1 Introduction

Rare earth fluorides have attracted the researchers in the recent times because of their ability to exhibit downconversion (DC) and upconversion (UC) luminescence processes, which also makes them suitable for various interesting applications like fingerprints [1], photocatalysis [2], drug delivery [3], medical imaging [4], biolabelling [5], protein conjugation [6], solar cells [7] and miRNA detection [8]. In contrast to the downconversion, upconversion (UC) materials emit high-energy photons, when excited by low-energy photons (anti-stokes process) [9]. In particular, upconversion materials have engrossed attention in both in vitro and in vivo optical bio-imaging applications due to their excitation in NIR (Near-infrared) range, with high photo-chemical stability and low auto-fluorescence, high signal-to-noise ratio, detection sensitivity, photo-bleaching and high light penetration depth in tissues. Hence, they are advantageous and safe to the human body and less harmful to living normal cells. The excited sources used in downconversion cause damage to biological tissues [10].

Rare earth fluorides such as binary REF3 and complex AREF4 (RE = rare earth; A = alkali), where rare earth elements can exhibit strong fluorescent emission through 4f–4f or 4f–5d transitions, alkali ions increase the complex cationic sites for growth and fluorides are used instead of Cl− or Br−, both are highly susceptible to moisture [11]. NaYF4 is considered as one of the excellent host materials for optically active trivalent lanthanide (Ln3+) ions, as it possesses a high refractive index and low phonon energy, which leads to the low probability of non-radiative decay than other oxide hosts and in most inorganic matrices. In addition, upconversion nanoparticles (UCNPs) of NaYF4 are alternatives to organic dyes and semiconductor quantum dot luminescent labels due to their attractive merits such as high quantum yields, narrow emission peak and long lifetimes [12]. The NaYF4 exits in two polymorphic forms namely cubic (α-) and hexagonal (β-) phases depending upon the synthesis conditions and methods [13]. On increasing the temperature above 160 ºC, the driving force leads to a phase transformation from α phase to β phase [14]. While increasing the reaction time, hexagonal nano/microtubes and microprisms are formed. α-NaYF4 is a metastable phase and transformation from α- NaYF4 to β- NaYF4 is irreversible till 673 °C [15]. Hexagonal β-NaYF4 is intense, minimize non- radiative multiphoton relaxation process [16] and maximize upconversion emission than cubic α-NaYF4 [17]. To increase the strength of NIR absorbance, Yb is co-doped with Er. In this case, Yb acts as sensitizer and Er act as activator. Here, excited light is primarily absorbed by Yb at 980 nm, and the energy of two excited Yb state is then transferred to one Er ion, resulting in emission exclusively on green and red spectral region [16]. The decay time constant of 520 nm, 540 nm and 654 nm emissions can be tuned in the range of microseconds to milliseconds [18,19,20,21].

From the earlier studies, various preparation methods have been employed to synthesize NaYF4 nanoparticles. Li et al. [22] used thermal decomposition method and showed promising applications in biolabels and displays. Han et al. [23] adopted precipitation followed by solvothermal method to form core–shell (precipitate) and hollow microsphere structure (solvothermal treatment), which demonstrates application in drug delivery fields. Ding et al. [24] have used microwave-assisted flux cooling method that gives β-NaYF4:Yb, Er microrods. The above synthesis procedures demonstrate mixed morphology, i.e. irregular cubic, hexagonal nanoparticle/microrod structures are formed. These procedures exhibited increased green emission, which is a shorter wavelength with short penetration depth. This suppresses its application in bio-imaging. To overcome the structural irregularities, hydrothermal method is preferred. Hence, pure and Yb, Er-doped β-NaYF4 particles were synthesized by acid-free hydrothermal method at 180 °C for 12 h with NH4F as fluoride source. The better uniformity, highly cryatalline nature with controlled morphology and stable β-NaYF4:Yb, Er particles were obtained via hydrothermal method. Sodium citrate which plays a double role as a co-ordination agent and structure directing agent can form strong complexes with Y3+ ions through co-ordination interaction [13]. Citrate is a ligand, which strongly absorbs and alters the mineral surface for mineral growth behaviour [25]. The variation of strain and crystallite size for each molar concentration was discussed. Further an attempt was made to correlate structural, optical, magnetic properties of synthesized materials and found HeLa cell viabilities using methyl thiazolyl tetrazolium (MTT) assay.

