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Using nitrilotriacetic acid (NTA) as the capping agent, β -NaYF 4 upconversion microcrystals (UCMCs) were successfully synthesized via a hydrothermal synthesis method. Systematic investigations were conducted to evaluate the influence of NTA/Ln molar ratio, NaF/Ln molar ratio, and pH on the morphology, size, and crystal phase characteristics of the UCMCs. Multiple characterization techniques, such as scanning electron microscope (SEM) and X-ray diffractometer (XRD), were employed to elucidate these effects. The experimental findings reveal that these parameters exert a substantial influence on the morphology and size of the UCMCs, with the crystal phase alterations primarily governed by variations in the NaF/Ln molar ratio. Furthermore, a comprehensive exploration of the growth mechanism of UCMCs was undertaken based on the modulation of reaction time. The emission spectra of the synthesized UCMCs were meticulously investigated, unveiling that microcrystals exhibiting larger size and enhanced crystallinity display more pronounced luminescent properties. Comparative analysis with UCMCs synthesized using ethylenediaminetetraacetic acid (EDTA) and sodium citrate (Na 3 Cit) as capping agents, which are extensively applied in diverse fields, demonstrated the superior luminescence properties of the newly obtained NTA-UCMCs. In addition, NTA-UCMCs were also employed for the fabrication of transparent and flexible fluorescent films. This research thus presents a novel approach for the synthesis of highly luminescent UCMCs, holding promise for various applications in practical contexts. [Display omitted] • β -NaYF 4 : Yb3+, Er3+ UCMCs were synthesized via hydrothermal method with nitrilotriacetic acid (NTA) used as a capping agent. • NTA/Ln, F/Ln and pH were observed to exert pronounced influences on the morphological characteristics and size of the UCMCs. • The upconversion emission intensity intricately linked to the morphology, size, and surface ligands of the UCMCs. • NTA-assisted synthesis was observed to result in UCMCs with a hexagonal prism morphology, displaying enhanced luminescence. • The combination of NTA-UCMCs and polyvinyl alcohol (PVA) resulted in the formation of a transparent fluorescent film. [ABSTRACT FROM AUTHOR]