The rapid industrialization and population growth have made environmental contamination a critical global issue. Pesticides like methyl parathion, glyphosate, and atrazine, heavy metal ions like Zn2+, Al3+, and Cu2+, anions like F−, NO3−, Cr2O72−, and PO42−,explosives like 2,4,6-trinitrophenol, 2,4-dinitrophenol, and nitrobenzene have all been swiftly discharged into water bodies from altered sources like the chemical, leather, dye, plastic, and pharmaceutical industries [1–3].The ecosystem and human health are severely impacted by these toxins. Zn2+, which is primarily found in plants and natural water, is one of these pollutants that can easily enter the human body through the food chain. In the human body, zinc is the second-most prevalent transition metal ion after iron, and it is essential for signal transmitters, structural or catalytic cofactors, and gene expression regulators [4, 5]. Numerous diseases, including Alzheimer's, epilepsy, amyotrophic lateral sclerosis, Parkinson's, hypoxic ischemia, and epilepsy, can be brought on by the human body having Zn2+ above allowable limit [4, 6]. Therefore, it is becoming more and more important in the fields of medical and environmental sciences to detect Zn2+ ions in drinking water in a sensitive and focused manner.
Although many analytical techniques have been developed to determine Zn2+ ions, including flame atomic absorption spectroscopy, voltammetry, mass spectrometry, inductively coupled plasma mass spectrometry, atomic absorption spectroscopy (AAS), and X-ray fluorescence spectrometry, the majority of these techniques are more expensive, take more time (especially when preparing the samples), and have low sensitivity [7–10]. Significant effort has been put into creating affordable, user-friendly fluorescence-based sensing systems that have great selectivity/sensitivity, simple response, and portability in order to overcome these drawbacks. The main objective today is to design simple, uncomplicated, inexpensive, and selective sensors to detect Zn2+ ions.
Due to their excellent emission properties, rapid response times, high sensitivity, selectivity, porosities, and potential for viable supramolecular interactions between the host frameworks and target analytes, luminescent metal-organic frameworks (LMOFs) have drawn a lot of attention over the past 20 years as excellent fluorescent sensors for detecting trace amounts of analytes[11, 12]. LMOF sensors for the accurate and sensitive identification of metal ions [13, 14], anions [15, 16], and explosives [17–21] have been the subject of numerous reports. The demand for the incorporation of suitable recognition sites in LMOFs to offer different receptor-target interactions has significantly increased, since they can improve the potential features of LMOFs for sensing applications [22]. Schiff bases sites are currently gaining attention as target-specific recognition sites for the production of LMOF-based sensors. Schiff base-decorated LMOF sensors are favorable due to the synergistic effects of the combination of MOF structural properties and target-specific recognition sites (e.g., low DLs, short response times, and good selectivity). In particular, the nitrogen atoms in Schiff base recognition sites—which serve as Lewis bases—interact with a target species via acid-base, hydrogen bonding, and/or coordination interactions to enhance the sensing capacity of LMOF sensors. Instead, the greater porosity and large surface area of LMOFs allow these identifying sites to engage with target species without difficulty, which significantly boosts the sensitivity.
Encouraged by the aforementioned factors, we created MIL-53-HNA, a novel LMOF sensor, using 2-(2-hydroxynapthyl-1-imine)terephthalic acid (H2HNA) Schiff base as an organic linker with Al3+ ion. For sensing Zn2+ ions, this MOF exhibits a superbly sensitive and selective fluorescent turn-on response. It was found that hydroxyl and imine sites within pore cages of MIL-53-HNA are capable of selective complexation with Zn2+ ions, which results in a significant increase in the fluorescence intensity. The proposed Zn2+ sensor has simple manufacture (just a one-step reaction) with superior sensing capabilities is a key benefit.