1 Introduction

Bacterial infections continue to pose a significant global health challenge, contributing to an estimated 13.7 million deaths annually worldwide [1]. The advancement of antibacterial materials and their extensive utilization across a range of sectors, including pharmaceuticals, medical devices, household products, and living environments, is imperative in addressing this menace to human health and well-being [2]. Antibacterial materials can be categorized into three main types on the basis of their source: natural antibacterial materials, organic antibacterial materials, and inorganic antibacterial materials. Among these categories, inorganic antibacterial materials stand out for their superior durability and stability and lower toxicity than the other two types. Notably, they also demonstrate a reduced likelihood of inducing bacterial resistance [3]. Among inorganic antibacterial agents, silver-based compounds have been widely utilized but are associated with biotoxicity and discoloration issues. Similarly, copper-based agents present challenges related to the impact of color on product appearance and limited application scenarios [4, 5]. In contrast, zinc oxide (ZnO) has emerged as a highly promising option because of its unique chemical stability, wide bandgap properties, and high exciton binding energy. The versatile nature of ZnO extends beyond its role as an effective antibacterial material; it has applications in the photocatalytic degradation of pollutants and has promoted advancements in photonics, laser technology, and sensor development, among others [6,7,8]. The wide availability of ZnO at an affordable price further enhances its appeal for diverse applications while maintaining excellent antibacterial activity and biocompatibility. As research continues to advance, the potential for new applications of ZnO as an antibacterial material remains dynamic, reinforcing its status as a valuable asset in addressing the ongoing challenges posed by bacterial infections.

The synthesis methods of ZnO, such as sol-gel, precipitation, solvothermal, hydrothermal synthesis, ultrasonic synthesis, and green synthesis [9, 10], face challenges in effectively controlling the morphology and microstructure of ZnO for simple large-scale production [11]. The ultrasonic-assisted synthesis of nanoscale ZnO represents a time-efficient, cost-effective, and straightforward approach suitable for laboratory-scale applications. The underlying mechanism of ultrasonic-assisted synthesis is cavitation, which harnesses the mechanical action of ultrasound to disperse substances in the solvent. Modifying ultrasonic conditions has the potential to yield diverse morphologies of ZnO to a certain extent [11]. Anadi et al. [12] demonstrated that the preparation of leaf-shaped ZnO via the ultrasonic method results in excellent antibacterial effects suitable for applications requiring substantial quantities of ZnO. The advantages of the ultrasonic method for sample preparation include good dispersion, rough surface characteristics, and effective control over the morphology and size of nanoscale ZnO, most importantly contributing to rapid onset and prolonged antibacterial action [13].

ZnO can serve as an antibacterial agent or coating in a variety of materials, including paint, ceramics, glass, rubber, and composite materials. However, the issue of aggregation due to its high surface energy significantly hinders its dispersibility and antibacterial effectiveness [4, 14, 15]. Improving its dispersibility presents a critical challenge in the synthesis of high-performance ZnO. Strategies for enhancing dispersibility include physical and chemical methods such as high-energy ball milling, ultrasonic treatment, surfactant utilization, and their combined application [16, 17]. Huang et al. [18] enhanced the dispersibility of ZnO particles by incorporating dispersants to produce a highly dispersible nanofluid, leading to significant improvements in the antibacterial performance as well as the formaldehyde and toluene removal capabilities of ZnO. Zhan et al. [19] developed a composite coating with superior dispersibility based on graphene oxide and ZnO via the hydrothermal method, which demonstrated excellent antibacterial and corrosion resistance properties. Sun et al. synthesized Fe-doped ZnO nanoparticles with different iron contents and then modified them with 3-aminopropyltriethoxysilane and polyethylene glycol 600. The resulting nanoparticles can disperse in water with good dispersity and improved stability [20]. However, obtaining these ZnO samples mostly involves the preparation of ZnO nanoparticles followed by surface modification using dispersants, which is cumbersome and time-consuming.

In this work, we propose a simplified procedure by adding dispersants during the synthesis of ZnO to achieve high dispersity. Furthermore, ultrasonic-assisted synthesis of ZnO allows for obtaining fully crystalline ZnO directly without the need for high-temperature calcination, thereby preserving the stability of the dispersant. The prepared ZnO exhibits excellent compatibility in waterborne coatings, maintains exceptional dispersion properties, and demonstrates effective antibacterial activity. We employed sodium hexametaphosphate (SHMP), sodium dodecyl benzene sulfonate (SDBS), and sodium polyacrylate (PAAS) as dispersants to facilitate the one-step synthesis of ZnO particles with ultrasonic assistance. The influence of incorporating dispersants on the morphology, structure, size, dispersibility, and antibacterial activity of the synthesized ZnO was comprehensively examined through characterization and performance testing. Furthermore, a comparative analysis with commercially available ZnO was conducted to elucidate the relationship between dispersibility and antibacterial properties. Subsequently, an in-depth exploration of the antibacterial mechanism of the material was undertaken. Finally, the self-synthesized ZnO was integrated into waterborne paint to evaluate its practical antibacterial efficacy.

