1 Introduction

Research on antibacterial, antifungal, antioxidant, anticancer, anti-aging, and anti-inflammation materials has evolved and flourished due to scientific and technological improvements as well as necessity [1]. The performance of the materials utilized in the preceding case is determined not only by structure, particle size, morphology, and composition, but also by the technique of preparation. Nowadays, metal (Ag/Au NPs) or metal oxide (TiO2, ZnO, NiO, CuO, and so on) NPs production incorporate algae, fungi, plants, bacteria, and microorganisms [2], which is also preferred over other physical or chemical procedures because of its benefits such as lower chemical risk, lower cost, environmental friendliness, high compatibility, and safety. Additionally, green synthesis process does not requires the use of any catalyst due to the phytochemicals present in the extract, such as alkaloids, flavonoids, tannin, and saponins which function as a reducing and capping agent, results in remarkable triumph in research world [2, 3].

In general, metal and metal oxides in nanoscale dimensions exhibit extraordinary qualities because of the high surface to volume ratio, so these materials were employed in semiconductors, chemical sensors, textiles, energy storage devices, cosmetics, electronics, and health care technologies, in the meanwhile extensive research is also on its way [4,5,6,7,8,9,10,11,12,13,14]. Due to low toxicity and size-dependent impressive property, zinc oxide, semiconducting material, has a wide range of applications in textile, food (as additives), agriculture, and cosmetics, particularly in life-saving drugs, paint, electronics, rubber industries, solar cells, electroluminescent devices, piezoelectric transducers, and many other consumer products which is also acknowledged as safe material by US Food and Drug Administration [15,16,17,18,19,20]. Because of its intriguing electrical, optical, biological, and chemical capabilities, several natural extracts were used to synthesize zinc oxide nanoparticles in addition to physical and chemical methods [21,22,23,24,25]. The anticancer property of ZnO NPs was examined against MG-63 human osterosarcoma cells using Rubia cordifolia leaf extract as the reducing agent by Natarajan Sisubalan et al. [26]; it also damages the cells through the reactive oxygen species (ROS) generation. C. fistula and M. azadarach mediated ZnO NPs shown better zone of inhibition against clinical pathogens such as Escherichia coli (E. coli) and S. aureus than traditional drugs [27]. Various other reducing and capping agents have also been utilized to investigate the therapeutic potential of zinc oxide nanoparticles including Cucumis melo [20], Rubia cordifolia [26], Acalypha indica [28], Corriandrum sativum [29], Areca catechu [30], Lippia adoensis [31], Camellia sinensis [32], Nerium Oleander [33], marine brown alga [34], Cassia auriculata [35], Garcinia xanthochymus [36], Psidium gujava [37], Mangifera indica [38], Monsonia burkeana [39], chitosan and alginate [40], Phlomis [41], and so on. Due to its small size and favorable interactions with biological membranes, receptors, nucleic acids, and protein acids, ZnO nanoparticles are frequently employed in biomedicine [42]. Robson Dias Wouters et al. [43] used Eucalyptus grandis extract to create ZnO NPs for testing photodegradation and antibacterial properties against tartrazine yellow dye and other infections. Zinc oxide-doped selenium oxide nanoparticles were reported by Husam Qanash et al. [44] using Alternaria alternata. Excellent antibiotic property was obtained against Staphylococcus aureus (28.33 mm) and better dye degradation obtained after 300 min for methylene blue.

Coconut, as a natural source and non-toxic material, plays a major role in medicinal field. Lignin, tannin, flavonoids, pentose, and cellulose are the important bioactive compounds present in the coconut [45]. Coconut milk is of the utmost importance in traditional Ayurveda treatment. It can be employed for the treatment of oral ulcers. Coconut milk contains anti-microbial properties in the gastrointestinal tract and it has the potential to control hyperlipidemia [46]. Sugar, dietary fiber, proteins, antioxidants, vitamins, and minerals are rich in coconut water. Disease like bronchitis, throat infections, gonorrhea, influenza, and can be treated using coconut water since it is rich in anti-fungal, anti-viral, anti-inflammatory, anti-diabetic, antibacterial, and anti-parasitic properties. It promotes insulin secretion in the body and promotes blood glucose utilization [45]. Bioconstituents like gibberellins GA1, GA3, GA5, GA6, GA7, GA9, GA12, GA13, trans-zeatin, trans-zeatin riboside, trans-zeatin O-gluoside, etc. are present in coconut water which is necessary for our daily life [45].

