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Label-free upconversion nanosensor for water safety monitoring of permanganate and dichromate ions

BMC Chemistry volume 19, Article number: 60 (2025) Cite this article

Abstract

Water contaminated with heavy metal ions poses serious threat to the human health and environment protection. It is imperative to develop analytical tools to detect heavy metal ions. Herein, we propose autofluorescence free SiO2 modified upconversion nanosensor for label-free and fast determination of MnO4− and Cr2O72− anions. The highly efficient and multi-colour upconversion luminescence (UCL) of UCNPs@SiO2 was effectively quenched by MnO4− and Cr2O72− anions with fast response time of 2 and 1 min, respectively. The UCNPs@SiO2 nanosensor exhibits linear detection ranges of 0.6–2000, 2–2000 µM with the LOD at 0.15, 0.04 µM for MnO4− and Cr2O72− anions, respectively. The nanosensor was successfully applied for real lake and tap water samples with satisfactory results. The UCNPs@SiO2 UCL nanosensor demonstrates autofluorescence free and fast determination of MnO4− and Cr2O72− anions with high sensitivity, good specificity, low LOD, and wide linear detection range, holding great potential for food and environmental sample sensing.

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Introduction

In current fast paced industrialization, it is of paramount importance to detect hazardous chemicals including heavy metal cations and anions discharged from industrial effluents because of their serious threat to human health and environment protection [1]. Industrial wastewater that contains numerous heavy metal ions contamination could pose significant challenge to the aquatic plants, animals as well as to humans due to their potential adverse effects. Among the hazardous chemicals, permanganate ion (MnO4−) is believed to be a potential carcinogenic chemical, which is frequently used as bleaching, disinfectant and redox reagents both in laboratory and industry [2, 3]. It is well-known that KMnO4 is widely used in Hummers Method for converting graphene oxide into graphene, which produces hazardous chemical species such as Mn2+, MnO4− and Mn2O7 [4]. KMnO4 is also used for water treatment to remove reductive, odorous, phenolic substances [5]. The trace amount of MnO4− can not only lead to the neurological disorder in human but also could impart disability [6]. World Health Organization (WHO) prescribed an upper consumption limit for total manganese in drinking water i.e., 0.1 mg L− 1 [4]. An excess of MnO4− is harmful and can lead to kidney and liver damage [7].

Besides MnO4−, the dichromate ion (Cr2O72−) is another type of extremely hazardous material because of the toxic, carcinogenic and mutagenic nature [8]. The high-valent chromium species Cr2O72− anions are widely utilized as chemical intermediates [9]. The Cr2O72− presence in the industrial wastewater dominates since the chromium-based materials are widely used both in industry and home as a decorative and protective paint to prevent automobile and home/offices furniture from wear and tear thereby improving the shelf life of the components used in daily life [10]. A rather serious problem arises when the Cr2O72− anions are freely discharged into the water sources without any processing after being used in mineral processing, printing, plating, and tanning. According to the WHO, the total amount of Cr concentration in consumable water should not exceed above 0.05 mg L− 1 [10]. Excess intake of this anion can cause haemorrhaging, asthma, kidney diseases, neurological disorders and cancer [11, 12].

The efficient detection and decontamination of MnO4− and Cr2O72− from water environments is thus highly desired yet challenging due to the coexistence of other heavy metals with them. The complex water environment containing different metal ions alongside MnO4− and Cr2O72− makes the detection and removal complicated because of the competition between luminescence and adsorption sites of the ions. It is difficult to fabricate reliable sensing platform to determinate MnO4− and Cr2O72− from such a complex environment considering significant high signal to noise ratio caused by other heavy metal ions [8]. Therefore, exploring effective ways to detect inorganic ions has attracted much attention. Thus, it is essential to quantify the traces of MnO4− and Cr2O72− onsite from drinking water, environment as well as from manufacturing industry, and ecological system. In this regard, developing novel probes with high sensitivity and selectivity to detect metal ions is of prime importance for human health. So far, a variety of analytical techniques have been employed to detect ions, like inductively coupled plasma mass spectrometry (ICP-MS) [13], atomic absorption spectroscopy (AAS) [14], liquid chromatography-tandem mass spectrometry [15], ion exchange [16], adsorption [17], and membrane separation [18]. However, these approaches are still limited, needing specialized and bulkier instruments, moreover, they are difficult to operate, expensive, and time-consuming [19].

Over the past decade, fluorescent studies have gained significant attention owing to superior sensitivity and selectivity, onsite detection, simple operation, and portability [20]. Fluorescent probes for sensing metal ions include metal organic frameworks (MOFs), carbon-based nanomaterials (graphene, nanotube, nanodots and quantum dots), and metal or metal oxide nanomaterials [21,22,23,24,25,26,27] etc. These materials are not adequate for metal ions detection because of their tendency to aggregate, limited quantum yield, poor selectivity, and weak photostability.

Upconversion nanoparticles (UCNPs), which convert near-infrared (NIR) light to emission ranging UV-to-visible wavelength, have attracted enormous attention in recent past on account of the distinct characteristics. These include sharp emissions with long luminescence lifetime, resistance to photobleaching, high photostability, biocompatibility and negligible autofluorescence [28,29,30]. UCNPs have demonstrated successful applications across various fields, including bioapplications for bioimaging, single-particle biomarking, and therapeutics, optical sensing for temperature, pressure, pH, and molecular detection, and lighting and displays applications for nanoparticle lasing, full-colour display, information storage and anticounterfeiting [31]. In recent years, the UCNPs have been successfully applied to detect antibiotics [32, 33], heavy metal ions [34] as well as pesticides [35]. In addition, the rational core-shell structural design of UCNPs not only generates protective shells for upconversion luminescence (UCL) enhancement, but also regulation stoichiometric composition within a specified scale for fine tuning emission colours and dynamic control of upconversion process. It provides significant flexibility, enabling the synergistic development of new multifunctional biosensor with tuneable properties [36, 37]. The as-synthesized UCNPs are normally capped by hydrophobic ligands. A further requirement for surface modification enables UCNPs for obtaining aqueous dispersion and preventing aggregation. Silica shell exhibits transparency, biocompatibility, non-toxicity, chemical inert, and hydrophilicity. SiO2 encapsulation results in SiO2-coated UCNPs both stable and well-dispersed in aqueous solution [38, 39]. Moreover, the silica shell can be further integrated with functional groups through silanization techniques.