2 Materials and methods

2.1 Chemicals

Sodium chloride (NaCl), Yttrium (III) Chloride hexahydrate (YCl3⋅6H2O, 99.99%, Sigma), Ytterbium (III) Chloride dehydrate (YbCl3⋅6H2O, 99.99%, Alfa), Erbium (III) Chloride anhydrous (ErCl3, 99.9%, Acer), Sodium citrate tribasic dihydrate (C6H5Na3O7.2H2O, SRL chem) and de-ionized water were used as starting materials without further purification.

2.2 Synthesis of β-NaYF4

NaYF4 was hydrothermally prepared using NaCl, NH4F and YCl3 as precursors along with sodium citrate as a stabilizing agent. 1 M of YCl3 was dissolved in de-ionized water to form a transparent solution. 4 M of NH4F was added to the solution and maintained under stirring for an hour to form a white solution. Then 1 M of NaCl and 0.5 M trisodium citrate were added and the solution was stirred vigorously for 3 h. After, the mixture was transferred to 100 ml Teflon-lined autoclave and maintained at 18 °C for 12 h and allowed to cool at room temperature, and then, the obtained pure NaYF4 particles were washed with de-ionized water, ethanol and acetone to remove unreacted products.

2.2.1 Synthesis of β-NaYF4: Yb3+, Er3

2X Yb and X Er-doped β-NaY(1 − 3X)F4 compound (X = 0.25,0.5,0.75,1,1.5,2%) was prepared via hydrothermal method. For β-NaYF4: Yb, Er (0.25, 0.5, 0.75, 1, 1.5, 2%) compound, required amounts of YCl3, YbCl3, ErCl3 were placed in 50 ml beakers separately. De-ionized water was added to obtain transparent solution. These transparent solutions were added together under constant magnetic stirring. 4 M of NH4F was added dropwise to form a white solution. Then, 1 M of NaCl and 0.5 M trisodium citrate were added, and then, the solution was stirred vigorously for 3 h. Finally, the mixture was transferred to Teflon-lined autoclave and maintained at 180 °C for 12 h and left undisturbed till it attains the room temperature. The products were collected by centrifugation. The above typical procedure was repeated for different molar concentrations.

2.3 Cell culture and cytotoxicity assay

Human cervical carcinoma cell lines (HeLa) were cultured in medium supplied with 10% inactivated Fetal Bovine Serum (FBS), 100 IU/ml of Penicillin and 100 μg/ml of streptomycin in 5% CO2 atmosphere at 37 °C until confluent. The cell was dissociated with TPVG solution. The monolayered cell culture was trypsinized (containing 10% FBS) in 96-well microtiter plate and 100 μl of HeLa cells (50,000 cells/well) was added. After 24 h, partial monolayer was formed. The 100 μl of test sample was added in different concentrations. The cells were incubated at 37 °C for 24 h in 5% CO2 atmosphere. The MTT was added to each cell well and incubated for 4 h at 37 °C in 5% CO2 atmosphere. The supernatant was removed and 100 μl of DMSO was added and shaken to solubilize the formazan. The absorbance was measured at a wavelength of 570 nm. The inhibition of cell growth by 50% (IC50) values for cytotoxicity test was derived from a non-linear regression analysis based on the sigmoid dose–response curves for each cell line. Drug-treated images of cell lines were captured by inverted biological microscope.