Some of the literatures based on Cocos nucifera are as follows: a flower-shaped NPs was reported by Farjana Rahman et al. [19] using Cocos nucifera leaf extract with the grain size of around 15 nm. Satheshkumar et al. [47] describe the photocatalytic and antibacterial activity of coconut water mediated curry leaves extract over ZnO NPs. The NPs exhibited excellent degradation property against methylene blue with 98.45% of efficiency in 60 min and good biological performance over different pathogens. The biological performance of ZnO NPs was examined by Priyatharesini P.I. et al. [48] using Cocos nucifera male flower extract. The antibacterial activity of silver nanoparticles was also enhanced by using Cocos nucifera leaf extract as the reducing agent against common human pathogens such as Staphylococcus aureus, Bacillus subtilis, Salmonella typhimurium, Escherichia coli, Pseudomonas aeruginosa, and Citrobacter freundii [49].

To the best of our knowledge, the originality of the current study attempts to synthesise zinc oxide nanoparticles for the first time using two traditional Indian beverages: coconut water and coconut milk. The biological performance, including its structural, optical, and morphological characteristics, as well as its antibacterial and anti-inflammatory properties, were studied.

2 Experimental method

2.1 Materials

The fresh coconut milk and coconut water collected from local agricultural field of Coimbatore, India were used to prepare zinc oxide nanoparticles. Zinc nitrate hexahydrate [Zn(NO3)2. 6H2O (99%)] and ethanol (AR grade) were procured from Himedia, Coimbatore, India.

2.2 Preparation and analysis of ZnO NPs

Fresh coconut milk and coconut water extract (100 mL each) are collected to make zinc oxide nanoparticles in the same approach as described in our previous research [20]. The collected extract is filtered through Whatmann filter paper and used further without refrigeration. In the meanwhile, 0.3 M of zinc nitrate (precursor) was dissolved in 100 mL of distilled water to prepare a solution. The prepared solution was added drop by drop to 100 mL of coconut water under constant stirring for 3 h. The changes in color appear due to the presence of flavonoids, alkaloids, glucose, tannins, etc., in natural extract and then dry the colloidal solution in the hot plate. Figure 1 depicts general experimental flowchart of ZnO NPs preparation. The collected raw sample was calcined in muffle furnace at 400 °C for 4 h. Later, the obtained powder was washed 3 to 5 times thoroughly using deionized water and utilized further for analysis. Same procedure was repeated to prepare zinc oxide NPs using 100 mL of coconut milk at same conditions. Further, the prepared samples were named as MZ (milk extract) and WZ (water extract) subjected to characterization.

Fig. 1
figure 1

Experimental setup of ZnO NPs

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2.3 Characterization

The formation of zinc oxide should be confirmed by recording Powder X-ray diffraction (XRD) for the prepared material on X’Pert PRO (BRUKER) diffractometer equipped with CuKα radiation λ = 0.1540 nm) in the range of 2θ = 10–80° with a step size of 0.0500°. The presence of functional groups was examined by attenuated total reflectance-Fourier transform-infrared spectroscopy in the frequencies range from 4000 to 400 cm−1 (BRUKER Alpha II) for pure coconut milk, coconut water extract and prepared nanoparticles. The vibrational and optical property of the nanoparticles was recorded using BWS415-785 portable laser Raman Spectrometer under excitation of 785 nm and Ultraviolet-Visible Diffuse Reflectance Spectroscopy (JASCO V 750: from 200 to 700 nm) respectively. The morphology and size of the materials was examined by scanning electron microscopy (SEM) using JEOL JSM 6390 along with Energy Dispersive Absorption X-ray spectrum (EDAX) and Transmission Electron Microscope (Tecnai G2 20 S-TWIN). Fluorescence spectrophotometer (Fluoromax Plus C- HORIBA) was used to determine the emission wavelength and luminescence (PL) property of the nanomaterial from 250 to 700 nm and Carl Zeiss used for XPS analysis under high vacuum with Al Kα excitation at 250 W.