In this study, a novel multi-shelled UCNPs@SiO2 nanoprobe was designed and developed for detection of MnO4− and Cr2O72− anions, via energy transfer (ET; Fig. 1). The oleate capped core-shell-shell-shell UCNPs (denoted as OA-UCNPs) were typically synthesized using a co-precipitation method. Subsequently, the UCNPs were modified with SiO2 layer (denoted as UCNPs@SiO2). In addition to the robust UV-to-visible UCL, the resulting UCNPs@SiO2 were dispersed well in water, chemically stable, and biocompatible. UCNPs@SiO2 showed significant reduction in UV and visible UCL after MnO4− addition, while extensively quenched UV UCL with the presence of Cr2O72−. The UCNPs@SiO2 based UCL sensor effectively detected MnO4− and Cr2O72− in real samples of lake and tap water. The proposed UCNPs@SiO2 UCL sensor demonstrates feasibility, sensitivity, and selectivity for the determination of MnO4− and Cr2O72− in environmental contaminants.

Fig. 1
figure 1

Schematics of the UCNPs@SiO2 based UCL sensor for MnO4− and Cr2O72−detection

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Materials and methods

Materials

All the raw materials and solvents were, obtained directly from the suppliers, utilized in current study without any further purification step. The high purity (≥ 99.9%) raw materials i.e., Y(CH3CO2)3⋅xH2O, Gd(CH3CO2)3⋅xH2O, Eu(CH3CO2)3⋅xH2O, Yb(CH3CO2)3⋅4H2O, Tm(CH3CO2)3⋅xH2O, Lu(CH3CO2)3⋅xH2O along with the ≥ 98% pure NaOH and NH4F were supplied by Sigma-Aldrich. The solvents oleic acid (90%) and 1-octadecence (90%) used in current study were supplied by Sigma-Aldrich. While the tetraethyl orthosilicate (TEOS, ≥ 99%), ammonium hydroxide (NH3⋅H2O, AR, 25-28%), cyclohexane (99.7%), methanol (AR), and N, N-dimethylformamide (DMF, AR, ≥ 99.99%) were supplied by Macklin, China. Ethanol (99.7%) and acetone (≥ 99.7%) were procured from Guangdong Guanghua Technology, China and Guangzhou brand, China, respectively. Igepal CO-520 (Polyoxyethylene (5) nonylphenyl ether, (C2H4O)n·C15H24O, n ~ 5) was supplied by Aladdin, China. The aqueous metal anions solution was prepared from their respective salts (analytic or molecular biology grade) such as NaCl, KBr, CH3COONa, NaHCO3, Na2CO3, NaNO3, Na2SO4, NaH2PO4·2H2O, Na2HPO4, Na3PO4, and KSCN. Ultrapure water was produced via Milli-Q purification system installed at university. Lake water and tap water were collected from the university town campus of Guangdong University of Technology (GDUT). The membrane with 0.22 μm pore size was supplied by the Jinteng Co., Ltd., Tianjin, China. The disposable sterile syringe was purchased from Jiangsu Kangyou medical equipment Co., Ltd, China.

Instruments

X-ray diffractometer (XRD, Malvern Panalytical DY735, UK) with a Cu Kα irradiation (λ = 1.5406 Å) and 2θ range of 10–90° with a scanning rate of 5° min− 1 was used to study crystal structure of the prepared powders. To evaluate the nanoparticle morphology and size, transmission electron microscopy (TEM, Hitachi HT7700, Japan; 100 kV) was used. For nanoparticle elemental mapping analysis, a high-resolution TEM (HRTEM, FEI TALOS F200S, Czech Republic; 200 kV) was operated under the scanning transmission electron microscopy (STEM) mode coupled with energy-dispersive X-ray spectroscopy (EDS). Fourier transform infrared (FT-IR) transmittance spectra in the 400–4000 cm− 1 range with a resolution of 1 cm− 1 were measured on a Nicolet 6700 spectrometer (Thermo-Fisher Scientific, USA). Ultraviolet-visible (UV-Vis) absorption spectra and the zeta potentials of the liquid samples were recorded through the Lambda 950 spectrophotometer (PerkinElmer, USA) and the Zeta potential analyzer (Malvern, UK), respectively. The UCL spectra of liquid samples were measured on a USB 2000 + spectrometer (Ocean Optics, USA) using an external fibre-coupled 980 nm diode laser (BWT Beijing Ltd., China). To measure the single particle UCL lifetime, 980 nm NIR laser (Laser 1, B & A Technology Co., Ltd.) coupled Edinburgh FLS980 spectrophotometer was used with 2.9 mW laser power, 971 Hz pulsed frequency and 650 nm spot diameter. The sample droplets were descended onto the glass slides for measuring the nanoparticle UCL lifetime.

Synthesis of UCNPs

Co-precipitation method was used to prepare the NaYF4:0.2Eu core nanoparticles as reported in [40] with some modifications. Briefly, 2 mL water solution of Y(CH3CO2)3 (0.2 M) and Eu(CH3CO2)3 (0.2 M) was added to a 50 mL round bottom flask containing oleic acid:1-octadecence (4:6 mL) solvents. Subsequently, the mixture was heated for 30 min at 160 ℃ until the lanthanide-oleate complex formation and then allowed to cool down to room temperature. The methanol solution containing NaOH (1 mmol) and NH4F (1.54 mmol) was then added to the resultant mixture and heated again for 30 min at 50 ℃ under constant stirring. Thereafter, the reaction temperature of the mixture was raised to 110 ℃ for methanol evaporation. The resultant mixture was then heated at 300 ℃ for 1 h under an inert (argon) environment. Finally, the resultant NaYF4:0.2Eu core nanoparticles were allowed to cool naturally and washed several times in different solvents (ethanol, methanol and cyclohexane). The NaYF4:0.2Eu core nanoparticles redispersed in 4 mL cyclohexane were stored at 4 ℃ in a refrigerator for further use.

To synthesize NaYF4:0.2Eu@NaYbF4:0.39Gd/0.01Tm (core-shell), NaYF4:0.2Eu@NaYbF4:0.39Gd/0.01Tm@NaYbF4 (core-shell-shell), and NaYF4:0.2Eu@NaYbF4:0.39Gd/0.01Tm@NaYbF4@NaLuF4 (core-shell-shell-shell) nanoparticles, similar procedures were employed, with the exception of precursor solution preparation that Y(CH3CO2)3 and Eu(CH3CO2)3 was replaced with Yb(CH3CO2)3, Gd(CH3CO2)3 and Tm(CH3CO2)3 for core-shell, Yb(CH3CO2)3 for core-shell-shell, and Lu(CH3CO2)3 for core-shell-shell-shell nanoparticles. The detail preparation procedures are supplied in the supplementary information.

