High-Performance Sensing of Formic Acid Vapor Enabled by a Newly Developed Nanofilm-Based Fluorescent Sensor

Molecular design and architectonics towards film-based fluorescent sensing

The past few decades have witnessed encouraging progress in the development of high-performance film-based fluorescent sensors (FFSs) for detecting explosives, illicit drugs, chemical warfare agents (CWAs), and hazardous volatile organic chemicals (VOCs), among others. Several FFSs have transitioned from laboratory research to real-world applications, demonstrating their practical relevance. At the heart of FFS technology lies the sensing films, which play a crucial role in determining the analytes and the resulting signals. The selection of sensing fluorophores and the fabrication strategies employed in film construction are key factors that influence the fluorescence properties, active-layer structures, and overall sensing behaviors of these films. This review examines the progress and innovations in the research field of FFSs over the past two decades, focusing on advancements in fluorophore design and active-layer structural engineering. It underscores popular sensing fluorophore scaffolds and the dynamics of excited state processes. Additionally, it delves into six distinct categories of film fabrication technologies and strategies, providing insights into their advantages and limitations. This review further addresses important considerations such as photostability and substrate effects. Concluding with an overview of the field's challenges and prospects, it sheds light on the potential for further development in this burgeoning area.

Enzymes encapsulated in organic-inorganic hybrid nanoflower with spatial localization for sensitive and colorimetric detection of formate

Formate is an important environmental pollutant, and meanwhile its concentration change is associated with a variety of diseases. Thus, rapid and sensitive detection of formate is critical for the biochemical analysis of complex samples and clinical diagnosis of multiple diseases. Herein, a colorimetric biosensor was constructed based on the cascade catalysis of formate oxidase (FOx) and horseradish peroxidase (HRP). These two enzymes were co-immobilized in Cu3(PO4)2-based hybrid nanoflower with spatial localization, in which FOx and HRP were located in the shell and core of nanoflower, respectively (FOx@HRP). In this system, FOx could catalyze the oxidation of formate to generate H2O2, which was then utilized by HRP to oxidize 2,2'-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid to yield blue product. Ideal linear correlation could be obtained between the absorbance at 420 nm and formate concentration. Meanwhile, FOx@HRP exhibited excellent detection performance with low limit of detection (6 μM), wide linear detection range (10-900 μM), and favorable specificity, stability and reusability. Moreover, it could be applied in the detection of formate in environmental, food and biological samples with high accuracy. Collectively, FOx@HRP provides a useful strategy for the simple and sensitive detection of formate and is potentially to be used in biochemical analysis and clinical diagnosis.

Recent Advances in Construction Strategies for Fluorescence Sensing Films

A year ago, film-based fluorescent sensors (FFSs) were recognized in the "IUPAC Top Ten Emerging Technologies in Chemistry 2022" due to their extensive application in detecting hidden explosives, illicit drugs, and volatile organic compounds. These sensors offer high sensitivity, specificity, immunity to light scattering, and noninvasiveness. The core of FFSs is the construction of high-performance fluorescent sensing films, which are dependent on the processes of "energy transfer" and "mass transfer" in the active layer and involve complex interactions between sensing molecules and analytes. This Perspective focuses on the latest strategies in constructing these films, emphasizing the design of sensing molecules with various innovative features and structures that enhance the mass transfer efficiency. Additionally, it discusses the ongoing challenges and potential advancements in the field of FFSs.

An Azomethine-H-Based Fluorogenic Sensor for Formic Acid

Formic acid (FA) is an important C1-containing feedstock that serves as a masked source of dihydrogen gas (H2). To encourage the adoption of cleaner (noncarbonaceous) energy sources, FA detection and sensing is thus of considerable interest. Here, we examine the use of a commercially available dye, azomethine-H (Az-H), for FA sensing. Solution studies confirm that FA quenches both the absorbance and the luminescence properties of Az-H. FA was additionally found to attenuate a known Az-H (E)-to-(Z) conformational change, suggesting an Az-H/FA interaction, possibly through hydrogen bonding; this phenomenon was probed using 1H NMR spectroscopy. Moving toward a solid-state sensor, the Az-H probe was incorporated into a gelatin-based matrix. On exposure to FA, the luminescence of this system was found to increase in a FA-dependent manner, attributed to the formation of stable hydrogen-bonded structures, facilitating a (Z)-to-(E) isomerization via imine protonation, allowing for production of the more luminescent (E)-isomer. This fluorogenic signal was used as a FA sensor with an estimated detection limit of ca. 0.4 ppb FA vapor. This work constitutes an important step toward a highly sensitive FA sensor in both the solution and solid state, opening new space for the detection of organic acids in differing chemical environments.