2 Materials and methods

2.1 Materials

Zinc acetate dihydrate, sodium hydroxide, and SHMP were purchased from Xilong Science & Technology Co., Ltd. SDBS was obtained from Shanghai Macklin Biochemical Technology Co., Ltd. PAAS (a 45% aqueous solution, Mw ≈ 4500) was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. The commercially available ZnO utilized in this study was supplied by Nanjing Tiansland Biotechnology Co., Ltd. Antibacterial and nonantibacterial waterborne paint were acquired from Shanghai Nippon Paint Co., Ltd. Staphylococcus aureus (S. aureus) ATCC 25,923 and Escherichia coli (E. coli) ATCC O157 were employed as representative gram-positive and gram-negative bacteria to assess the antibacterial efficacy of ZnO.

2.2 Preparation of ZnO

ZnO was prepared via a previously reported method with some modifications [11]. Specifically, a peristaltic pump was used to slowly add 180 mL of 0.3 M sodium hydroxide solution into a glass beaker containing 180 mL of 0.15 M zinc acetate solution at a flow rate of 12 mL/min. The beaker was then placed in an ultrasonic cleaner (40 kHz, KH-100B, Kunshan Hechuang Ultrasonic Instrument Co., Ltd.) for 120 min while stirring at a speed of 60 rpm. Throughout the reaction process, the colorless solution gradually turned milky white, and the resulting precipitate was separated by centrifugation before being dried at 80 °C for 24 h. The obtained product was recorded as pristine ZnO. Additionally, three different dispersants, SHMP, SDBS, and PAAS (2%, w/v), were dissolved in the zinc acetate solution prior to performing the above procedure. This results in the preparation of three groups of ZnO with distinct dispersants: ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS.

2.3 Characterizations

The crystal structure of the samples was characterized via a Bruker D2 Phaser X-ray diffractometer (XRD, Cu target, λ = 1.5418 Å) with a scanning speed of 2°/min over an angular range of 20°~80°. The surface morphology and structure of the samples were examined via transmission electron microscopy (TEM, accelerating voltage of 200 kV; FEI Talos F200X G2). Water was used as a dispersant to prepare an aqueous solution of each sample, which was sonicated for 10 min and finally tested by TEM. The FTIR transmission spectrum of the sample was measured in the range of 400 cm− 1~4000 cm− 1 on a Thermo Scientific Nicolet iS20 instrument. The thermal properties of the samples were evaluated via a Netzsch TG 209 F3 Tarsus instrument at a heating rate of 20 °C/min within the temperature range of 30–800 °C. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo Scientific K-Alpha instrument. The test sample was fed into the analysis chamber at a pressure of less than 2.0 × 10− 7 mbar, with a spot size of 400 μm, an operating voltage of 12 kV, a filament current of 6 mA, and a full-spectrum scanning flux energy of 150 eV in steps of 1 eV. UV‒Vis measurements in the wavelength range of 200 ~ 800 nm were conducted via a Shimadzu UV‒3600i Plus spectrophotometer in the integrating sphere mode. The particle size and zeta potential of the samples were analyzed via the Malvern Zetasizer Nano ZS90 dynamic light scattering technique.

2.4 Dispersibility characterization

ZnO (0.05 g) was weighed and transferred into the test tube, followed by the addition of 10 mL of distilled water. The test tubes were shaken vigorously to ensure thorough mixing and then placed vertically in an ultrasonic cleaner for 30 min to facilitate complete dispersion of the ZnO. Once the ultrasound was completed, the test tube was positioned on a rack to initiate timing. This marks the beginning of the sedimentation process. The height difference between the clarified interface and total liquid level was measured every 12 h, allowing the sedimentation rate (%) to be calculated as follows:

Sedimentation rate (%) = (height of clarified interface of ZnO solution/height of total liquid level) ×100.

Additionally, for comparison, the supernatant from each test tube was collected every 12 h, and the absorbance within the wavelength range of 200 ~ 600 nm was measured via a UV‒visible spectrophotometer (L9, Shanghai Yidian Analytical Instrument Co., Ltd.).