2.4 Characterization

Powder X-ray Diffractometer was used to analyse the phase and crystal structure using CuKα radiation (BRUKER D-8 ADVANCE). FT-IR spectra were recorded with FTIR-Thermo Nicolet 6700 instrument using KBr pellet technique. Raman spectroscopy was used to study the vibrational modes of the powder samples. The powder samples were excited at a wavelength of 785 nm by a semiconductor diode laser (Renishaw inVia Raman Microscope). UV–VIS–NIR Spectrophotometer was used to study optical absorption of the sample, in the wavelength range 200 to 2500 nm with maximum resolution = 0.1 nm (Shimadzu UV-3600 Plus). Spectrofluorometer was used to study the emission spectra of the given samples, source_xenon lamp 450 W with range 180–1550 nm resolution 0.2 nm (Flurolog-FL3-111). HRSEM-Scanning Electron Microscope for morphological analysis Resolution: 3 nm @30 kV HV mode (HITACHI S-3400 N). TEM analysis was carried out by JEOL 3010 High-resolution Transmission Electron Microscope (HRTEM). VSM was used to measure the magnetization of samples as function of field.

3 Results and discussion

3.1 XRD analysis

Figure 1 shows the powder X-ray Diffraction (XRD) pattern of pure and Yb, Er-doped β-NaYF4 which confirms the formation of hexagonal phase and is in good agreement with the JCPDS data (card no. 28–1192, a = 5.960 Å, c = 3.510 Å, Space group P63/m). A shift in diffraction peaks towards lower angle may be attributed to the substitution of ion having a larger radii [26] Yb (0.93 Å) and Er (0.89 Å) into the host lattice Y (0.82 Å), which leads to expansion of the crystal on doping (Fig. 2). Particularly, 1.5%Yb, 0.75% Er-doped β-NaYF4 shows a shift towards higher angle which may due to the contraction in crystal structure. The variation in lattice constant is due to strain [25, 27]. Thus, the peak shift confirms the incorporation of dopant into β-NaYF4 lattice. The lattice constant, unit cell volume, strain and average crystallite size have been tabulated (Table 1). From the table and Fig. 2, 0.5%Yb, 0.25% Er dopant concentration has shown decreased crystallite size and peak intensity along with peak broadening when compared to other concentrations. All other dopant concentrations have decreased peak intensity than host. It is well known that a decrease in unit cell volume will cause a shift towards larger angle and vice versa.

Fig. 1
figure 1

XRD pattern of pure and Yb, Er-doped NaYF4 particles

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Fig. 2
figure 2

Maximum intensity plane (110) of pure and Yb, Er-doped NaYF4(110) plane

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Table 1 The average crystallite size of β-NaYF4: Yb, Er, lattice strain, cell volume and lattice parameters
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Strain and crystallite size of the prepared nanoparticles were calculated using the Williamson–Hall (W–H) plot method,

$$\beta_{(hkl)} \cos \theta_{(hkl)} \; = \;\left( {0.9* \, \lambda } \right)/D\; + \;4\varepsilon \;\sin \theta_{(hkl)},$$
(1)

where λ is the wavelength of the X-ray radiation (CuKα = 1.5406 Å), K is constant taken as 0.9, β is the line width at half maximum height (FWHM), θ is the diffracting angle and ε is the strain associated with the nanoparticles [28]. The W–H Plot is shown in Fig. 3a–g. The slope of line gives the lattice strain (ε) and intercept (Kλ/β cos θ) gives grain size (D). From the figure, it is found that all the synthesized compounds exhibit tensile strain. The variation in lattice strain value is mainly due to the influence of dopant on the host lattice and depends on the synthesis method, particle size and shape. There is a small variation in the lattice constant due to the presence of strain in the host lattice. Thus, XRD analysis confirmed the incorporation of dopant in host NaYF4 [28].

