in medical diagnosis, biopharmaceuticals and other aspects[115][116][117][118][119]. For example, the lack of ascorbic acid (AA) in the human body may lead to scurvy, and excessive ascorbic acid may cause diarrhea and even urinary calculi. Tan et al. Designed a dual-signal sensing strategy of fluorescence and magnetic resonance imaging based on carbon dots and MnO2 nanosheets. When AA is not present, the fluorescence of carbon dots will be quenched by MnO2, which is due to the overlap of spectral bands between the two to make IFE effective[115]. If AA exists, the reduction of MnO2 to Mn2+,IFE by AA is inhibited, the fluorescence of carbon dots is restored, and the relaxation time of protons in water is sharply reduced due to the release of Mn2+, and the magnetic resonance signal changes significantly, thus realizing the highly sensitive detection of AA. Other small molecular compounds with strong absorption bands, such as curcumin and methotrexate, can be used as absorbers. Gong et al. And Sun et al. Used curcumin and methotrexate as fluorescent absorbers in the IFE system, respectively, and used carbon dots as fluorescent donors to realize the simple construction of the IFE sensing system and their highly sensitive detection[118][119].

3 Conclusion and prospect

The fluorescence analysis method based on IFE has the advantages of high sensitivity, good selectivity, simple and flexible operation, on-site and portable detection, and has a wide range of applications in many fields. In this paper, the research progress of IFE in environmental detection in recent years is summarized, including the detection of heavy metal ions, anions and small molecular environmental pollutants, and the important role of nanomaterials in IFE sensing system is analyzed. It was found that IFE has the following trends in environmental detection: (1) For the target analytes with strong absorption bands in environmental pollutants, the development of fluorescent donors with specific spectra can easily achieve high sensitivity and accurate detection, but attention should be paid to avoid interference. (2) It is helpful to improve the fluorescence quenching efficiency and detection sensitivity by further increasing the spectral overlap area between the absorber and the fluorophore, or by using the synergistic effect of IFE and other sensing mechanisms and signal amplification technologies. (3) The ratiometric fluorescence detection method has been paid more and more attention, which can minimize the interference of background factors and improve sensitivity and reliability; Multi-mode detection has attracted much attention, such as the synergistic effect of fluorescence and colorimetric dual-mode detection, which is conducive to improving sensitivity, reducing detection limit and expanding detection range. (4) The IFE-based fluorescent probe can be assembled with other matrix materials to prepare a composite material, which can enhance the stability and response speed to analytes while basically maintaining the original excellent characteristics of the fluorescent probe.Furthermore, the reaction system is transferred from the liquid phase to the solid phase carrier by using the idea of similar fluorescent test paper for reference, which can improve the portability of use and the ability of real-time quantitative in-situ detection, and is an important development direction of IFE in environmental detection.
Although some progress has been made in the application of IFE in environmental monitoring, there are still some challenges: (1) The materials that can be used as phosphors and absorbers need to be further explored and improved.The quantum yield and stability of fluorescent nanomaterials need to be improved, and other materials with high extinction coefficient need to be actively explored as absorbers besides noble metal nanomaterials. (2) In the complex actual environment, it is easy to be affected by external environmental changes and coexisting interference substances, which leads to the reduction of detection sensitivity, and it is necessary to consider improving the selectivity and reliability of materials and methods. (3) At present, the detection target of IFE-based fluorescence analysis methods is generally single, and there are few reports on the simultaneous detection of multiple targets, so it is necessary to fully explore and utilize the advantages of IFE sensing system and related nanomaterials. In addition to the field of environmental detection, IFE has also been widely reported in biomarker detection and disease diagnosis, so IFE-based fluorescent sensors have broad application prospects.

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Funding

National Basic Research Program of China(2015CB352100)

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