Silica coating of UCNPs

By using a reverse microemulsion method, the SiO2 shell was coated onto the as-synthesized oleate capped core-shell-shell-shell UCNPs (OA-UCNPs) [41, 42]. Typically, 1.4 mL of OA-UCNPs in cyclohexane, was mixed with 8.6 mL of cyclohexane, and 0.4 g of CO-520 surfactant and stirred for 10 min. To form a water-in-oil microemulsion, 1.6 g of CO-520 surfactant and 80 µL of NH3⋅H2O were then added and sonicated for 30 min. Then, the mixture was constantly stirred for 48 h at room temperature after the addition of TEOS (40 µL). Post stirring for 48 h, the acetone was used to precipitate the SiO2 coated UCNPs (UCNPs@SiO2). The final product was washed twice with ethanol/water (v: v = 1:1) via centrifugation for 20 min at 9500 rpm speed. The UCNPs@SiO2 was finally dispersed in water (7 mL, 10.0 mg/mL) and kept in a refrigerator for further use.

UCL determination of anions

To achieve optimum UCL based sensing performance of UCNPs@SiO2 towards MnO4− and Cr2O72−, the optimal sensing time was investigated. The UCL spectra were measured for UCNPs@SiO2 (1.0 mg/mL) in presence of MnO4− (20, 2000 µM) and Cr2O72− (1000 µM) anions, respectively, mixed at different stirring times. The stirring time for MnO4− and Cr2O72− was (2, 5, 10, 15, 20, 30, 40, 60, 180, and 360 min) and (1, 2, 4, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 40, 50, 60, 120, and 180 min), respectively.

By fixing the optimized time, i.e., 2 min for MnO4− and 1 min for Cr2O72−, the MnO4− and Cr2O72− concentration dependent UCL spectra of UCNPs@SiO2 (1.0 mg/mL) were recorded where the concentrations of MnO4− and Cr2O72− were (0.6, 0.8, 2, 5, 10, 20, 50, 80, 100, 150, 200, 500, 800, 1000, 1400, 1600 and 2000 µM) and (2, 5, 10, 20, 100, 200, 500, 800, 1000, 1500 and 2000 µM), respectively.

To evaluate the selectivity and anti-interference properties of UCNPs@SiO2 nanoprobes for determination of MnO4−, the UCL spectra of UCNPs@SiO2 (1.0 mg/mL) containing different anions (including 800 µM of MnO4−, Cl−, Br−, CH3COO−, HCO3−, CO32−, NO3−, SO42−, H2PO4−, HPO42−, PO43−) at the optimized stirring times (2 min) were measured. Also, the UCL spectra of UCNPs@SiO2 (1.0 mg/mL) were measured when MnO4− (800 µM) was coexisted with 800 µM of Cl−, Br−, CH3COO−, HCO3−, CO32−, NO3−, SO42−, H2PO4−, HPO42−, PO43− individually or in combination with all other anions (800 µM of Cl−, Br−, CH3COO−, HCO3−, CO32−, NO3−, SO42−, H2PO4−, HPO42−, PO43−) under the aforementioned optimal conditions to check the anti-interference properties of the UCNPs@SiO2. All the UCL spectra of the UCNPs@SiO2 were carried out at room temperature.

To evaluate the selectivity and anti-interference performance of UCNPs@SiO2 sensor for determining Cr2O72−, the UCL spectra for UCNPs@SiO2 (1.0 mg/mL) containing different anions (including 2000 µM of Cr2O72−, Cl−, Br−, CH3COO−, NO3−, SO42−, SCN−, H2PO4−, HPO42−, and PO43−) were measured at the optimized stirring time (1 min). Furthermore, the UCL spectra of UCNPs@SiO2 (1.0 mg/mL) in the presence of Cr2O72− (2000 µM) coexisted with 2000 µM of Cl−, Br−, CH3COO−, NO3−, SO42−, SCN−, H2PO4−, HPO42−, and PO43− individually or in combination with all other anions (2000 µM of Cl−, Br−, CH3COO−, NO3−, SO42−, SCN−, H2PO4−, HPO42−, and PO43−) under the aforementioned optimal conditions. All the UCL spectra of the UCNPs@SiO2 were carried out at room temperature.

Determination of anions in lake and tap water sample

To check the performance of the UCNPs@SiO2 probes in real samples sensing application, different types of water (tap and lake) was collected and the presence of the anions was assessed. The lake water was injected using disposable sterile syringes and filtered through a 0.22 μm membrane to remove insoluble particles according to the described procedures [33]. For analysis, different concentrations of MnO4− (1, 5, 20, 100, 500, 1000 and 2000 µM) or Cr2O72− (10, 50, 100, 500, 1000 and 1500 µM) were added to the 100-fold ultrapure water diluted lake water. On the other hand, the obtained non-filtered tap water was 100-fold diluted with ultrapure water and loaded with MnO4− (1, 5, 20, 100, 500, 1000 and 2000 µM) or Cr2O72− (10, 50, 100, 500, 1000, and 1500 µM). The UCL spectra were measured for the real samples of UCNPs@SiO2 (1.0 mg/mL) with anions using 980 nm laser excitation.

Results and discussion

Characterization of UCNPs

We developed the NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4@NaLuF4 core-shell-shell-shell UCNPs for UCL covering the UV, blue, and red emission. The 980 nm NIR excitation extracted by Yb3+ ions was upconverted for Tm3+ UV and blue emission, which was transferred through the Gd sublattice [40]. Interfacial energy transfer from Gd3+ (inner shell) to Eu3+ (core), is responsible for the red UCL [43, 44]. An extra NaYbF4 sensitizer layer is designed to obtain the UCL with great enhancement [45], as demonstrated in previous publication of our group [35]. The inert outermost NaLuF4 shell is to protect the upconversion process [32]. Recently, Chen group [46] developed NaYF4:Yb, Er UCNPs with surface attaching DNAzymes labelled with BHQ1 dye for detection of Cu2+ ions at the single-nanoparticle-level with LOD of 220 pM. Ding et al. [47] developed poly acrylic acid functionalized NaYF4:Yb, Er/2,2-bipyridine system for ciprofloxacin sensing in the 0.05–1000 ng/mL range and 0.13 ng/mL LOD. However, the proposed core-shell-shell-shell UCNPs present several significant benefits, such as relatively small particle size for nanosensor application, robust strong multicolor UCL covering the UV-Vis region, significantly enhanced UCL due to the extra NaYbF4 sensitizer layer combined with the inert protection shell (NaLuF4) as outmost layer.