Fig. 3
figure 3

a–gW–H Plot of pure and Yb, Er-doped NaYF4

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3.2 FT-IR analysis

The functional groups of pure and Yb, Er-co-doped β-NaYF4 compounds were identified by the FT-IR spectra (Fig. 4). The strong transmission peak at ~ 3449 cm−1 could be attributed to –OH stretching vibration mode. The peaks at ~ 2924 cm−1 and ~ 2854 cm−1 are assigned to the asymmetric and symmetric stretching vibrations of methylene group (–CH2), respectively. The sharp peaks at ~ 1628 cm−1 and ~ 1438 cm−1 are corresponding to asymmetric and symmetric stretching vibrations of the carboxylic group (COO−)[29]. The band at ~ 1094 cm−1 is assigned to C–O stretching vibration of co-ordinating metal cation [30].

Fig. 4
figure 4

FT-IR spectra of pure and 2% & 4% Yb, Er-doped NaYF4

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3.3 Raman studies

The vibrational modes of the synthesized compounds were investigated in order to identify the non-radiative relaxation. Raman spectral analysis was used to predict the vibrational phonon modes of β-NaYF4 and Yb, Er-doped NaYF4 (Fig. 5). The peaks between 150 and 750 cm−1, 1000 and 1600 cm−1 that were clearly visible and distinct were attributed to the host lattice NaYF4 vibrational features [6, 31]. Peaks between 450 and 750 cm−1 were assigned to the vibrational frequencies of the Na–F pair of atom. The non-radiative losses increase as particle size decreases. The presence of –OH band between 1450 and 1650 cm−1 was ascribed to the phonon frequencies of Y–OH [20]. All the bands at wavenumbers 500–1000 cm−1 were attributed to vibrations from organic capping agent citrate. The presence of citrate on surface of particles was indicated by strong –COOH out of plane deformation mode in 988–1112 cm−1 region [32]. Reduced peak intensity was found between 1000–1600 cm −1 except 1% Yb, 0.5% Er and 2% Yb, 1% Er samples which may due to the variation in lattice parameters. The peak at 2502 cm−1 was not observed in pure but present in the doped samples. The variation in peak intensity may due to induced strain. Particularly 2% Yb, 1% Er-doped NaYF4 sample has shown increased peak intensity at 2502 cm−1. Hence, Raman spectra confirm structural purity of the prepared samples.

Fig. 5
figure 5

Raman spectra of pure and Yb, Er-doped NaYF4 with excitation wavelength at 785 nm

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3.4 Surface morphology analysis

3.4.1 SEM

SEM images [shown in Fig. 6(a–r)] reveal the formation of uniform periodic array of hexagonal plates and smooth flat surface with cracked ends. The diameter of pure and Yb, Er-doped β-NaYF4 was found to be ~ 2 to 4 μm and the thickness was ~ 0.6 to 1 μm. The formation of hexagonal β-NaYF4 was related to dissolution and re-nucleation process, which greatly influences the intrinsic structure factor of initial seed and leads to anisotropic growth along reactive direction. The growth of crystal might be explained by Ostwald ripening process, which suggests the growth of large crystal from smaller sized particles. These smaller particles shrink and accelerate in the solution to initiate the growth of larger crystals under hydrothermodynamical condition. It is widely used for template-free synthesis of inorganic nano/microstructures in recent years [25]. Energy dispersive X-ray (EDX) analysis indicated the presence of Na, Y, F, (Fig. 6s–u) Er, Yb and also revealed that the obtained particles have Na/Y/F ratio of 1:1:4 and 1,2% Yb, 0.5, 1% Er, which confirmed the formation of stoichiometric β-NaYF4 and β-NaYF4:Yb, Er, respectively [9]. Hence, hydrothermal approach shows high yield of compounds.

Fig. 6
figure 6figure 6

a–c SEM images of as prepared NaYF4 particles, d–f image of 0.5%Yb, 0.25%Er concentration, g–i image of 1%Yb, 0.5%Er concentration, j, k image of 1.5%Yb, 0.75%Er concentration, l, m image of 2%Yb, 1%Er concentration, n–p image of 3%Yb, 1.5%Er concentration, q, r image of 4%Yb, 2%Er concentration, s Edax image of NaYF4 particles, t Edax image of 1%Yb, 0.5%Er concentration, u Edax image of 2%Yb, 1%Er concentration

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3.4.2 TEM

TEM images clearly confirmed the formation of crystalline β-NaYF4 hexagonal plates. The samples display high particle size uniformity. All products were of hexagonal shape and have a uniform size shown in Fig. 7.

