1 Introduction

Textile dye wastewater is one of the highly addressed problems of the world environmental concerns. Most of the industries use different dyes to colour their fabrics. Effluents are highly coloured and degradation of the dyes in water bodies releases secondary pollutants into the environment. It may also affect photosynthetic activities of aquatic vegetation by reducing the diffusion of sun light. Among the different types of dyes, azo dye, benzidine dyes, etc., are non-biodegradable and/or undergo some partial degradation to form harmful by-products which are carcinogenic in nature [1]. Most of the industries withdrawn the uses of benzidine and benzidine based dyes for colouring their fabrics [2], but still it is used in some of the processing industries. One among such renowned dye, Acid Orange 7 (AO7), is widely used in textile, paper, food industries, etc. When effluent containing the dye is released into the environment, causes severe adverse effects such as damages to skin, eyes, digestive system, respiratory irritation, aesthetic issues, mutagenic and carcinogenic effects. So the researchers takes more attention towards the treatment of such potential pollutants by different treatment processes like electrocoagulation, adsorption, photocatalyst, membrane treatment, advanced oxidation process, etc., to make the sustainable environment [3, 4]. Table 1 explores the details of percentage of degradation of AO7 dye by various methods.

Table 1 Percentage of degradation of AO7 by various methods
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Cancer has high morbidity and mortality worldwide. Melanoma is an aggressive type of skin cancer and its characteristics include multi drug resistance, high relapse rate and low survival rate. Nowadays, apart from traditional chemotherapy and surgery in the treatment of cancer, nanoparticles are also used due to their selective targeting capability [10]. Among various nanomaterials, zinc oxide nanostructure has gained importance due to its opto-electrical, antimicrobial, anti-proliferative, catalytic and photochemical properties [11]. Zinc Oxide (ZnO) is the second most abundant metal oxide and is considered a “Generally Recognised As Safe (GRAS)” compound by US Food and Drug Administration. It is vital for various cellular mechanisms and also helps in maintaining cellular homeostasis, hence its biocompatibility.

Among the different materials used for environmental concern, ZnO is a semiconductor with a wide direct-band gap of 3.3 eV. ZnO and ZnO-related materials have attracted more and more attention over the past decades because of its applications in various fields such as photocatalyst, semiconductors, solar cells and pharmaceuticals. Many methods for synthesis of zinc oxide nanostructures have been reported in wet chemistry such as thermal decomposition, sol–gel techniques and precipitation [12].

Most preferably the adsorbent material choice is taken based on the easy availability, more adsorption, cost effective one. The present study focussed on the cost effective synthesis and application of nanomaterial for the treatment of carcinogenic dyes. Unexpectedly the property of synthesized zinc oxide nanostructures explores its application to the pharmaceutics against anticancer activities (antiproliferative efficacy against human melanoma).

2 Materials and Methods

2.1 Materials

The chemicals used in this present research work such as zinc nitrate (Zn(NO3)2), sodium hydroxide (NaOH) and ethanol were of analytical grade with 99% purity, purchased from Sigma-Aldrich. Commercially available Acid orange 7 (AO7) dye was used without further purification (Table 2). 0.1 M NaOH and 0.1 M HCl were used to adjust the desired initial pH values of reaction mixtures.

Table 2 Colour index of Acid Orange 7 dye
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2.2 Synthesis

The nanomaterial was synthesized by taking exactly 100 mL of 0.3 mol Zn(NO3)2 in a double-necked round bottom flask and stirred continuously using a magnetic stirrer (REMI) at 100 rpm. To the reaction mixture 0.4 mol NaOH (S1) was added within a time span of 20 min under constant stirring. Then, the reaction mixture was heated in an oil bath for 5 h at 90 °C with constant stirring. After heating, the reaction mixture was allowed to cool naturally at room temperature. The white precipitate formed was separated by centrifuge at 5000 rpm for 10 min. The precipitate was washed thoroughly with double-distilled water and then washed several times with pure ethanol. Then, the precipitate was dried at 80 °C for 12 h in a hot-air oven. Finally, the precipitate was calcined at 550 °C for 1 h in a muffle furnace.