2.4 Antibacterial assay

The antibacterial activity of the prepared ZnO NPs was tested against two Gram negative bacteria: Klebsiella pneumonia and Escherichia coli, and two Gram-positive bacteria: Staphylococcus aureus and Streptococcus pneumonia using disc diffusion method based on Clinical Laboratory Standards Institute (CLSI) [50]. Using a sterile swab, 100 mL of a fresh culture containing 1 × 108 CFU mL−1 of bacteria was dispersed onto Mueller Hinton Agar (MHA) plates.

The bacteria were extended on nutrient agar to obtain its strain. The sterile filter paper of 6-mm diameter which contains samples of three different concentrations (~ 1, 1.5, and 2 mg mL−1) were spread on the surface of the inoculated agar plate and incubated for 24 h at 37 °C to study the bacterial stain [50]. For positive control, standard antibiotic amoxicillin was used and DMSO used as s negative control.

2.5 Anti-inflammation study

Acetyl salicylic acid, bovine serum albumin (BSA), was purchased from Sigma Aldrich, USA. 10X PBS was purchased from Himedia, India.

To measure protein denaturation inhibition, the method of Mizushima, Kobayashi, and Sachin S. Sakate et al. [51, 52] was slightly modified. To the test samples, 500 mL of 1% bovine serum albumin were added (500, 250, 100, 50, and 10 mg mL−1). After 10 min at room temperature, this combination was heated for 20 min at 50 ± 2 °C. After the resultant solution had reached room temperature (34 °C), absorbance at 660 nm was measured. Acetyl salicylic acid was used as a positive control. Using the following formula, the experiment’s percent inhibition for protein denaturation was calculated:

$$\mathrm{percentage}\;\mathrm{of}\;\mathrm{Inhibition}=100-\left(\left(\mathrm A1-\mathrm A2\right)/\mathrm A0\right)^\ast100$$
(1)

where

A1:

absorbance of the control

A2:

absorbance of the test sample

A0:

absorbance of the positive control.

The IC50 values were calculated from the dosage response curve when the concentration achieves 50% of a maximum scavenging capability. Every test and analysis were run in triplicate and averaged.

2.6 Statistical analysis

Statistical analysis was accomplished by one-way ANOVA with Tukey’s multiple comparisons test using P < 0.05 (95% confidence interval) as a statistical significance threshold unless mentioned specifically. All statistical analysis was performed in triplicate using graph-pad prism software-VI.

3 Results and discussion

Zinc oxide nanoparticles synthesized using coconut milk and coconut water are named as MZ and WZ respectively. Figure 2 depicts the XRD pattern of synthesized ZnO NPs using coconut milk and water. Both the samples exhibit a highly crystalline well-defined diffraction pattern. The diffraction pattern observed at 31.7, 34.3, 36.1, 47.4, 56.5, 62.8, 66.4, 67.8, and 69.0° and 31.7, 34.4, 36.2, 47.5, 56.5, 62.8, 66.4, 67.9, and 69.0° corresponds to (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes for MZ and WZ NPs respectively. Both the pattern indicates the formation of hexagonal wurtzite structure (JCPDS: 36–1451) without any other additional peaks [53]. When compared to MZ, WZ exhibits more peak broadening, which suggest that the material formed smaller crystals. Using Debye-Scherer formula (D = 0.9λ/β cos θ, which takes its usual meaning), an average crystallite size of produced zinc oxide nanoparticles was calculated as 25.5 and 11.3 nm for coconut milk and water respectively. Compared to earlier reports by Archana P et al. [20], Shabnam Fakhari et al. [54], Yasser A., and Selim et al. [55], the prepared ZnO using coconut water shows lesser crystallite size. ZnO NPs reported by Farjana Rahman et al. [19] using Cocos nucifera leaf extract exhibit the average crystallite size as 16.6 nm. The micro-strain of the produced material was calculated as 0.0013 (MZ) and 0.0032 (WZ) using the equation, lattice strain = β sin θ/4 [20]. Due to more strain, the broadening and minute shift in the XRD peak occurs along with the reduction in particle size.