2.5 Antibacterial properties

The antibacterial properties of ZnO were evaluated according to GB/T 21,510 − 2008. Initially, the bacteria were activated, and a bacterial suspension was prepared. Various aqueous solutions, including commercially available ZnO, pristine ZnO, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS, with different concentrations (0 g/L, 1 g/L, 3 g/L, and 5 g/L), were selected. Subsequently, 0.5 mL of sample solution and 0.5 mL of bacterial suspension with a diluted concentration of 10− 4 were separately added to Petri dishes for testing against E. coli and S. aureus via the plate culture method. Additionally, to investigate whether the three dispersants possess antibacterial activity, a solution of dispersants (2%, w/v) used in ZnO synthesis was prepared as a control, along with a bacterial mixture with the same proportions as the abovementioned samples. Furthermore, under dark and light conditions (a fluorescent light source inside the incubator providing light intensity between 1000 lx and 3000 lx with a white light wavelength), the antibacterial properties of different ZnO samples at a concentration of 5 g/L were examined [21]. All the materials utilized in this study were autoclaved, and all the experiments were conducted on a clean bench in a sterile environment. The Petri dishes were cultured at an incubation temperature of 37 °C for 24 h, followed by colony counting analysis via the icount20 colony counter from Hangzhou Xunshu Technology Co., Ltd. The results obtained from these experiments represent an average value on the basis of at least three repetitions. The calculation method for the antibacterial rate (%) is as follows:

Antibacterial rate (%) = (the number of colonies in the control group - the number of colonies in the experimental group)/the number of colonies in the control group ×100.

2.6 Antibacterial mechanism analysis

2.6.1 Zinc ion release

To investigate the antibacterial mechanism of ZnO, the release of zinc ions into bacterial cultures was assessed in accordance with previous methods [22]. The samples examined in this experiment were pristine ZnO and ZnO-PAAS, and the experimental procedures for both samples were identical. Typically, 5 mL of E. coli or S. aureus bacterial suspensions were mixed with 5 mL of ZnO in a sterile tube and cultured at 25 °C and 150 rpm in a water-bath shaking incubator for 8 h to obtain a solution containing Zn2+. After centrifugation, the resulting supernatant was supplemented with nitric acid to a concentration of 5% (v/v) and stored for use. The concentration of Zn2+ was determined via an inductively coupled plasma emission spectrometer (ICAP-6300, Thermo Fisher Scientific, UK), from which the actual concentration of Zn2+ was calculated.

2.6.2 Reactive oxygen species (ROS) analysis

Thirty microlitres of a 1 mg/mL ZnO solution was added, and 30 µL of the trapping agent 5,5-dimethyl-1-pyrrolin-N-oxide (DMPO) was added. A mixture of deionized water and methanol at a concentration of 100 mM was used to generate long-lasting free radical products, namely, DMPO-OH and DMPO-O2−. These radicals are relatively stable in methanol and water, respectively. After thorough mixing, an appropriate amount of the mixture is absorbed into capillary tubes, which are then placed into the sample chamber of an electron paramagnetic resonance spectrometer (EPR, Bruker EMX PLUS, Germany) fitted with quartz tubes. The •OH and •O2− tests were conducted in darkness and under a light source (a 300 W xenon lamp) for 10 min, respectively.

2.7 Application of ZnO in waterborne paint

In accordance with GB/T 21,866 − 2008, the antibacterial properties of paint after the addition of ZnO were investigated via the antibacterial determination method and the antibacterial effect of the coating (paint film). Four plastic plates measuring 50 mm × 50 mm were prepared as the bottom plates. In addition to the control group, the commercially available paint with an antibacterial effect, the original nonantibacterial paint supplemented with commercially available ZnO at a concentration of 3% (w/v), and the original nonantibacterial paint supplemented with ZnO-PAAS at a concentration of 3% (w/v) were coated on the plastic plate twice and dried for 7 days each. The paint film on the test plate was completely dry. Subsequently, 1 mL of bacterial suspension diluted to a ratio of 10− 3 was added to the surface of each plastic plate, which was spread evenly, covered with polyethylene film, and cultured in a sterilized plate at 37 °C for 24 h. A lotion containing normal saline with NaCl at a concentration of 0.85% and supplemented with Tween 80 at a concentration of 0.2% was subsequently used to elute the plate. Following elution, inoculation into nutrient agar culture media was performed with 1 mL of lotion per culture dish, which was then placed in an incubator set at 37 °C for 24 h before colony counting.