Colorimetric Aerogel Gas Sensor with High Sensitivity and Stability

The detection of formic acid vapor in the usage environment is extremely important for human health and safety. The utilization of metal-organic frameworks (MOFs) for the detection of gaseous molecules is an attractive strategy. However, the rational design and construction of MOF-based gas sensors with high sensitivity and mechanical stability remain a significant challenge. In this study, a simple approach is reported to fabricate colorimetric aerogel sensors assembled from MOF particles via ice template-assisted methods. As the aerogel sensor with staggered lamellae structures significantly provides a high air-volume intake of flowing gas, it generates a sufficient probability of contact reactions for highly mobile target molecules. Additionally, it enhances the mechanical stability by providing stress resistance between the staggered lamellae structures. Compared to conventional film sensors for the detection of formic acid molecules, aerogel sensors exhibit an 8-fold lower limit of detection, 15-fold better sensitivity at low concentrations, 34-fold faster response time, and higher stability. This approach shows great potential for rapid and real-time detection of target molecules as well as superior performance in the structural construction of various gas-sensitive materials.

Imine Bond-Based Fluorescent Nanofilms toward High-Performance Detection and Efficient Removal of HCl and NH3

A new kind of imine bond-based fluorescent nanofilm was developed as multifunctional materials for high-performance detection and efficient removal of hydrogen chloride (HCl) and ammonia (NH3). The flexible, uniform, and photochemically stable nanofilms as prepared showed fast (<1 and <0.5 s), sensitive (<150 ppb and <1.5 ppm), and selective response to HCl and NH3, respectively, and the removal efficiencies to HCl and NH3 are 187.5 and 37.5% (w/w), respectively. A reversible earthy-red to green fluorescence color change upon adsorption of NH3 or HCl enabled visualized monitoring of the two gases in the air. Mechanism studies revealed that the adsorption of HCl is a result of hydrogen bond formation between the analyte and the imine groups. Adsorption of NH3, however, is a result of chemical reaction with the pre-adsorbed HCl. The applicability of the detection and removal strategies as developed was further verified by conducting the tests on real-life or simulated scenarios.

Ratiometric Fluorescent Sensor Based on Tb(III) Functionalized Metal-Organic Framework for Formic Acid

Formic acid is a common chemical raw material, the effective detection of which is of importance to food safety and environmental quality. In this work, the lanthanide functionalized dual-emission metal-organic framework (TH25) was prepared as a ratiometric fluorescent sensor for formic acid. This ratiometric sensor has a good detection performance with high selectivity, sensitivity, and reproducibility. Together with a low limit of detection of 2.1 ppm, these characters promise the ability to sense at low levels as well as a practical detection ability. This work provides ideas for the design and synthesis of effective chemical sensors for organic acids.

Mechanistic insights to lactic and formic acid toxicity on benthic oligochaete worm Tubifex tubifex

Lactic and formic acid are two commonly found monocarboxylic organic acids. Lactic acid is discharged into the water bodies as acidic industrial effluent from the food, cosmetic, chemical, and pharmaceutical industries, whereas formic acid is discharged from various paper, leather tanning, and textile processing industries. The present study investigated the toxicity of both organic acids upon the benthic oligochaete worm Tubifex tubifex. The 96-h median lethal concentration (LC₅₀) values for lactic and formic acid are determined as 143.81 mg/l and 57.99 mg/l respectively. The effects of two sublethal concentrations (10% and 30% of 96 h LC₅₀) of these acids on differential expression of oxidative stress enzymes are investigated. The comparative analysis of acute toxicity demonstrates that formic acid exposure is more detrimental to T. tubifex than lactic acid. The in silico structural analysis predicts that formic acid can interact with cytochrome c oxidase of the electron transport system and thereby inhibits its functionality and induces reactive oxygen species production. Integrated biomarker response (IBR) analysis illustrates that overall oxidative stress of formic acid to T. tubifex is significantly higher than that of lactic acid, which supports the structural analysis. It is concluded from this study that toxicokinetic-toxicodynamic and species sensitivity distributions studies are helpful for ecological risk management of environmental toxicants.