Fig. 2
figure 2

XRD pattern and W-H plot (inset) of ZnO nanoparticles

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Furthermore, the Williamson-Hall (W-H) method (Fig. 2) was also used to determine the crystallite size for the MZ (26.5 nm) and WZ (12.05 nm) samples, which is likewise closer to the size determined using the Scherer formula. It is confirmed by both methods that WZ’s crystallite size is lesser than that of MZ’s. Due to the existence of compressive strain, MZ and WZ exhibit positive slope [56].

ATR-FTIR spectrum of the pure coconut water and coconut milk was presented in Fig. 3a. The presence of strong, broad O-H stretching (3319 cm−1), amine group (2917, 2857 cm−1), alkyne (2114, 2108 cm−1), strong aldehyde (1740 cm−1), alkene (1639 cm−1), C-H bending mode (1458 cm−1), and C-O stretching mode (1150, 1110 cm−1) were inferred from the spectrum [47, 57].

Fig. 3
figure 3

ATR-FTIR pattern of a coconut milk (M) & coconut water (W), and b ZnO nanoparticles

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Coconut milk and coconut water-mediated ZnO nanoparticle FTIR spectrum was exhibited in Fig. 3b. The broad absorption peak appeared for MZ and WZ at 3424 and 3426 cm−1 associated with O-H stretching mode of hydroxyl groups and -NH amine groups. The asymmetric C-H stretching bond was identified around 2930 cm−1 [58]. Due to H-O-H bending, vibration of water molecules a weak band observed near 2330 cm−1. The characteristic peak at 1621 cm−1 inferred the presence of C = O and the presence of aromatic stretching vibrations (C-N) was confirmed by observing a sharp peak at 1021 cm−1 for both the samples respectively [11, 24, 59]. The absorption band near 1400 cm−1 implicit C-C the stretching mode [60]. The characteristic bands observed at 461, 465, and 865 cm−1 ascribed to stretching vibrations of Zn-O respectively [24].

The formation of highly crystalline hexagonal wurtzite structure of ZnO NPs was further confirmed by Raman analysis in the range of 0–1500 cm−1. The characteristic peaks of ZnO NPs are depicted in Fig. 4a which illustrates C6V point groups and it corresponds to A1 + 2B1 + E1 + 2E2 representation [61], where A1 and E1 signify polar mode of Raman and Infrared active region. Raman is active and infrared is inactive in the polar mode E2, but both are inactive in the polar mode B1. Furthermore, transverse optical (TO) and longitudinal optical (LO) phonons are the two modes that are distinct from A1 and E1 modes. Both low- and high-frequency modes are present in E2. High intense peak of E2 was observed near 430 in both the cases due to lattice oxygen vibrations and a small peak observed near 320 cm−1 because of multi-phonon mode (E2M). The small broad peak observed at 372, 526 and 655 cm−1 are due to defects at phonon mode. The transverse optical mode and the combination of the transverse and longitudinal optical mode (TO + LO) with A1 symmetry responsible for the very small, weak, and broad peak that was seen at 975 and 1078 cm−1 [62]. The intensity of WZ decreases as compared to MZ may be due the presence of least amount of defects, and the result also suggesting the presence of highly crystalline nature of ZnO NPs. To study the size distribution of the prepared particles, DLS analysis taken and presented in Fig. 4b. The average particle size was observed as ~ 170 and 143 nm for MZ and WZ sample.

Fig. 4
figure 4

Raman spectrum (a) and DLS (b) of ZnO nanoparticles

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Figure 5 revealed the UV absorption spectra of pure coconut water, milk extract and ZnO NPs. A clear absorbance peak observed for W at 268 nm, whereas a small hump notified for M at 282 nm. ZnO NPs: WZ has a significant absorption peak towards blue shift at 363 nm and MZ exhibits a peak at 374 nm, which is closer to the light absorption range (360 − 380 nm) of zinc oxide nanoparticles [59, 63]. The shift in the peak towards lower wavelength denotes photo exciton of electron from the valence band to the conduction band. As illustrated in Fig. 5, the optical band gap energy (Eg) was estimated by plotting a graph between (αhυ)2vs (hυ) and the intercept of the tangent that extends towards the x-axis will give Eg. WZ (2.78 eV) exhibits a smaller optical gap than MZ (3.07 eV) and Archana P et al. [20], suggesting that the phytochemical in the extract will contribute to the formation of a potent optical coupling between zinc and oxygen atoms. The reduction in particle size, lattice strain, and oxygen deficiencies play a major role in tuning the optical band gap, which results in enhancing the photocatalytic activity of the prepared semiconducting material.