Similarly, the synthesis was performed with varying the concentrations of NaOH viz., 0.8, 1.0, 1.2 and 1.4 mol and the product obtained were named as S2, S3, S4 and S5, respectively. Finally, the products obtained were stored in airtight containers until further use.

2.3 Characterization

A Shimadzu XRD-6000 powder X-ray diffractometer was used to record the X-ray diffraction (XRD) patterns of the samples. Ni-filtered Cu Kα radiation was used at a tube voltage of 40 kV with tube current of 200 mA. Raman spectra measurements were made with a Bruker RFS 27 multi-RAM stand-alone FT-Raman spectrometer under the spectral range 4000 to 50 cm−1at a resolution of 2 cm−1. The source used was a 1064 nm Nd-YAG laser. Field emission scanning electron micrographs of the samples were obtained with a FEI Quanta FEG 200 scanning electron microscope (SEM). The energy dispersive X-ray (EDX) spectra were recorded using a FEI Quanta FEG 200 SEM with EDX attachment. The diffuse reflectance spectra of the samples were recorded with a PerkinElmer Lambda 950 UV/Vis/NIR spectrometer in the wavelength 175 to 3300 cm−1. Surface area and porosity of the sample was scrutinized by nitrogen adsorption/desorption analysis (BET) using a Micrometrics ASAP 2020 apparatus at a relative pressure (P/P0) range of 0.04–0.9.

2.4 Batch Adsorption Studies

The performance of synthesized sorbent materials (S1, S2, S3, S4 and S5) were analysed by batch experimental studies. A total of 100 mg of adsorbent dose was mixed with 100 mL of AO7 solution at different initial concentrations viz., 25, 50, 75 and 100 ppm at different temperature ranging from 30, 35, 40 and 45 °C. The pH of the reaction mixture was adjusted from 2, 4, 6, 8, 10, 11and 12 by 0.1 M hydrochloric acid and 0.1 M sodium hydroxide. The reaction mixtures were agitated using a temperature-controlled Remi-make orbital shaker. The concentration of the residual dye in the clear supernatant solution at different time intervals from 0 to 100 min was estimated by measuring the absorbance using an Elico-make BL130- UV–Vis spectrometer. The dye degradation efficiency was estimated at the λmax value of 483 nm at different time intervals. All the adsorption experiments were performed in duplicate, and the maximum deviation allowed between the experiments was only 4%. The percentage of decolourization of dye was calculated as,

$${\rm{Colour \,removal}}\left( \% \right) = \left( {\frac{{C_{0} - C_{t} }}{{C_{0} }}} \right)x 100$$
(1)

where C0 is the initial concentration of the dye solution and Ct is the concentration of the dye after the time interval t.

2.5 Antiproliferative Studies

The human melanoma cancer cell line (A-375) was procured from the National Centre for Cell Sciences (NCCS), India. The cancer cells were allowed to grow in Dulbecco’s modified Eagle medium (DMEM) with 2 mM of L-glutamine, and the cells were maintained with a balanced salt solution (BSS) that consisted of 1.5 g L−1 Na2CO3, 0.1 mM nonessential amino acids, 1 mM sodium pyruvate, 2 mMl-glutamine, 1.5 g L−1 glucose, 10 mM (4-(2-hydroxyethyl)-1-piperazine ethane sulfonic acid) (HEPES) and 10% fetal bovine serum (FBS) (Gibco, USA). Additionally, the cell was maintained at 37 °C with 5% CO2 in a humidified CO2 atmosphere. The pictorial representation is shown in Fig. 1.