Polymeric Emissive Materials Based on Dynamic Covalent Bonds

Dynamic covalent polymers, composed of dynamic covalent bonds (DCBs), have received increasing attention in the last decade due to their adaptive and reversible nature compared with common covalent linked polymers. Incorporating the DCBs into the polymeric material endows it with advanced performance including self-healing, shape memory property, and so forth. However, the emissive ability of such dynamic covalent polymeric materials has been rarely reviewed. Herein, this review has summarized DCBs-based emissive polymeric materials which are classified according to the different types of DCBs, including imine bond, acylhydrazone bond, boronic ester bond, dynamic C-C bond, as well as the reversible bonds based on Diels–Alder reaction and transesterification. The mechanism of chemical reactions and various stimuli-responsive behaviors of DCBs are introduced, followed by typical emissive polymers resulting from these DCBs. By taking advantage of the reversible nature of DCBs under chemical/physical stimuli, the constructed emissive polymeric materials show controllable and switchable emission. Finally, challenges and future trends in this field are briefly discussed in this review.

Film-based fluorescent sensor for visual monitoring and efficient removal of aniline in solutions and gas phase

Aniline has attracted much concern for its long degradation half-life and huge toxicity to the environment and human beings. Therefore, the development of a multi-functional device for visual detection and efficient removal of aniline was highly anticipated. In our work, the small-size Eu@UiO-66(COOH) was obtained by post-synthesis modification (PSM), and then the film-based fluorescent sensor was prepared by crosslinking reaction. The films not only showed incredible mechanical stability and potential for large-scale preparation, but also have excellent fluorescence response to aniline in solutions and gas phase. As the concentration of aniline increased, the fluorescence of films gradually increased at 350 nm, while the fluorescence gradually quenching at 620 nm, and the detection limits (LOD) of aniline in water and air were 0.27 ppb and 0.086 ppb, respectively. In addition, the adsorption performance of the film for aniline has also been confirmed and the maximum adsorption capacity was 32.6 mg/g, which is a strong guarantee for the realization of ultra-trace detection and toxicity reduction of aniline. In summary, the multi-functional film sensor has been designed for ultra-trace detection and efficient removal of aniline in solutions and gas phase, and have significant value for pollutant treatment, ecological restoration and early prevention.

Interfacially confined preparation of fumaronitrile-based nanofilms exhibiting broadband saturable absorption properties

Interfacial nanofilms with nonlinear optical (NLO) properties were prepared via confined dynamic condensation of 4,4'-methylenedianiline (MDA) with the synthesized 2,3-bis(4-(bis(4-formylphenyl)amino)phenyl)fumaronitrile (BTFA). Investigated using the open-aperture Z-scan technique, BTFA showed reverse saturable absorption ascribed to the synergetic mechanisms of two-photon and excited-state absorption. In contrast, the as-prepared nanofilms demonstrated broadband saturable absorption within the spectral range of 720∼1700 nm. The characteristics of nonlinear absorption coefficient (β) decreased along with increasing the incident pulse intensity. Taking advantage of the flexibility and post-machinability properties, the folding layers of the nanofilms offered the feasibility to fine-tune the specific NLO responses. The optimal β value was found to be -10.1 cm/MW for eight-layer nanofilm as well as the normalized transmittance increased up to 35-fold at 800 nm. Utilized as a conceptual saturable absorber, the representative modulation depth and saturation intensity were observed to be around 2.4% and 7.37 GW/cm² at 800 nm, respectively, comparable to traditional two-dimensional (2D) materials. Aiming to clarify the possible underlying physical processes, a four-level model was employed to illustrate the fast relaxation of the excited states. Present work demonstrates that proper design of building blocks combined with interfacially confined dynamic condensation enables rational development of high-performance NLO materials.