NaYF4:Eu core nanoparticles were prepared for successive growth of the layers of NaYbF4:Gd/Tm, NaYbF4, and NaLuF4 using coprecipitation method [40]. Fig. S1a-c shows representative TEM images of the core (NaYF4:Eu), core-shell (NaYF4:Eu@NaYbF4:Gd/Tm) and core-shell-shell (NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4) nanoparticles, respectively. The nanoparticles are well-dispersed, uniform in size, and have a regular spherical shape. The particle size distribution histograms are shown in the Fig. S1d-f, which shows the particle diameter of 17.0 nm, 27.0 nm, and 32.9 nm, respectively. The TEM image of the core-shell-shell-shell NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4@NaLuF4 UCNPs presented in Fig. 2a, shows that the UCNPs are monodispersed, uniform in size, and have a regular spherical shape. The particle size distribution histogram is shown in the Fig. S2, with an average diameter of 39.5 nm. The TEM image of the UCNPs displayed in Fig. 2(b-h) along with corresponding elemental mapping, indicating that the (Y3+, Eu3+), (Gd3+, Tm3+) and Lu3+ are present in the core, inner shell, and outermost shell, respectively, which is consistent with the designed composition. The EDS result shown in Fig. 2i is in line with the elemental mapping and the designed UCNPs composition. Through HRTEM of a single core-shell-shell-shell nanoparticle, see Fig. 2j, the lattice fringes are found to exhibit d-spacing of 0.520 nm that corresponds to the (100) plane arises from NaYF4 hexagonal structure. To further investigate the structure of the as-synthesized UCNPs, the XRD patterns of the UCNPs (Fig. 2k), core, core-shell, core-shell-shell nanoparticles (Fig. S3), reveal main characteristic peaks at 17.2° (100), 30.0° (110), 30.89° (101), 43.55° (201) and 53.79° (211) of a NaYF4 hexagonal structure (JCPDS No. 16–0334), which are consistent with the lattice d-spacing result indicated in Fig. 2j.

The UCL spectra of the NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4@NaLuF4 UCNPs carried out at room temperature under 980 nm excitation can be seen in Fig. 2l. The prominent UCL in the UV to visible region centered at 345, 361, 450, 477 and 646 nm, are readily assigned to the Tm3+ transitions of 1I6 → 3F4, 1D2 → 3H6, 1D2 → 3F4, 1G4 → 3H6, 1G4 → 3F4, respectively [40, 48]. The prominent Eu3+ peaks were observed at 510 (5D2 → 7F3), 525 (5D1 → 7F1), 535 (5D1 → 7F2), 555 (5D1 → 7F3), 590 (5D0 → 7F1), 615 (5D0 → 7F2) and 695 nm (5D0 → 7F4) [49]. The UCL peak related to the 6P7/2 → 8S7/2 transition of Gd3+ was detected at 312 nm [50]. The UCL intensity was similar for NaYF4:Eu@NaYbF4:Gd/Tm core-shell nanoparticles in comparison with NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4 core-shell-shell nanoparticles with outmost NaYbF4 sensitization layer, while ~ 100 folds stronger for NaYF4:Eu@NaYbF4:Gd/Tm@NaLuF4 core-shell-shell nanoparticles with outmost NaLuF4 protection layer than NaYF4:Eu@NaYbF4:Gd/Tm core-shell nanoparticles, indicating the inert NaLuF4 layer capable of eliminating the non-radiative energy losses related surface defects and quenching [51]. Inserting of NaYbF4 sensitization layer between NaYbF4:Gd/Tm and NaLuF4 shells can significantly enhance the UCL intensity ~ 1500-fold, as also illustrated in the inset of Fig. 2l showing the NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4@NaLuF4 core-shell-shell-shell UCNPs with significantly enhanced UCL than NaYF4:Eu@NaYbF4:Gd/Tm@NaLuF4 core-shell-shell nanoparticles under irradiation with a 980 nm laser.

Fig. 2
figure 2

TEM image of the (a) core-shell-shell-shell NaYF4:0.2Eu@NaYbF4:0.39Gd/0.01Tm@NaYbF4@NaLuF4 UCNPs, (b) randomly chosen core-shell-shell-shell UCNPs for elemental analysis, (c–h) elemental mapping and (i) EDS spectrum of the core-shell-shell-shell UCNPs; (j) HRTEM image of individual core-shell-shell-shell nanoparticles (d-spacing = 0.520 nm); (k) XRD pattern of the core-shell-shell-shell UCNPs; (l) UCL spectra of NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4@NaLuF4 in comparison with NaYF4:Eu@NaYbF4:Gd/Tm, NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4 and NaYF4:Eu@NaYbF4:Gd/Tm@NaLuF4 nanoparticles (in cyclohexane) measured under 980 nm laser excitation, where the right inset shows the photographs of the as-synthesized NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4@NaLuF4 UCNPs (denoted as CSSS) and NaYF4:Eu@NaYbF4:Gd/Tm@NaLuF4 (denoted as CSS(Lu)) upon 980 nm irradiation

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Surface SiO2 modification of UCNPs

The obtained core-shell-shell-shell OA-UCNPs are hydrophobic in nature. On the other hand, silica shell could provide good hydrophilicity to UCNPs. Therefore, we introduced SiO₂ layer on to the UCNPs surface, to facilitate the transformation of hydrophobic OA-UCNPs to hydrophilic UCNPs@SiO₂. Furthermore, the SiO2 layer protects the UCNPs from the adverse environmental conditions [52]. Figure 3a, represents a typical TEM image of the UCNPs@SiO2. The UCNPs@SiO2 are monodispersed with regular spherical shape with the particle size about 44.6 nm (Fig. S4). The SiO2 shell exhibits integration and uniformity, with the shell thickness approximately 2.6 nm. The FT-IR spectrum of OA-UCNPs, as shown in Fig. 3b, exhibits several absorption peaks at (2926, 2853 cm− 1) and (1562, 1463 cm− 1) that could be attributed to the characteristic asymmetric and symmetric stretching vibrations of CH2 and carboxylic (-COO) groups, respectively [48, 53], suggesting the presence of OA capping ligand on the UCNPs surface. In contrast, after encapsulation of UCNPs with SiO2, the characteristic absorption at 2926, 2853, 1562, and 1463 cm− 1 was significantly reduced, indicating successful removal of OA from UCNPs surface. UCNPs@SiO2 exhibit prominent absorption band at 3433 cm− 1 due to the O-H stretching frequency from silanol group (Si-OH). In addition, the other absorption bands due to the Si-O-Si (1074 and 795 cm− 1), Si-OH (963 cm− 1), and Si-O (458 cm− 1) were observed [32]. Figure 3c shows the result of the zeta potential analysis. The surface potentials (ζ) for the water-soluble negatively charged UCNPs@SiO2 was found to be -12.9 mV, which agrees with previously reported result [32], confirming the SiO2 surface modification. Upon addition of the analyte anions, the UCL of UCNPs@SiO2 was declined significantly in presence of MnO4− (Fig. 3d), while the UV UCL of Tm3+ at 345, 360 nm was seriously quenched by Cr2O72− (Fig. 3d), indicating that the UCNPs@SiO2 UCL is feasible for the sensing of MnO4− and Cr2O72− anions.