Fig. 1
figure 1

Pictorial representation of cell culture

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The cytotoxic effect of the synthesized compound (IC50) was tested by using MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. The cancer cell line (A-375) was allowed to grow (1 × 104 cells/well) in a 96-well plate for 48 h in order to obtain a confluence of 80%. It was followed by the replacement of the culture medium with a fresh medium containing various concentrations of synthesized compounds, and the cells were incubated for a further 48 h. The culture medium was removed and 100 µL of the MTT solution was added to each of the plates and incubated at 37 °C for 4 h. After removal of the supernatant, 10-min incubation was carried out adding 50 µL of DMSO to every well. The formazan crystals were dissolved. The optical density was measured at 620 nm in an ELISA multi-well plate reader (Thermo Multiskan EX, USA). The optical density (OD) value was used for estimating the percentage of viability using formula (2).

$${\text{\% of viability }} = \frac{{\text{OD value of experimental sample}}}{{\text{OD value of experimental control}}}{ } \times 100$$
(2)

The selected human melanoma cancer cells (A-375) were allowed to grow on cover-slips (1 × 105 cells/cover slip), and the cells were incubated with the synthesized ZnO nanostructures at different concentrations and fixed in ethanol–acetic acid solution (3:1, v/v). The cover slips were gently mounted on glass slides for morphometric analysis. Three monolayers were micrographed in each experimental group. The morphological alterations of the cells were analyzed using a Nikon (Japan) bright-field inverted light microscopy at 40 × magnification.

3 Results and Discussion

3.1 Characterization of ZnO Nanostructures

Figure 2 presents the EDX spectra of samples S1, S2, S3, S4 and S5. They show the presence of only zinc and oxygen and indicate the purity of the samples.

Fig. 2
figure 2

EDX spectra of ZnO nanostructures S1, S2, S3, S4 and S5

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The powder XRD patterns of samples S1, S2, S3, S4 and S5 (Fig. 3) uniformly display diffraction peaks at 31.72°, 34.39°, 36.24°, 47.46°, 56.49°, 62.77° and 67.84°. These reflections correspond to 1 0 0-, 0 0 2-, 1 0 1-, 1 0 2-, 1 1 0-, 1 0 3- and 1 1 2-planes of hexagonal wurtzite crystalline lattice and the peak intensities are in agreement with the JCPDS Card No. 36–1451 for S1, S2, S3 and S4 ZnO samples and JCPDS 79–2205 for S5 ZnO sample. All the ZnO nanocrystals show intense 101—diffraction peak indicating preferential crystallographic 101—orientation. In particular, the sample S5 displays highly intense 101- peak, which may be due to hollow hexagonal tube-like structure, as seen from FE-SEM (vide infra). The average crystallite sizes of S1, S2, S3, S4 and S5 deduced from the XRD data are 26, 17, 22, 25 and 33 nm, respectively.

Fig. 3
figure 3

XRD Spectra of ZnO nanostructures S1, S2, S3, S4 and S5

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Raman spectroscopic analysis using the shift in bulk phonon frequencies are useful to infer structural disorder, defects like oxygen deficiency and surface impurities and phonon capturing in ZnO nanostructures [13]. The E2 (high) peak is the characteristic optical phonon frequency for the perfect wurtzite hexagonal phase of ZnO crystal and is observed at 437 cm−1 [14]. This peak fits to the space group C6v4(28). The Raman E2 (high) frequencies of the five synthesized ZnO nanostructures, shown in Fig. 4, have been compared with the theoretical [15, 16] and experimental values of bulk ZnO. The results indicate the gradual increase of the lattice defects on moving from sample S1 to sample S5. The ZnO nanostructure S5 shows maximum defects theoretically as well as comparatively. The proposition is in agreement with a couple of recent reports on Fe -doped ZnO films [17]. The Raman spectra of ZnO nanostructures S4 and S5 show the B1 peak at 297 cm−1 as broad and less intense, implies the existence of lattice defects in the nanostructures [18].