Fig. 5
figure 5

Absorption spectrum of a coconut milk, b coconut water, c ZnO, and d, e Tauc plot of ZnO nanoparticles

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XPS analysis was used to examine the surface chemistry and oxidation state of the produced ZnO NPs. Figure 6a depicts the presence of Zn (2p) and O (1 s) in the nanomaterials from XPS analysis. Two strong symmetric signals found at 1021 eV and 1044 eV correspond to Zn 2p3/2 and Zn 2p1/2 by splitting up of Zn 2p associated to ZnO matrix with the energy difference of 23.1 and 23.03 eV for MZ and WZ respectively as shown in Fig. 6b [64]. Further, Fig. 6c shows the doublet splitting of O (1 s) symmetric signals. Due to the presence of O2− and Zn-O bond, the peak observed at 530.32 and 530.50 eV for both materials [65]. The characteristic signal observed at middle binding energy: 531.64 and 531.96 eV corresponds to loosely-bound oxygen present on the ZnO surface, adsorbed O2, or OH hydroxyl group. The obtained results are closely similar to the results obtained by Chandrasekaran Karthikeyan et al. [64] and E. De la Rosa et al. [65] for zinc oxide NPs.

Fig. 6
figure 6

XPS spectrum of MZ and WZ (a) wide scan spectra and high-resolution spectra of (b) Zn 2p (c) O 1 s

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The defects present in the crystal system will be analyzed by photoluminescence (PL) spectra, which is also important to analyze the biological performance. At an excitation wavelength, the electrons in the valence band are shifted to the conduction band and then returned to the valence band, resulting in the generation of a photoluminescence signal. The excitation wavelength for MZ and WZ are 320 nm and shown in Fig. 7. In the visible region, zinc vacancy-Vzn, oxygen vacancy-Vo, zinc interstitials-Zni, and oxygen interstitials-Oi defects are observed [40]. Oxygen-based defects play a vital role for biological performance of the material. The emission spectrum corresponds to MZ are 362, 388, 417, 443, 479, and 525 nm; whereas for WZ, it corresponds to 390, 419, 447, 482, and 520 nm respectively. The peak shifted towards red emission for WZ in comparison to MZ. The near band edge emission disappears in WZ may be due to distortion. Obtained peaks are similar to earlier report by Chandrasekaran Karthikeyan et al. [40].

Fig. 7
figure 7

Photoluminescence emission of ZnO nanoparticles

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The morphology of the particles plays a fascinating role in determining the capability of the nanoparticles. Figure 8a–h implicit the SEM, TEM, SAED pattern, and EDAX spectrum of ZnO NPs using coconut milk and coconut water extract. The prepared materials indicate a clear spherical shape with homogenous and uniform grain boundaries from SEM analysis. Both samples exhibit a nano-sized particles from 20 to 80 nm and maximum number of particles ranges between 50 − 60 nm (MZ) and 40 − 50 nm (WZ). The formation of NPs include following mechanism: Zn2+ ions present in the precursor Zn(NO3)2.6H2O coordinate with OH− molecule and converted to Zn(OH)42− while heating at 100°C and later the ions translated to ZnO NPs due to phytochemicals present in the coconut extract [40]. Zinc oxide nanoparticles prepared using coconut water shown reduction in particle size than particles obtained from coconut milk, which supports average crystallite size obtained from XRD results. The Transmission Electron Microscopic (TEM) images of the ZnO NPs are shown in Fig. 8b and f. Figure 8b revealed the formation of uniformly distributed small spherical shape particles of MZ, whereas the presence of spherical particles along with and rod shape morphology was identified for WZ from Fig. 8f. The average particle size was calculated as ~ 10 (MZ) and ~ 8 nm (WZ) respectively. The SAED pattern of lattice resolved TEM data indicates the formation of corresponding planes of ZnO NPs in highly crystalline arrangement with hexagonal wurtzite structure as shown in Fig. 8c and g. The quantitative elemental analysis of MZ and WZ was examined from EDAX spectrum as shown in Fig. 8d and h. Both the spectra exhibit only zinc and oxygen peak corresponding to purity of the material as inferred from XRD spectrum. The atomic percentage was observed as 17.25% (Zn) and 83.75% (O) for MZ and WZ respectively.