3 Results and discussion

3.1 XRD of the synthesized ZnO samples

Figure 1 shows the XRD patterns of pristine ZnO, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS. The results showed that the diffraction peak positions of the four samples were almost the same. However, the intensity of the diffraction peaks of the three kinds of ZnO with added dispersant was significantly higher than that of pure ZnO. It is because the dispersant effectively prevents agglomeration of ZnO nanoparticles and reduces mutual adhesion between particles. This dispersing effect makes the particles more homogeneous and increases the number of crystalline surfaces exposed to X-rays, resulting in stronger diffraction peaks and higher crystallinity in the XRD patterns. The diffraction peaks located at 2θ values of 31.78°, 34.47°, 36.36°, 47.47°, 56.62°, 62.75°, and 67.89° are attributed to the (100), (002), (101), (102), (110), (103), and (112) crystal planes, respectively [23]. Comparing the XRD patterns of the four synthesized samples with the standard pattern, no significant differences were found, indicating that the products are ZnO with a hexagonal crystal structure. The peak shape is very sharp, and the strongest peak is obtained at 2θ = 36.36°, indicating that the sample has a good crystallinity [23]. The crystal size of ZnO samples was calculated according to the Debye-Scherrer equation [24],

$$\:D\:=\:\frac{K\lambda\:}{\beta\:COS\theta\:}$$

where D is the crystallite size, K is Scherrer constant (0.89), λ is the X-ray wavelength, β is the integral half width, and θ is the Bragg angle. The diffraction peaks of the XRD spectra of the samples were fitted to the Gauss function using Origin software, which was fitted until convergence, and the corresponding half-peak widths were found and substituted into the formula to calculate the grain sizes under different diffraction angles, and the average value was finally calculated as the final grain size of the samples. The calculated average crystal sizes of pristine ZnO, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS are 19.6, 13.0, 10.7, and 10.5 nm, respectively. It can be concluded that the crystal size decreases with the addition of dispersants, which is consistent with previous studies [25].

Fig. 1
figure 1

XRD patterns of the synthesized ZnO

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3.2 TEM of the synthesized ZnO

Figure 2 shows TEM images of the synthesized ZnO samples, with high-magnification images displayed in the upper right corner for each sample. In Fig. 2(a), pristine ZnO is distributed in clusters with smooth edges, with an average size of approximately 330 nm [26]. Given the irregular shape of each sample, the DM software was employed to determine the maximum length as a representative measure of ZnO particle size, followed by calculation of the average value as the standard size. The number and distribution of different particle sizes in various samples were assessed at a scale of 1000 nm by analyzing approximately 100 particles. Subsequently, for each sample, we computed both the average particle size and generated a corresponding histogram depicting the particle size distribution [27]. Figure 2(b-d) shows that, compared with those of pristine ZnO, the morphologies of ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS are distinct and that they have needle-like shapes characterized by sharp edges. This phenomenon can be attributed to crystal growth along fixed crystal planes, facilitated by ultrasonic assistance and dispersants [28]. Simultaneously, the dispersants contribute to the growth process while ensuring a certain degree of dispersion between grains, resulting in the observed crystal morphology depicted in the figure. The average particle sizes calculated for ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS were approximately 336.1, 281.3, and 351.8 nm, respectively [26]. These TEM images demonstrate that each sample possesses a complete internal crystal structure with relatively clear lattice fringes; noticeable gaps are also evident at the edge of these fringes, which unequivocally confirms the successful synthesis of various ZnO materials [29]. Ultrasound-assisted preparation can alter the structure of ZnO. Zhang et al.‘s study [30] revealed the different morphologies achieved through this method, which resulted in smaller sizes and surface roughness than those of mechanical stirring methods.

Fig. 2
figure 2

TEM images of ZnO samples at different scales and particle size distribution of ZnO samples ((a) pristine ZnO; (b) ZnO-SHMP; (c) ZnO-SDBS; and (d) ZnO-PAAS)

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3.3 FTIR of the synthesized ZnO