Fig. 3
figure 3

(a) TEM image and (b) FT-IR transmittance spectra of UCNPs@SiO2, with later in comparison with OA-UCNPs; (c) Zeta potential of UCNPs@SiO2, and (d) UCL spectra of UCNPs@SiO2 (1.0 mg/mL), UCNPs@SiO2 (1.0 mg/mL) with presence of MnO4− (500 µM) or Cr2O72− (1000 µM) in aqueous solution under 980 nm laser excitation

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UCNPs@SiO2 sensor optimization for the anion detection

To achieve efficient determination of anions using UCNPs@SiO2 sensor, we investigated the optimal experimental conditions for the UCNPs@SiO2 with the anions. The UCNPs@SiO2 were dispersed (Fig. S5a) in different solvents including water, ethanol, methanol, and DMF, respectively, and the solvent-dependent UCL was measured. The UCNPs@SiO2 exhibit the similar UCL intensity (Fig. S5b) for dispersion in water, ethanol, methanol, and DMF, respectively. On the other hand, similar as shown in Fig. 3(a) for TEM of UCNPs@SiO2, Fig. S6 shows the TEM of UCNPs@SiO2 prepared in repeated batch for one week storage in water, which also exhibit a uniform morphology with SiO2 layers modification on surface of UCNPs, further indicating water dispersion of UCNPs@SiO2 remains stable without aggregation. Combining consideration of the MnO4− and Cr2O72− anions mostly showing presence in wastewater, the water was then chosen as the optimal solvent for sensing. The UCNPs@SiO2 sensor was then incubated with MnO4− and Cr2O72− anions in water and the UCL was investigated to study the time-dependent UCL quenching at different stirring time. The MnO4− addition at concentrations of 20 µM (Fig. S7a) and 2000 µM (Fig. S7c) significantly quenched the UCL of UCNPs@SiO2 at a 2-minute incubation time, and the corresponding time-dependent relative UCL intensity with presence of 20 µM (Fig. S7b) and 2000 µM (Fig. S7d) MnO4− concentration exhibits negligible change after 2 min incubation between UCNPs@SiO2 and MnO4−. On the other hand, upon addition of Cr2O72−, the UCNPs@SiO2 show significantly quenched UCL (Fig. S8a) at 1 min of incubation, and progressively enhanced quenching until 7 min incubation time, then reach negligible change after 7 min incubation time, as shown in Fig. S8b for the corresponding time-dependent relative UCL intensity. With increasing the incubation time between 1 to 7 min, the enhanced UCL quenching of UCNPs@SiO2 upon addition of Cr2O72−, may be attributed to some possible interferences, such as pH change for UCNPs@SiO2 sensor, highly chemical oxidation capability for Cr2O72−. To eliminate the possible interferences as listed above, the incubation time was thus optimized at 1 min for UCNPs@SiO2 sensor with Cr2O72−. Therefore, we chose water as the optimal solvent, and 2 and 1 min as the optimal time for incubating UCNPs@SiO2 with MnO4− and Cr2O72− anions in water, respectively.

Sensing for MnO4 −

To assess the UCNPs@SiO2 sensor sensitivity for determination of MnO4− anion, the UCL of the designed UCNPs@SiO2 sensor was measured under the optimal conditions in presence of various concentrations of MnO4− anion. Figure 4a shows the UCL spectra of UCNPs@SiO2 in wavelength range of 300–400 nm against the different concentrations of MnO4− (0.6–2000 µM) in the homogeneous assay. As the concentration of MnO4− increased (0.6–2000 µM), the intensity of UCNPs@SiO2 UCL main peaks due to the 1I6 → 3F4 (345 nm), 1D2 → 3H6 (361 nm), 1D2 → 3F4 (450 nm), and 1G4 → 3H6 (477 nm) transitions of the Tm3+ ions was significantly quenched. The UCNPs@SiO2 sensor exhibits promising sensing behaviour for MnO4− where the UCL intensity was significantly quenched (95%) after addition of 2000 µM of MnO4− (Fig. 4a). A major UCL quenching, about 10%, was observed in presence of MnO4− at low concentration of 2 µM (Fig. 4a). To explore the UCL quenching behaviour towards MnO4−, a Stern-Volmer curve (Fig. 4b) was plotted using the equation, \(\:\frac{{I}_{0}}{I}=1+{K}_{sv}\left[c\right]\), where I0, and I represent the integrated intensity of strong Tm3+ UCL at 345 and 360 nm for UCNPs@SiO2 without and with the presence of an analyte, respectively. [c] is the concentration of the analyte in mole (as the quencher), and \(\:{K}_{sv}\) is Stern-Volmer constant (M−1) [54, 55]. At low MnO4− concentration range, a Stern-Volmer plot (Fig. 4b) shows a nonlinear behaviour while an upward trend was observed with increasing MnO4− concentration. The nonlinearity of the Stern-Volmer plot indicates the energy transfer among UCNPs@SiO2 and MnO4− or it may be related to both the static and dynamic quenching [56]. For the UCNPs@SiO2 sensor towards MnO4− determination, the UCL quenching efficiency (Fig. 4c), is defined as [(I0–I)/I0]. Figure 4c shows the efficacy of UCNPs@SiO2 UCL quenching against the logarithm of MnO4− concentration, which exhibits well-fitted linear relationships of y = 0.0952 x + 0.0626 with correlation coefficients R2 = 0.9944 in the 0.6–80 µM concentration range of the MnO4− (Fig. 4d) and y = 0.4903 x − 0.6863 with R2 = 0.9974 for MnO4− concentration ranging 80-2000 µM (Fig. 4e), respectively. As revealed in Fig. 4f, by further plotting c/[(I0–I)/I0] against c [57], with c referring to the concentration of MnO4−, good linear relationships were established with y = 4.2315 x + 14.596 (R2 = 0.9844) for the MnO4− concentration in range of 0.6–80 µM (inset of Fig. 4f) and y = 0.9256 x + 273.3 (R2 = 0.9981) for the MnO4− concentration ranging 80-2000 µM (Fig. S9), indicating that UCNPs@SiO2 is qualified for determination of MnO4− ion in wide 0.6–2000 µM concentration range. The limit of detection (LOD) of UCNPs@SiO2 toward MnO4− can be calculated using 3 σ/S [35], where σ is the relative standard deviation of blank sample obtained from 5 measurements, while S represents the calibration curve slope, with a LOD of 0.15 µM, demonstrating the high sensitivity of the synthesized MnO4− sensor i.e., UCNPs@SiO2. In comparison with the reported fluorescent methods for sensing of MnO4− as listed in Table S1, the proposed UCNPs@SiO2 sensor exhibits the excellent detection sensitivity with least LOD and wide detection range. Enabled with the NIR excited UCL for improved signal-to-noise ratio and autofluorescence free background, the proposed UCNPs@SiO2 upconversion nanosensor can be potentially used for sensitive MnO4− detection in aqueous samples.