Fig. 7
figure 7

TEM images of β-NaYF4 particles

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3.5 Growth mechanism

Figure 8 shows the schematic diagram of α-NaYF4 and α, β-NaYF4:Yb, Er. The cubic phase formation of pure α-NaYF4 involves the reaction and nucleation of the precursors Y3+ and F− in the aqua media. The product formed was confirmed by the white solution. NaCl and trisodium citrate (Cit3+) were added, which resulted in the formation of cubic α-NaYF4 as depicted in SEM image (Fig. 8a). The substitution of dopant [Yb3+, Er3+] ions leads to the formation of α-NaYF4: Yb, Er. The SEM image (in Fig. 8b) depicts both spherical and cubic morphology after dissolution and re-nucleation process. When the temperature is maintained at 180 °C, the small metal fluoride crystallites nucleate and quickly congregate to cubic phase spherical nanoparticle in order to reduce the surface energy. After dissolution and re-nucleation process, the metastable cubic phase possesses the dominating anisotropic growth. So α-phase particles release monomers at higher rate along with high-degree supersaturation. These monomers have strong interaction due to Brownian motion. The rate constant of the reaction between particles and monomers results in sudden nucleation of hexagonal β-NaYF4: Yb, Er seed. Thus, the phase transition from cubic α to a hexagonal array of β-NaYF4: Yb, Er happens and such a formation is mainly due to the Ostwald ripening growth process [25]; here the smaller crystals detach, diffuse and get deposited on large crystal resulting in the growth of large crystal. Prolonged time of 12 h leads to the formation of hexagonal β-NaYF4: Yb, Er. microplates with concave top end facets and, hence, more stable hexagonal crystalline phases emerge.

Fig. 8
figure 8

Schematic diagram for the formation of α, β-NaYF4: Yb, Er

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3.6 UV–VIS–NIR spectra studies

The UV–VIS–NIR absorption spectra of the pure and (Yb, Er)-doped NaYF4 compounds are shown in Fig. 9. The absorption peak occurs in the wavelength region of 200–1200 nm which could be attributed to f–f transitions. The characteristic absorption peaks of Er3+ are observed at 378, 407, 448, 489, 523, 544, 652, 800 and 975 nm corresponding to electron transitions from 4I15/2 → 4G11/2 (~ 378 nm), 4I15/2 → 2H9/2 (~ 407 nm), 4I15/2 → 4F5/2 (~ 448 nm), 4I15/2 → 4F7/2 (~ 489 nm), 4I15/2 → 2H11/2 (~ 525 nm), 4I15/2 → 4S3/2 (~ 544 nm), 4I15/2 → 4F9/2 (~ 653 nm) and 4I15/2 → 4I9/2 (~ 800 nm) and 4I15/2 → 4I11/2 (~ 975 nm), respectively. The absorption peaks occurring at NIR wavelength range (975 nm) correspond to the 2F7/2 → 2F5/2 electronic transition [33]. The green and red upconversion luminescence dominates the visible light spectra [34]. It further indicates the effect of incorporation of Yb, Er into β- NaYF4 system. In general, optical absorption spectra depend on the shape, size, strain and vacancies present in the sample. Wang et al. [35] observed blue shift for nanoparticles and nanorod, which is due to quantum confinement effect.

Fig. 9
figure 9

UV–VIS–NIR absorbance spectra of pure and β-NaYF4: Yb, Er particles

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Calculation of optical energy gap of compounds from the absorbance spectra using Woods and Tauc’s function [36] is given as

$$\left( {\alpha h\nu } \right)^{n} = A\left( {h\nu - E_{g} } \right),$$
(2)

where α is the linear absorption co-efficient of the materials, hν is the energy of the photon, ‘A’ is proportionality constant of characteristic material, and n equals either ½ for indirect transition or 2 for direct transitions (Fig. 10) [37].