Fig. 8
figure 8figure 8

SEM (a, e), histogram (inset), TEM (b, f), SAED (c, g), and EDAX (d, h) spectrum of MZ and WZ NPs

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Currently antibacterial therapy has been significantly influenced by the rise and proliferation of multidrug-resistant infections. The usage of natural extract which acts as a reducing and capping agent to prepare NP-attracted pharmacist. These prepared NPs will be treated with different bacteria/pathogens in various concentrations to identify its growth inhibition. In this scenario, the precursor and prepared nanoparticles were treated against Staphylococcus aureus, Streptococcus pneumonia, Klebsiella pneumoniae, and Escherichia coli as shown in Fig. 9(a-j). The pure zinc nitrate hexahydrate exhibits least zone of inhibition against different pathogens and shown in Fig. 9e. The antibacterial activity is influenced by the material’s concentration as well as the particle size and type of metal oxide being employed. The electrostatic interaction developed in the smaller sized particles causes them to adhere more readily to the bacterial cell walls and form a radical, which results in disturbing the protein structure of the bacteria [11]. The electrostatic interaction between positively charged Zn ions and negatively charged O has a propensity to enter the cell wall, which causes protein leakage, DNA and protein molecule breakage, and ultimately results in the death of the bacteria (Fig. 9l) [40]. It is often worth to note that smaller sized NPs will have effective antibacterial activity, as it can easily entered into the cell membrane and destroy it. The formation of tiny-sized ZnO NPs when utilizing coconut water is confirmed by XRD and SEM examination. Furthermore, the mechanism of reactive oxygen species (ROS) plays an important role in destructing the cell membrane of the bacteria. High reducing and oxidizing properties are exhibited by the electrons in the conduction band and the holes in the valence band, which also contribute to the creation of free radicals (.OH and.O2−). These free radicals are known to damage cells by interfering with DNA, amino acids, lipids, carbohydrates, and proteins [26]. The general mechanism of ROS generation under UV-Vis irradiation is as follows:

Fig. 9
figure 9figure 9

Antibacterial activity of ZnO NPs using coconut milk (a − d), coconut water ( g− j), and histogram of zinc nitrate hexahydrate and prepared NPs (e, f, k). Schematic representation of antibacterial mechanism (l)

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$$\begin{array}{c}\mathrm{ZnO}+\mathrm{h\upsilon }\to {\mathrm{e}}^{-}+{\mathrm{h}}^{+}\\ {\mathrm{e}}^{-}+{\mathrm{O}}_{2}\to {{}^{\bullet }{\mathrm{O}}_{2}}^{-}\\ \begin{array}{c}{\mathrm{e}}^{-}+{{}^{\bullet }{\mathrm{O}}_{2}}^{-}+{2\mathrm{H}}^{+}\to {\mathrm{H}}_{2}{\mathrm{O}}_{2}\\ 2{\mathrm{e}}^{-}+{{}^{\bullet }{\mathrm{HO}}_{2}}^{-}+{\mathrm{H}}^{+}\to {}^{\bullet }\mathrm{OH}+{\mathrm{H}}^{+}\end{array}\end{array}$$

M. Karthikeyan et al. [53] and Chandrasekaran Karthikeyan et al. [40] clearly explains the mechanism of ROS formation for ZnO NPs and emphasize the cell membrane damage that leads to cell death. In the current study, the produced ZnO NPs exhibit good antibacterial action against E. coli at all concentrations, and both materials demonstrated good antibacterial activity against all four pathogens and were similar to commercial amoxicillin at higher doses. Table 1 summarizes that the produced NPs outperformed previous findings leveraging different natural extracts in terms of antibacterial activity.