We investigated the selectivity of the proposed UCNPs@SiO2 sensor to detect MnO4− by monitoring the UCL response to several anions, including Cl−, Br−, CH3COO−, HCO3−, CO32−, NO3−, SO42−, H2PO4−, HPO42−, PO43−, and MnO4−. Figure 4g and Fig. S10a display the UCNPs@SiO2 UCL responses for the different anions at a fixed concentration of 800 µM. Among these anions, only MnO4− exhibited considerable quenching effect on UCL intensity of UCNPs@SiO2 with quenching efficiencies of ~ 75%, whereas other anions showed no apparent effect on UCL response of UCNPs@SiO2. This suggests that the UCNPs@SiO2 sensor exhibits high selectivity towards MnO4− sensing. Moreover, the UCNPs@SiO2 sensor was subjected to an anti-interference experiment for the detection of MnO4− by introducing MnO4− into the system concurrently with the addition of other individual or mixed anions. The UCNPs@SiO2 sensor, in the presence of MnO4− of 800 µM (Fig. 4h and Fig. S10b), did not show a substantial difference compared to the system together with other individual anions (800 µM). Figure 4i and Fig. S10c, illustrate the comparable UCL quenching in intensity for UCNPs@SiO2 sensor when MnO4− (800 µM) was added, both with and without different mixed anions including Cl−, Br−, CH3COO−, HCO3−, CO32−, NO3−, SO42−, H2PO4−, HPO42−, and PO43− at equal concentration (800 µM). This proves that the other anions show negligible effect on the UCL of UCNPs@SiO2 sensor to detect MnO4−. These results reveal the good selectivity of UCNPs@SiO2 sensor for detecting MnO4−. Thus, UCNPs@SiO2 could be considered as a promising candidate for detecting MnO4− anion in aqueous solution.

Fig. 4
figure 4

(a) MnO4− (0.6–2000 µM) concentration dependent UCL spectra of UCNPs@SiO2 in aqueous solution, (b) plot of I0/I against MnO4− concentration (Stern-Volmer), (c) the UCL quenching efficiency of UCNPs@SiO2 against the logarithmic concentration of MnO4−, (d) the UCL quenching efficiency showing the linear relationship plot for MnO4− concentration in range of 0.6–80 µM, (e) linear relationship plot of UCL quenching efficiency for MnO4− concentration in the 80–2000 µM range, (f) relationship between c/[(I0–I)/I0] versus MnO4− concentration, inset showing the well-fitted linearity for MnO4− concentration in the 0.6–80 µM range, (g) UCL intensity of UCNPs@SiO2 with addition of various anions (800 µM) in aqueous solution; Anti-interference plot of UCNPs@SiO2 sensor for MnO4− detection, (h) UCL intensity of UCNPs@SiO2 in presence of the MnO4− (800 µM) mixed with other individual anions (800 µM); (i) UCL intensity of UCNPs@SiO2 in the presence of MnO4− (800 µM) mixed with 800 µM other anions. (Note: integrated intensity of UCNPs@SiO2 UCL at 345 and 361 nm was used.)

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Sensing for C2O7 2−

Figure 5a shows UCL spectra at the different Cr2O72− concentration under 980 nm laser excitation. With increase in the Cr2O72− concentration (2–2000 µM), the intensity of UCNPs@SiO2 was significantly quenched for the UCL at 345, and 361 nm, which are attributed to the 1I6 → 3F4 and 1D2 → 3H6 transitions of Tm3+ ions. The UCNPs@SiO2 sensor exhibit superior sensitivity behaviour for Cr2O72− through the quenching of ~ 74% UCL intensity when 2000 µM of Cr2O72− anion was added (Fig. 5a). For 10 µM of Cr2O72− concentration, the major UCL quenching (10%) was observed (Fig. 5a). To investigate the UCL quenching behaviour towards Cr2O72− determination, Fig. 5b illustrates the Stern-Volmer plot extracted from the UCL intensity shown in Fig. 5a. The Stern-Volmer plot displays an appreciable nonlinear behaviour with an upward curvature with increase in Cr2O72− concentration in range of 2-2000 µM (Fig. 5b), revealing an energy transfer from UCNPs@SiO2 to Cr2O72− or can be ascribed to both the static and dynamic quenching [56]. For the UCNPs@SiO2 sensing probe toward Cr2O72− determination, the quenching efficiency for UCNPs@SiO2 UCL at 345 and 361 nm against the logarithmic concentration of Cr2O72− is shown in Fig. 5c, which shows well-described linear relationship of y = 0.1474 x − 0.0395 with R2 = 0.9948 for the concentration of Cr2O72− in range of 2−500 µM (Fig. 5d) and y = 0.6737 x − 1.4801 with R2 = 0.9972 for Cr2O72−concentration ranging 500–2000 µM (Fig. 5e), respectively. By further plotting c/[(I0–I)/I0] against c [58], with c denoting the concentration of Cr2O72−, well-fitted linear relationships were accomplished with y = 2.9397 x + 73.66 (R2 = 0.9961) for Cr2O72− concentration in range of 2–500 µM (inset of Fig. 5f), and y = 0.8368 x + 1013.1 (R2 = 0.9979) for Cr2O72− concentration ranging 500–2000 µM (Fig. S11), indicating that UCNPs@SiO2 is qualified for determination of Cr2O72− ion in wide 2–2000 µM concentration range. The LOD of UCNPs@SiO2 for Cr2O72− ion was calculated to be 0.04 µM via the 3 σ/S formula [33], indicating the highly sensitive determination of Cr2O72− by the UCNPs@SiO2 sensor. The proposed UCNPs@SiO2 sensor towards Cr2O72− demonstrates the superior LOD and wide linear detection range than those existing fluorescent sensor materials, as shown in Table S1. With the advantages of NIR responsive UCL for negligible autofluorescence and improved signal-to-noise ratio, the UCNPs@SiO2 upconversion nanosensor holds the promise for sensitive determination of Cr2O72− for environmental monitoring and food safety.