Fig. 10
figure 10

Tauc’s Plot of pure and Yb, Er-doped NaYF4

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3.7 PL analysis

Room temperature photoluminescence behaviours of pure and Yb, Er-co-doped β-NaYF4 compounds, excited at 980 nm, are depicted in Fig. 11a. The visible emission spectra display two emission bands under NIR excitation, which may be attributed to 4f–4f transition of Er3+ ions. The green emission at 525 and 540 nm are due to the electronic transitions 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 of the Er3+ ions. The dominant red UC luminescence bands centred at 655 nm and 670 nm are assigned to the Er3+:4F9/2 → 4I15/2 transitions [27]. The intensity of red emission increases monotonously while taking 4% Yb, 2% Er as shown in Fig. 11b. The UC emission intensities and the ratio of various emissions are influenced by the doping level, excitation power, preparation temperature [38], impurities, surface ligands, solvent and defects [16]. The peak intensity of 0.5% Yb, 0.25% Er and 1.5% Yb, 0.75% Er are the next intense red emission after 4%Yb, 2%Er concentration and pure. This may be ascribed to the reduction in strain and crystallite size [30]. This variation in peak intensity confirms the dopant incorporation into host lattice. Janjua et al. [15] reported non-radiative relaxation might be the reason for the enhanced red emission. In addition, phase transition and thermal vibration of crystals, at higher temperature of about 773 K, generates disorder and promotes multiphoton cross-relaxation derived from Er, such as Er: 4S3/2 to 4F9/2 and Er: 4I11/2 to 4I13/2, resulting in a quenched green emission. Enhanced red output with upconversion of Yb,Er,Mn-tridoped NaYF4 nanoparticles was reported by Tian et al. [39]. Increased red emission affords deep penetration into tissues, which offers wider applications in vivo imaging especially magnetic resonance (MR) imaging probes and drug carriers for intracellular drug delivery applications.

Fig. 11
figure 11

a, b Photoluminescence spectra of pure and Yb, Er-doped NaYF4 compounds with excitation wavelength at 980 nm

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3.8 Magnetic studies

The magnetization curve (M–H) of pure and Yb, Er-co-doped NaYF4 compounds at 300 K is shown in Fig. 12. Apart from luminescent properties, NaYF4: Yb, Er particles respond to magnetic field. Synthesized β-NaYF4: Yb, Er particles exhibit paramagnetic behaviour in contrast to host β-NaYF4, which exhibits weak ferromagnetic nature at low magnetic field and diamagnetic behaviour at higher magnetic field (Fig. 12 inset) [40, 41] may be because of the vacancies or defects present in the sample. In general, the magnetic property of the non-magnetic compounds depends on defects/vacancies, surface morphology, core/shell interactions and the presence of excess charge carriers via dopants. As the dopant is introduced into β-NaYF4, there is a transition from diamagnetic to paramagnetic. The concentration at 4% Yb, 2% Er: β-NaYF4 significantly shows higher saturation magnetization than others and has good paramagnetic nature. From the literature, it was observed that the Mg2+ ion, Yb, Er-tridoped NaGdF4 core–shell structure exhibits paramagnetic signature [42]. In addition, the effect of Gd concentrations on the paramagnetic property of NaGdF4 was also examined. Wang et al. [43] successfully achieved the tunable upconversion luminescence of NaYF4:Yb3+, Er3+ combined with Fe3O4 particles as a sensitive DNA detection system. The NaYF4:Yb3+ (30%)/Gd3+ (40%)/Tm3+ (2%) nanorods are paramagnetic at room temperature. Furthermore, magnetic behaviour of the particles is size dependent, i.e. the size of ferromagnetic particles is reduced to a critical value, and the magnetic property is no longer ferromagnetic but superparamagnetic. As such, nanorods possess superparamagnetic property at 2 or 5 K with saturation magnetization (Ms) of about 60 emu g−1. Hence, they have potential application in magnetic resonance dual-mode fluorescent labels for bio-imaging applications. Wong et al. [44] demonstrated the synthesis of paramagnetic KGdF4:Tm3+ 2%, Yb3+ 20% core and KGdF4:Tm3+ 2%, Yb3+ 20%KGdF4 core–shell nanoparticles with magnetization at 20 kOe (at 293 K) is ~ 0.8 emu/g, have application in bio-separation. However, the diamagnetic to paramagnetic transition of non-magnetic β-NaYF4 compounds may be due to vacancies/defects and excess charge carriers.