Table 1 Comparison of antibacterial activity of ZnO NPs
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By denaturant protein, the produced materials’ anti-inflammatory activity was evaluated. The development of tissue damage during inflammation and arthritic reactions both entail protein denaturation. Figure 10 depicts the triplicate inhibition percent for protein denaturation of the prepared ZnO NPs calculated using Eq. (1) at different concentrations (10, 50, 100, 250, and 500 µg mL−1). In the presence of pathogenic microbes, trapped foreign particles, inflammatory cytokines, and epithelial cells reduce the production of thymic stromal lymphopoietin which responsible for anti-inflammatory activity [70]. It was found that percentage of inhibition increases with increase in concentration and maximum percentage obtained as 89.7% at 500 µg mL−1 for WZ than MZ (30.89%). The findings shows that prepared zinc oxide nanoparticles were successful in preventing thermally induced albumin denaturation at all concentrations, demonstrating their potential to regulate protein denaturation related to inflammation. The current work exhibits better zone of inhibition as compared to earlier reports [71, 72]. Due to the presence of phenolic compounds in the surface (inferred in FT-IR), maximum inhibition for protein denaturation occurs. In other words, no additional molecule exists between the nanoparticles and the protein, which explains why higher phenolic compounds on the surface have stronger anti-inflammatory effect [70, 73]. The anti-inflammatory results exhibited the IC50 value for MZ and WZ as 113.5 and 47.99 μg mL − 1 respectively. IC50 of WZ is also nearer to standard acetyl salicylic acid.

Fig. 10
figure 10

Inhibition percentage of albumin denaturation of ZnO NPs

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The anticancer studies of MZ and WZ were conducted on the breast cancer cell line MDA-MB-231 and presented in Table 2. The NPs were tested from 10 to 50 μg/mL concentrations. After 24 h of exposure, the IC50 values of MZ and WZ NPs against MDA-MB-231 cells were determined to be 30 and 40 μg/mL, respectively (with statistical significance at p ≤ 0.05). Several factors contribute to the toxicity of these nanomaterials. One of the factors is protein adsorption, which refers to binding proteins onto the surface of the nanoparticles. The adsorption of proteins can influence the behavior and interaction of the nanoparticles with cells and tissues, affecting their overall toxicity. The dissolution rate of the nanoparticles is another essential factor. If the nanoparticles dissolve in the surrounding medium, they may release their constituent elements or ions. In this case, releasing Zn2+ ions may contribute to the observed toxicity. Producing reactive oxygen species (ROS) is another potential mechanism of toxicity. Nanoparticles can induce the generation of ROS, which are highly reactive molecules that can cause oxidative stress and damage to cells [64]. This oxidative stress can lead to various cellular dysfunctions and contribute to the anticancer effects of the nanoparticles.

Table 2 Anticancer property of MZ and WZ
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4 Conclusion

Numerous natural extracts were used to locating ZnO NPs’ hidden capabilities. As a result, we succeeded in producing ZnO NPs incorporating a green, environmentally friendly method that employed coconut milk and water extract. With a reduced crystallite size for WZ (11.3 nm) and no secondary or impure phases forming, XRD measurements indicate the formation of pure ZnO NPs with highly crystalline nature. The FT-IR study revealed a stretching vibration at 461, 465, and 865 cm−1 along with other phenolic chemicals, confirming the Zn-O bond’s presence. The optical band gap is smaller for WZ (2.78 eV) than MZ (3.07 eV) as calculated from Tauc plot. The presence of zinc, oxygen, and surface imperfections is clearly analyzed from XPS and photoluminescence spectrum. The development of spherical-shaped particles with a maximum number of particles at 50 nm and purity for both samples was deduced from the SEM and EDAX analysis. TEM analysis insists the formation of rod and spherical shape particles for WZ also indicates that it will exhibit a better biological performance than MZ. The antibacterial activity of ZnO was seen to rise with concentration due to the production of enhanced reactive oxygen species (ROS) using coconut milk and coconut water, and maximum potential obtained for Escherichia coli (21.5 ± 0.25 mm). Coconut water (89.7%) had the highest anti-inflammation result compared to coconut milk-assisted ZnO. Additionally, WZ shown superior anticancer activity against breast cancer cell (MDA-MB-231). These outcomes led us to the conclusion that coconut water is a superior chemical than milk for clinical applications, and ZnO NPs made from both coconut milk and water are a highly potent substance for biomedical applications.