Interfacially confined preparation of copper Porphyrin-contained nanofilms towards High-performance Strain-Pressure monitoring

The development of ultrasensitive, durable and anti-jamming strain-pressure sensors that can precisely distinguish different motions or deformations is crucial for health diagnosis and disaster monitoring, but it also remains a challenge. In this study, a self-standing, highly flexible and uniform nanofilm (CuPTFA) was fabricated via interfacial covalent condensation of a copper complex of 5,10,15,20-tetra(4-carboxyphenyl)-porphine tetrahydrizides (Cu-TPPNHNH₂) and tris-(4-formylphenyl)-amine (TFA). The film was used as a sensing layer in a strain-pressure sensor with a sandwich-like structure of Au/I₂@CuPTFA/Au. The sensor had a gauge factor exceeding 10000, a strain range of linear response of 3-7%, reliable output signals after 5000 times of continuous use, and extraordinary noise-screening capability. The strain-pressure sensor can differentiate moderate muscle hyper-spasm from subtle swaying, walking and vigorous sporting activities, with great potential for real-life applications.

Conformationally tunable calix[4]pyrrole-based nanofilms for efficient molecular separation

Preparation of nanofilms which are able to reject water-soluble low molecular weight organic compounds in nanofiltration remains to be a challenge. Herein, we report a new kind of self-standing, defect-free, robust, centimeter-sized and thickness controllable calix[4]pyrrole (C[4]P)-based nanofilms with excellent molecular sieving performance in nanofiltration. The nanofilms were prepared via confined dynamic condensation of the tetra-benzoyl-hydrazine derivative of calix[4]pyrrole (CPTBH) with 1,3,5-benzenetricarboxaldehyde (BTC) at the air/dimethyl sulfoxide (DMSO) interface. Nanofiltration tests under 2 bar pressure with porous polyethylene terephthalate (PET) as the support and a CsF treated CPTBH-BTC nanofilm (∼100 nm) as the selective layer depicted a water permeance of 15 L m^-2h^-1 bar^-1 and a methanol permeance of 45 L m^-2h^-1 bar^-1. High rejection rates (>95%) were found in aqueous solution for most of the tested dyes and pharmaceuticals. Remarkably, the composite membrane also demonstrated good separation performance in aqueous phase to some amino acids and organic dyes with molecular weights around 200 g/mol. High-performance nanofiltration in methanol was also realized. In this case, the molecular weight cutoff value is ∼ 800 g/mol. These findings showed that introduction of macrocyclic hosts is an effective way to develop nanofilms with high solvent permeance but low molecular weight cutoff value.

Film Nanoarchitectonics of Pillar[5]arene for High-Performance Fluorescent Sensing: a Proof-of-Concept Study

Substrates play crucial roles for the sensing performances of fluorescent films owing to their effect on the formation of a fluorescent adlayer. However, no such film has been developed through synthesizing a substrate with a defined structure. We herein report a kind of self-standing, uniform, and thickness tunable pillar[5]arene-based nanofilms to serve as substrates for fabricating fluorescent sensing films. In comparison with a glass plate, the pillar[5]arene-based nanofilms can ensure spatial and electronic isolation of immobilized fluorophores and circumvent aggregation-caused quenching in a film state. For conceptual proof, a formic acid fluorescent sensing film was developed through simple loading of a fluorophore, a 4-azetidine-1,8-naphthalimide derivative of cholesterol (NA-Ch), onto the prepared nanofilm. Sensing performance studies demonstrated that the fluorescent film showed a sensitive, fast, and highly selective response to formic acid in air with a detection limit of lower than 2.8 mg m^-3 and a response time of less than 3 s. Moreover, the sensing is fully reversible and highly repeatable. Further studies showed that the film sensor can be used for fast determination of methanol acidity via vapor sampling. Clearly, innovation of substrates with defined structures can be taken as an effective and efficient way to develop new sensing films via combination with known fluorophores.