Fig. 12
figure 12

Magnetization curve of pure and Yb, Er-doped NaYF4 compounds at 300 K

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3.9 MTT assay

Finding the cytotoxicity of β-NaYF4: Yb, Er compound is one of the key factors determining whether they could be applied in biomedical field. Once the sample has been successfully synthesized, their biocompatibility was evaluated using cell lines, in which cell viabilities were assessed. In this present work, toxic potentiality of the β-NaYF4: Yb, Er sample in HeLa cells was determined by MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay at 24 h incubation. Different concentrations (12.5, 25, 50, 100, 200, 400 μg/mL) of the prepared (4% Yb, 2% Er) sample were employed. The cellular viability at 12.5 μg/ml concentration showed a slight but non-significant decrease. Then, cellular viability decreased gradually with increase in concentration of the β-NaYF4: Yb, Er sample (Fig. 13). Thus, the samples exhibit concentration-dependent loss of viability, especially higher toxicity to HeLa cell lines at higher concentration of sample (200–400 μg/mL). The sample below 200 μg/mL concentration exhibits possible applications in imaging, drug delivery and anticancer activity (detection and diagnosis of cancers). The IC50 value is obtained at a sample concentration of 82 μg/mL. The microscopical images after treatment of HeLa cells with different sample concentrations are shown below (Fig. 14). It is already reported that the higher concentration of upconversion nanoparticles along with anticancer drug (cisplatin) displays higher toxicity to cancer cells especially on human cervical carcinoma cell line (HeLa cells) and reduced toxicity at lower concentrations of the same cell line than the actual cancer drug [38, 39, 45]. Xiong et al. [46] estimated the cellular viability and have shown the low toxicity to HeLa cell lines until 400 μM concentration.

Fig. 13
figure 13

Cell viability values (%) estimated by MTT assay versus incubation concentrations of 0–400 μg/ml

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Fig. 14
figure 14

Microscopical images after treatment of HeLa cells (a 25 μg/ml, b 50 μg/ml, c 200 μg/ml and d control)

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4 Conclusion

β-NaYF4 and Yb, Er-doped β-NaYF4 hexagonal plates with different molar concentrations were synthesized successfully by hydrothermal method. Formation and growth mechanism has been explained. Powder XRD analysis has been used to determine the values of lattice parameters, average crystallite size, strain, cell volume, and it confirmed the formation of hexagonal phase. The stretching and bending modes of the compounds were analysed by FT-IR Spectra. The presence of vibrational modes has been examined by Raman analysis. Morphological evolution of hexagonal plates was identified by electron microscopic images and explained by Ostwald ripening process. The UV–VIS–NIR spectra of pure and Yb, Er-doped β- NaYF4 show the absorbed peaks of host, Yb and Er. Photoluminescence studies revealed the doping effects of Yb, Er into the host matrix. Especially the concentration of 4% Yb, 2% Er-doped β-NaYF4 enhanced the red emission. HeLa cells were not enough to survive at the higher concentration used in the study. Paramagnetic properties make Yb, Er-doped upconverting β-NaYF4 a promising material for future biomedical application especially at cellular-level bio-imaging and bio-separation.