Fig. 4
figure 4

Raman spectra of ZnO nanostructures S1, S2, S3, S4 and S5

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Sample S1 was prepared from 1:2 molar mixtures of precursors, and Fig. 5a shows morphology as a rectangular nanorod. The image suggests that rods are bundled prior to the separation. It is likely that the lowest concentration of sodium hydroxide enabled such formation. The length and width of the rods are nearly 400 nm and 30 nm, respectively. Figure 5b displays the plant leaves—like nanostructure (S2), which was obtained from 1:4 molar mixtures of the precursors. The large—scale plant leave—like structure is formed at the bottom of the collection. Most of the structures are uniform with length of upper leaves as less than 250 nm and the length of bottom leaves more than 400 nm. All the leaves are tri-centred and look like the apex of the tri—leaf cannabis plant and the thickness of each leaf is about 30 to 40 nm.

Fig. 5
figure 5

SEM images of ZnO nanostructures S1, S2, S3, S4 and S5

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Figure 5c depicts the morphology of the sample S3 synthesized with precursors at a molar ratio 1:6. The structure has many thin nanosheets joined collectively to form a single 3D flower—like aggregate. Figure 5d shows the SEM image of S4, obtained using the precursors in the molar ratio 1:8. The morphology of S4 imitates the morphology of S3, that is, 3D flower-like aggregate, and the nanosheets are of thickness ~ 30 nm. The SEM image of S5, the sample derived from the precursors at a molar ratio of 1:10, looks like hollow hexagonal nanotubes (HHNT), as displayed in Fig. 5e. The HHNT, compared to the other nanostructures S1, S2, S3 and S4, shows a good aspect ratio and exhibits a thickness of 20 to 30 nm. The crystal growth mechanism of HHNT is likely to be formation of nanosheet intermediates followed by rolling over. However, the crystal growth mechanism of high–aspect–ratio hollow cylinders is not yet clear. Irrespective of the nano-ZnO morphology, all the synthesized samples (S1-S5) display the crystal pattern of hexagonal wurtzite ZnO.

The Kubelka–Munk (K-M) equation was used to calculate the band gap energies of the prepared samples from their reflectance spectra as shown in Fig. 6. Using the K-M equation, it is observed graphically that the band gap energies of S1, S2, S3, S4 and S5 are 3.189, 3.199, 3.222, 3.258 and 3.183 eV, respectively. The HHNT (S5) possesses the lowest band gap of 3.183 eV, with an absorption maximum of 397 nm in the UV region. Hence, S5 has been chosen for further studies.

Fig. 6
figure 6

Graphical representation of modified Kubelka–Munk: [F(R)hυ]2 Vs. a band gap energy and b corresponding absorption spectra of the ZnO samples S1 to S5

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Brunauer-Emmett-Teller (BET) gas sorption measurement was done to study the specific surface areas and the porous nature of the adsorbent ZnO nanostructure. It is evident from Fig. 7 that the nitrogen adsorption/desorption isotherm is in accordance with type IV isotherm given by IUPAC classification and the mesoporous nature of the ZnO nanostructure was confirmed from the P/P0 graph, where the adsorption/desorption curve increases from 0.6–1.0 and the pore diameter is approximately 50 nm. Similar results were explained by Shamhari et al. [19], Ismail et al. [20]. Mesoporous nature and the larger pore size of the ZnO nanostructures support the easy diffusion of dye molecule and hence can be used as a good adsorbent for wastewater treatment.

Fig. 7
figure 7

Protonation of dye species at lower pH

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3.2 Adsorption Studies

3.2.1 Effect of pH on Adsorption and Desorption

The pH of the adsorbate solution is one of the influential parameters in adsorption studies, and hence, the experiments are carried out at different initial of pH from 2, 4, 6, 8, 10, 11 and 12. The adsorption of contaminant on ZnO nanostructures is susceptible to the surface charge of the adsorbent and speciation of the contaminant in solution, both of which are influenced by the initial pH of the adsorbate solution [21, 22]. This results in electrostatic attraction/repulsion among the sorbent and solute interface. The pKa value of AO7 (pKa = 8.26) indicates that the extent of ionization of dye molecules is very low. Although the dye ionizes feasibly, shows better adsorption in a neutral/basic solution (pH > 7).