To investigate the selectivity of the proposed UCNPs@SiO2 sensor for detecting Cr2O72−, the UCL response to different anions, including Cl−, Br−, CH3COO−, NO3−, SO42−, SCN−, H2PO4−, HPO42−, PO43−, and Cr2O72− was monitored. Figure 5g and Fig S12a, show the UCL response of the UCNPs@SiO2 sensor in presence of the above anions in aqueous solution at the fixed 2000 µM concentration. Among the above anions, only Cr2O72− exhibited considerable quenching effect on UCL intensity of UCNPs@SiO2 with quenching efficiencies of ~ 74%, whereas other anions showed no apparent effect on UCL response of UCNPs@SiO2. This suggests that the UCNPs@SiO2 system exhibits excellent selectivity towards Cr2O72− sensing. Furthermore, the UCNPs@SiO2 sensor was subjected to an anti-interference experiment for the detection of Cr2O72− by introducing Cr2O72− into the system concurrently with the addition of other individual or mixed anions (2000 µM). The UCNPs@SiO2 sensor, in the presence of Cr2O72− (Fig. 5h and Fig. S12b), exhibited no significant difference when compared to the system together containing other individual anions (2000 µM). Figure 5i and Fig. S12c, illustrate the similar UCL quenching in intensity for the UCNPs@SiO2 sensor when Cr2O72− was added (2000 µM), both with and without different mixed anions such as Cl−, Br−, CH3COO−, NO3−, SO42−, SCN−, H2PO4−, HPO42−, and PO43− at equal concentration (2000 µM). These results indicate that the other anions exhibit negligible effect on the UCL response of UCNPs@SiO2 to detect Cr2O72−. In summary, the excellent sensitivity and high selectivity of the UCNPs@SiO2 sensor towards Cr2O72− make them to be potential for detecting ionic pollutants in wastewater.

Fig. 5
figure 5

(a) Cr2O72− (2–2000 µM) concentration dependent UCL spectra of UCNPs@SiO2 in aqueous solution, (b) Stern-Volmer plot (I0/I versus Cr2O72− concentration), (c) the UCL quenching efficiency of UCNPs@SiO2 against the concentration of Cr2O72−, the linear relationship plot of the UCL quenching efficiency for Cr2O72− concentration in the (d) 2–500 µM and (e) 500–2000 µM range, (f) plot of c/[(I0–I)/I0] versus concentration of Cr2O72−, inset showing simulated linearity correlation for Cr2O72− concentration in the 2–500 µM range, (g) UCL intensity of UCNPs@SiO2 with addition of various anions (2000 µM) in aqueous solution; Anti-interference properties of the UCNPs@SiO2 to detect Cr2O72− in aqueous solution, (h) UCL intensity of UCNPs@SiO2 with Cr2O72− (2000 µM) together with other individual anions (2000 µM), and (i) UCL intensity of UCNPs@SiO2 with the addition of Cr2O72− together with other mixed anions at a fixed 2000 µM concentration. (Note: integrated intensity of UCNPs@SiO2 UCL at 345 and 361 nm was used.)

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Sensing for anions in real lake and tap water

To explore the applying feasibility of the UCNPs@SiO2 sensor to determine MnO4− and Cr2O72− in food and environmental real samples, we conducted spike experiments in two different types of water (lake and tap) obtained from GDUT. Known concentration of MnO4− (1-2000 µM) and Cr2O72− (10-1500 µM) was added to UCNPs@SiO2 and their UCL was measured. The UV UCL peaks (345 and 361 nm) were quenched upon increasing the lake water (Figs. S13a, and S14a) and tap water (Figs. S13b, and S14b) contamination with MnO4−, and Cr2O72−, respectively. The reliability of UCNPs@SiO2 as MnO4− and Cr2O72− anion sensor and its practical application was explored via a technique of standard addition and recovery test. Table 1 presents that the recovery rates of MnO4− are 92.5–109.0% and 91.3–114.4% for lake and tap water, respectively. Table 2 displays that the recovery rates of Cr2O72− are 91.0–106.1% and 98.0–107.8% for lake and tap water, respectively. The relative standard deviations (RSD) for all real sample measurements are below 5.1%, indicating that the reproducibility of the UCNPs@SiO2 in real sample analysis is robust and reliable. Thus, the proposed UCNPs@SiO2 nanosensor shows impressive performance considering both the practical and reliable sensing aspects and holds promise for the on-site determination of MnO4− and Cr2O72− anions in real samples.

Table 1 Determination and recovery of MnO4− in lake and tap water
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Table 2 Determination and recovery of Cr2O72− in lake and tap water
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Mechanism of MnO4 − and Cr2O7 2− sensing

In general, the Förster resonance energy transfer (FRET) refers to a non-radiative phenomenon occurring on the basis of overlap between donor emission and the acceptor absorption, relative orientation of donor-acceptor dipoles, and the donor-acceptor distance ~ 1–10 nm [59, 60]. On the other hand, the inner filter effect (IFE) is known as a radiative energy transfer process, requiring that the excitation/emission of the donor overlapped with the acceptor absorption [61]. The FRET process will affect the excited-state dynamics of the energy donor, decreasing its luminescence decay lifetime, which differentiates FRET from IFE process. UV-Vis absorption spectra were recorded for various anions and UCNPs@SiO2 before and after addition of MnO4− and Cr2O72−. MnO4− exhibits two prominent absorption peaks in the UV-Vis region, specifically peaking at 311 and 525 nm (Fig. 6a), which could be ascribed to O → Mn charge transfer [62]. Cr2O72− anion has distinct absorption observed at 355 nm (Fig. 6a), in line with the reported optical characteristics of Cr (VI) [63]. The UV absorption at 290 nm for Cr2O72− is attributed to the O → Cr charge transfer [64]. As shown in Fig. 6b, the UCNPs@SiO2 display negligible absorption, whereas the MnO4− added UCNPs@SiO2 (UCNPs@SiO2@MnO4−) and Cr2O72− added one (denoted as UCNPs@SiO2@Cr2O72−) exhibit characteristic absorption peaks for MnO4− and Cr2O72−, respectively. The UCL of UCNPs@SiO2 at 345 and 361 nm shows significant overlap with 355 nm Cr2O72− absorption. On the other hand, the UCL of UCNPs@SiO2 at 345, 361, 450, and 477 nm shows complete overlap with the 311 and 525 nm absorption bands of MnO4−.