Figure 8 shows the variation of AO7 adsorption with respect to the dye bath pH. The point of zero charge (pHzpc) suggests the pH value required for the sorbent surface to possess a neutral charge. The pHzpc of ZnO nanostructures are about 10.1 ± 0.6. The anionic dye species [AO7]− shows an increase of adsorption with increasing pH from 2.0 to 8.0. The maximum AO7 dye adsorption of 97.5% over the adsorbent ZnO nanostructure is found to be at pH 8. On increasing the pH above pHzpc of ZnO, the sorbent surface acquires a negative charge and reveals electrostatic repulsion of the dye molecule because of the anionic nature of the dye and decrease of dye adsorption on ZnO [23]. Figure 9 depicts the pictorial representation of AO7 adsorption at lower pH. At lower pH, the protons may attach with any of the nitrogen present in the azo double bond of the dye and hence gets protonated. Thus, a repulsive force is generated between protonated AO7 dye with anionic ZnO nanostructures resulted in decrease in adsorption of dye species. In addition, the part dissolution of ZnO nanostructures in acidic solutions decreases the number of active sites of ZnO nanostructures. Similar observations were reported in the literature for other acidic dyes [24, 25].

Fig. 8
figure 8

Variation of AO7 adsorption/desorption with respect to the dye bath pH

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Fig. 9
figure 9

Pictorial representation of AO7 adsorption at lower pH

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Desorption studies of AO7 loaded on ZnO nanostructures are conducted to identify the nature of interaction between the sorbent and the sorbate molecules. AO7-loaded ZnO is shaken well with distilled water by varying its pH from 2 to 12 (Fig. 8). At lower pH, maximum amount of AO7 is eluted. Hence, acidic water with lower value of pH is identified as the suitable eluent for regenerating spent ZnO and substantiates the weak interaction of AO7 on ZnO surface. As acidic medium may hamper the ZnO nanostructure, 5% NaOH solution and ethanol are used to estimate the recyclability. The regeneration efficiency is found to be good and reaches about 80% up to 5 cycles, after which it decreases with every successive cycle.

3.2.2 Effect of Temperature

Temperature is another important factor for adsorption, and the equilibrium was studied by adsorption thermodynamics by varying the temperature from 30° to 45 °C. Figure 10 depicts the adsorption capacity of ZnO (100 mg) for AO7 dye at the initial concentration of 50 mg L−1 as a function of temperature from 30° to 45 °C and it clearly substantiates that an increase in the temperature of the dye bath decreases the adsorption of AO7 dye from 48.68 mg g−1 to 43.34 mg g−1. This implies that nature of the sorption process is exothermic. A possible reason is that an increase in temperature decreases the adsorptive force among AO7 species and active sites on the ZnO surface [26].

Fig. 10
figure 10

Effect of temperature on adsorption of AO7 dye

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3.2.3 Adsorption Kinetics

The adsorption kinetics is analyzed using the experimental data by employing common kinetic models, namely Langergren pseudo-first-order and Ho and McKay pseudo-second-order kinetic models. These kinetic data provide useful information about the sorption pathways and their related mechanism [27,28,29]. From the experimental data, the values qe and k1 are calculated by plotting qt vs. t for nonlinear and log(qe−qt) vs. t for linear expression for the pseudo-first-order kinetic model. Similarly, for pseudo-second-order, qe and k2 are calculated by nonlinear curve fitting with t/qt vs. t and linear fitting with qt vs. t.

The calculated kinetic results of linear and nonlinear models are tabulated in Table 3 with respect to the initial concentration of AO7 dye (from 25 to 100 mg/L) and dye bath temperature (from 30° to 45 °C). It is inferred that, while increasing initial dye concentration from 25 to 100 mg/L, the pseudo-first-order rate constant increased from 0.0762 to 0.1023 min−1 for the linear model and decreased from 0.1069 to 0.0654 min−1 for the nonlinear model, respectively. Similarly, under different temperatures, k1 value increases from 0.0912 to 0.0972 min−1 for the linear model, and for nonlinear model, it decreases from 0.0803 to 0.0546 min−1. Thus, it is clearly seen that the nonlinear model shows a coherent variation, whereas no sequential variation of rate constant is found in the linear model with respect to concentration and temperature of the dye bath.

Table 3 Pseudo-first-order and second-order kinetic parameters for AO7-adsorption on nano-ZnO
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It is observed that the nonlinear form of pseudo-first-order and pseudo-second-order fits represent the experimental data in a better way when compared with linear forms. The qe(exp) values always increase as a function of initial concentration of dye (from 25.00 to 90.67 mg/g), and the qe(cal) values (from 24.84 to 96.67 mg/g) also follow the same trend as Shukla et al. [30].

Furthermore, the qe(cal) values of the pseudo-first-order kinetic model harmonize with the qe(exp) values than in the case of the pseudo-second-order kinetic model. In addition, the values of the correlation coefficient r2 for pseudo-first-order kinetics are relatively higher than the pseudo-second-order kinetics. Hence, from kinetic analysis, it is concluded that the best fit kinetic model to explain the adsorption of AO7 dye onto ZnO surface is the nonlinear form of pseudo-first-order model [31].

3.2.4 Adsorption Isotherm

The isotherm model provides knowledge about the affinity between the adsorbate and adsorbent, which enables us to predict the nature and mechanism of interaction [32]. In this study, the equilibrium data of AO7 adsorption on ZnO were analyzed by employing the isotherms of Langmuir and Freundlich.

Table 4 represents the isotherm parameters for the removal of AO7 dye by ZnO adsorbent. For the linear model, the value of Langmuir constant (bL) increases from 0.0107 to 0.0198 (L mg−1) as the dye bath temperature is raised from 30 °C to 45 °C, and for the nonlinear model, the bL constant decreases from 0.2665 to 0.0877 L mg−1 for the same temperature range. In the case of the nonlinear model, the exothermic nature of the adsorption process is supported by the decrease in bL value with increase in temperature from 30 °C to 45 °C. Out of the two models, the Freundlich model (0.9825–0.9897) is slightly better in respect of r2 as compared to the Langmuir isotherm (0.9786–0.9866). The nonlinear data exhibit the best fit with high r2 value than the linear data. The sorption process is favorable if the value of “n” lies in the Freundlich threshold range, that is, n = 1 to 10 [33]. Hence, it is proposed that the adsorption process of the present study is multilayered and occurs on the heterogeneous surface of the sorbent material.

Table 4 Parameters of AO7 adsorption Isotherm
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3.2.5 Adsorption thermodynamics

For validating the nature of adsorption of AO7 onto the ZnO surface, the thermodynamic parameters such as free energy (ΔG) and enthalpy (ΔH) are estimated. In this study, Langmuir constant bL (L mg−1) is modified to kL(mol L−1) by involving the molecular weight of AO7 [34] and Eq. 3 is employed to evaluate the free energy.

$$\Delta G = - RTlnk_{L}$$
(3)

where the gas constant, R = 8.314 J K−1 mol−1, T is the temperature of dye bath in Kelvin and kL is the Langmuir adsorption equilibrium constant (mol L−1). The value of kL indicates the affinity between the adsorbent and the adsorbate. The negative value of ΔH, − 31.06 kJ/mol signifies physical adsorption and exothermic in nature. Furthermore, the negative ΔG values − 3.44, − 3.625, − 3.977 and − 5.235 kJ/mol with respect to the temperature from 30, 35, 40 and 45 °C, respectively, suggest the spontaneity of the adsorption process. Value of ΔS − 113.16 J/Kmol shows less randomness of the adsorption process because of association of AO7 on ZnO nanostructure [34].