To further understand the energy transfer processes, under 980 nm laser excitation, the UCL lifetime decay for Tm3+ at 450 nm was assessed, both in the presence and absence of MnO4− (Fig. 6c). Considering the lifetime decay as non-monoexponential, the average decay lifetime of the UCNPs@SiO2 sensor was calculated via the following Eq. [65, 66],

$$\:{\tau\:}_{\text{a}\text{v}\text{g}}=\frac{{\int\:}_{0}^{\infty\:}tI\left(t\right)\text{d}t}{{\int\:}_{0}^{\infty\:}I\left(t\right)\text{d}t}$$

where I(t) is the UCL intensity at given time t after ceasing the excitation. For UCNPs@SiO2 sensor, the average lifetime (𝜏avg, ms) of 450 nm Tm3+ UCL is found to be 0.178 ms. Upon MnO4− addition, a 1D2 UCL of the Tm3+ decayed faster with decreasing τavg by 0.104 ms for UCNPs@SiO2 with MnO4− (50 µM) and by 0.065 ms for UCNPs@SiO2 with MnO4− (2000 µM). This drastic change in UCL lifetime experimentally indicates that the non-radiative FRET from UCNPs@SiO2 to MnO4− took place [32, 60], which agrees with the nonlinear trend in the Stern-Volmer plot and confirms the occurrence of combined static and dynamic UCL quenching. To calculate the FRET efficiency (E) following equation [60], \(\:E=1-\frac{{\tau\:}_{\text{D}\text{A}}}{{\tau\:}_{\text{D}}}\), is used, where τDA and τD are the UCNPs@SiO2 (donor) lifetime with and without MnO4− acceptor, respectively. The extent of efficient FRET for the UCNPs@SiO2 sensor is determined to be 42% and 63% for MnO4− concentration of 50 and 2000 µM, respectively. The enhancement in the FRET efficiency of the sensor with the increase in MnO4− content is in line with the two different linear 0.6–80 and 80–2000 µM concentration ranges for MnO4− sensing, as demonstrated in Fig. 4d and e. Furthermore, the TEM results showing the diameter of core-shell NaYF4:Eu@NaYbF4:Gd/Tm nanoparticles of 27.0 nm (Fig. S1b and e) and NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4@NaLuF4 UCNPs@SiO2 of 44.6 nm (Fig. 3a and Fig. S4), indicates the least FRET distance from Tm3+ in the inner NaYbF4:Gd/Tm shell to MnO4− at ~ 8.8 nm, which further support the FRET process from UCNPs@SiO2 to the analyte quencher (Tm3+ 345, 361, 450, 477 nm UCL quenched by MnO4−, Tm3+ 345, 361 nm UCL quenched by Cr2O72−). On the other hand, the IFE process occurs with luminescence lifetime remaining unchanged by introduction of energy acceptor [67], thus IFE should also be possible for the operational energy transfer mechanism for UCNPs@SiO2 sensor detecting anions (MnO4− and Cr2O72−). Taken all together, it is plausible that the UCNPs@SiO2 upconversion nanosensor could involve both FRET and IFE mechanisms to detect MnO4− and Cr2O72− anions.

Fig. 6
figure 6

(a) UV-Vis absorption spectra of different anions, MnO4− (~ 1 mM), Cr2O72−(~ 0.5 mM), others are 0.1 ~ 3.33 mM; (b) UV-Vis absorption spectra of the UCNPs@SiO2 (1.0 mg/mL), UCNPs@SiO2 with MnO4− (0.5 mM) and UCNPs@SiO2 with Cr2O72− (0.5 mM), contrasted with UCL spectrum of UCNPs@SiO2 (1.0 mg/mL) at 980 nm laser excitation in aqueous solution; (c) The UCL lifetime decay of UCNPs@SiO2 with and without MnO4− addition

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Conclusions

The autofluorescence free UCNPs@SiO2 UCL nanosensor was successfully developed for label-free and fast determination of MnO4− and Cr2O72− anions. The surface SiO2 modification enables UCNPs with good water solubility and biocompatibility. The highly efficient and multi-colour UCL of UCNPs@SiO2 was effectively quenched by MnO4− and Cr2O72− anions with fast response time of 2 and 1 min, respectively. The UCNPs@SiO2 nanosensor exhibits linear detection ranges of [0.6–80 µM and 80–2000 µM], [2–500 µM and 500–2000 µM] with the LOD at 0.15 µM and 0.04 µM for MnO4− and Cr2O72− anions, respectively. The UCNPs@SiO2 nanosensor with good sensing capability, demonstrated satisfactory recognition and determination of MnO4− and Cr2O72− in real lake and tap water samples. UCL lifetime decay analysis reveals the energy transfer from UCNPs@SiO2 to anions through the combination of FRET and plausible IFE processes. The proposed UCNPs@SiO2 UCL nanosensor offers autofluorescence free and rapid determination of MnO4− and Cr2O72− anions with high sensitivity, good specificity, low LOD, and wide linear detection range, holding great promising for reliable environmental monitoring and food sample detection applications.

Data availability

The datasets supporting the conclusions of this article are included within the article and its additional file.

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Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Nos. 22075052, 62335008, 62122028), Guangdong Basic and Applied Basic Research Foundation (Nos. 2023A1515012988, 2023B1515040018) and Science and Technology Planning Project of Guangdong Province (No. 2022A0505020005). The authors would like to acknowledge United Arab Emirates University for the financial support with the Grant Code-G00004525 and fund code 12S171.

Funding

This work was financially supported by the National Natural Science Foundation of China (Nos. 22075052, 62335008, 62122028), Guangdong Basic and Applied Basic Research Foundation (Nos. 2023A1515012988, 2023B1515040018) and Science and Technology Planning Project of Guangdong Province (No. 2022A0505020005). The authors would like to acknowledge United Arab Emirates University for the financial support with the Grant Code-G00004525 and fund code 12S171.

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Authors and Affiliations

  1. Key Laboratory of Clean Chemistry Technology of Guangdong Regular Higher Education Institutions, Guangdong Engineering Technology Research Center of Modern Fine Chemical Engineering, School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, 510006, China

    Kuncheng Xu, Hongli Wen, Abdur Raheem Aleem & Murugavelu Marimuthu

  2. Guangdong Provincial Laboratory of Chemistry and Fine Chemical Engineering Jieyang Center, Jieyang, 515200, China

    Hongli Wen

  3. Analysis and Test Center, Guangdong University of Technology, Guangzhou, 510006, China

    Wei Song

  4. Centre for Optical and Electromagnetic Research, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, 510006, China

    Wang Chen & Qiuqiang Zhan

  5. Department of Physics, United Arab Emirates University, Al-Ain, Abu Dhabi, 15551, UAE

    Deshmukh Abdul Hakeem & Saleh T. Mahmoud

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  1. Kuncheng Xu
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  2. Hongli Wen
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  3. Wei Song
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Contributions

KX contributed to the preparation, characterization, sensing performance of the work, and generation of the figures. HW accomplished the conception or design of the work. WS and WC performed UCL and UCL lifetime assay. ARA, QZ, and STM contributed to participation of experiment design. Drafting the article was prepared by HW, MM and DAH. HW did the critical revision of the article and final approval of the version to be published. All authors read and approved the final manuscript.

Corresponding authors

Correspondence to Hongli Wen, Murugavelu Marimuthu or Deshmukh Abdul Hakeem.

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Xu, K., Wen, H., Song, W. et al. Label-free upconversion nanosensor for water safety monitoring of permanganate and dichromate ions. BMC Chemistry 19, 60 (2025). https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s13065-025-01415-3

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BMC Chemistry

ISSN: 2661-801X

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