3.3 Antiproliferative Efficacy Against Melanoma Cancer Cells

3.3.1 Cytotoxic Assay

The influence of the synthesized nanostructured ZnO on human melanoma cancer cells has been studied using MTT assay. Figure 11 indicates the in vitro cytotoxic effect of the synthesized ZnO (0–25 μg mL−1) on the selected A-375 melanoma cancer cells. The experimental results validate that all the synthesized nanomaterials dose-dependently inhibit the proliferation of the cancer cells in a significant manner. Sample S5 shows a higher efficacy in controlling the proliferation of the human melanoma cancer cells, with the lowest IC50 concentration (8 μg mL−1), when compared with the other samples. Though S5 (obtained with 1:10 precursors) exhibits a significant anticancer potency, it is not able to reach the effectiveness of the conventional drug doxorubicin. A previous study reveals that the synthesized ZnO nanorods are able to exhibit cytotoxicity of about 22 and 20 μg mL−1 against liver cancer cells for 24 h and 48 h, respectively [35]. Another study stated that the ZnO nanoparticles show an IC50 concentration of 10 μg mL−1 against the MCF-7 breast cancer cells [36]. An additional report depicts that the inhibitory concentration of DOX-ZnO nanoparticles is 0.125 μg mL−1 for MCF-7 and HT-29 cells [37].

Fig. 11
figure 11

Cytotoxic efficacy of synthesized ZnO nanostructures against human melanoma cancer cells

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3.3.2 Analysis of Morphological Alteration Upon Treatment

After MTT analysis of the five synthesized nanostructured ZnO, S5 (1:10) shows a significant cytotoxic inhibitory activity and is further evaluated for cancer cell morphological alteration potential. The alterations occur in the cancer cell morphology upon treatment with the synthesized ZnO at different concentration, as shown in Fig. 12. The control cells do not show any noticeable changes in their morphology, whereas the nanostructured ZnO-treated cells display appreciable changes on treatment including cell shrinkage, membrane blebbing and formation of floating cells.

Fig. 12
figure 12

Apoptotic analysis of synthesized zinc nanostructures using A375 cells with AO/EtBr a Control b-f samples. Scale bar- 50 µm

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These alterations occur based on the concentration in which the cells have been treated. Thus, the antiproliferative effect of the synthesized nanostructures may be linked with its ability to disrupt the morphology of the cells. According to a previous study Ashokan et al. [35], the treated cells exhibit severe morphological changes including formless shape, detached and floating from the cell surface at higher concentrations. Zinc oxide nanoparticles induce apoptosis in melanoma cells by increasing ROS and this effect may be mediated by the ROS-dependent induction of p53 in treated cells [38]. This clearly demonstrates the central role of ZnO materials in ZnO-induced apoptosis of cancer cells. Further studies are warranted to ascertain the potential usefulness of zinc oxide nanorods to correcting the dysregulation of apoptosis in the therapy of melanoma.

4 Conclusion

Different morphological ZnO nanomaterials have been successfully synthesized by sol–gel method. Characterisation of synthesized material using SEM, EDAX, XRD, BET and Raman spectra ensures the nanostructure crystal pattern of hexagonal wurtzite ZnO. Based on the characterization studies sample S5 has been chosen for the decolourisation of AO7 dye by adsorption method. From the kinetic analysis data, confirm the adsorption of AO7 dye onto ZnO surface is nonlinear form of pseudo-first-order model. Isotherm studies showed that Freundlich isotherm explains the adsorption process more clearly than Langmuir adsorption isotherm. The thermodynamic analysis reveals that the process is spontaneous, exothermic and less randomness. Furthermore, the identified nano-ZnO extends its application towards the pharmaceutical application. S5 (nanorod) exhibits increased anti-melanoma cancer efficacy when compared with the other synthesized nanostructures. Thus, the synthesized nanorod is found to have anticancer properties and is also an efficient adsorbent of hazardous dyes from aqueous dye solution.