The FTIR spectra of pristine ZnO, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS are shown in Fig. 3. The stretching vibrational modes of the Zn‒O bonds in all four samples exhibit distinctive bands with constant intervals, approximately at 417 cm− 1 and 902 cm− 1. A series of small characteristic peaks appear in the wavenumber range from 1150 cm− 1~1650 cm− 1, which can be attributed mainly to double bonds such as C = C, C = O, and C = N [31], primarily originating from dispersants. The wide peaks ranging from 2700 cm− 1 to 3300 cm− 1 correspond to the stretching vibration of the C–H bond [32]. However, only pristine ZnO exhibited a small characteristic peak at a wavelength of 2930 cm− 1, whereas ZnO-SDBS presented a peak at a wavelength of 2923 cm− 1 within this range. Owing to the ultrasonic synthesis method employed for ZnO synthesis, acoustic cavitation disrupts material surface tension and viscosity forces, resulting in the dispersion of nanoparticles within the solvent. Consequently, chemical bonds between water molecules breakdown, and hydroxyl radicals with strong oxidizing abilities are generated, promoting oxidation reactions involving C‒H bonds [33]. The characteristic peaks observed in the wavenumber range from 3200 cm− 1 to 3750 cm− 1 can be attributed to the stretching absorption peaks of the hydroxyl group (-OH) on the surface of ZnO.

Fig. 3
figure 3

FTIR spectra of pristine ZnO, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS

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3.4 TGA curves of the synthesized ZnO

Figure 4 shows the TGA results of the four ZnO samples. As shown in Fig. 4(a), the TGA curve can be delineated into four distinct stages: first, below 120 °C, all four samples exhibit varying degrees of mass reduction due to the evaporation of absorbed water and solvent [34]; second, namely, at temperatures between 120 ℃ and 400 ℃, there was a significant rise in the weight loss of all four samples. This indicates that the samples started to undergo thermal degradation, with a majority of the organic matter undergoing oxidation and decomposition; third, between 400 °C and 650 °C, the thermal decomposition process reaches completion for all four samples, leading to gradual stabilization of mass [33]; and finally, above 650 °C, residual carbon volatilization becomes the primary source of mass loss [35]. Notably, pristine ZnO, ZnO-SHMP, and ZnO-SDBS exhibit stable mass loss rates at this stage; however, as depicted in Fig. 5(b), ZnO-PAAS shows a pronounced decrease at 745.8 °C due to the dissociation of its main chain caused by irregular breaking of C‒C bonds under heating conditions [36].

Fig. 4
figure 4

Weight loss (a) and DTG diagrams (b) of pristine ZnO, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS

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3.5 XPS of the synthesized ZnO

The elemental composition of the four samples was further compared by XPS analysis, and the results are shown in Fig. 5. It was analyzed by Avantage software, using C 1s of contaminated carbon (284.8 eV) for charge calibration, and then XPS plots were drawn by procedures such as split-peak fitting and peak addition. In Fig. 5(a), the full-scan XPS spectra of synthesized four samples are presented, primarily comprising Zn, C, and O without any discernible unreacted Zn precursor [22]. However, miscellaneous peaks of Na 1s were observed for the three samples with added dispersants, which may be attributed to the presence of elemental Na in the three dispersants. Moreover, no additional contaminants are detected, indicating that a thorough synthesis process is consistent with the XRD results. Figure 5(b) displays the high-resolution Zn 2p XPS spectrum after fitting for four samples; two peaks of ZnO-PAAS are observed at 1041.8 eV and 1021.08 eV, corresponding to the 2p1/2 and 2p3/2 electrons of Zn2+, respectively. The separation of these double spectrum lines confirmed the successful synthesis of dispersant-containing ZnO [37]. Furthermore, Fig. 5(c) presents the high-resolution C 1s XPS spectrum after fitting for both samples; three consistent peaks attributed to C-O, C-C, and C-N are observed in four cases. As shown in Fig. 5(d), the fitted XPS spectra reveal that all four samples exhibit two peaks: One binding energy peak is related to the hydroxyl group adsorbed on the ZnO surface, and the other binding energy peak is related to the chemical absorption of O [38]. Evidently, the surface hydroxyl groups of ZnO-PAAS and ZnO-SDBS exhibit higher strength compared to pristine ZnO and ZnO-SHMP. These surface hydroxyl groups may play a crucial role in facilitating enhanced generation of ROS through interactions with oxygen and water molecules [39].

Fig. 5
figure 5

XPS spectra of pristine ZnO, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS ((a) full spectra; (b) Zn 2p; (c) C 1s and (d) O 1s)

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3.6 UV‒vis spectroscopy of the synthesized ZnO

UV‒visible diffuse reflection can be used to characterize the light absorption properties of semiconductor materials. Figure 6(a-b) shows the absorption spectra and optical band gaps of pristine ZnO, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS, which were measured via the UV‒visible diffuse reflection method. The characteristic absorption peaks of each sample indicate the successful synthesis of the ZnO particles [40]. At the same time, the characteristic peaks of ZnO red shifted after the addition of dispersant, which may be due to the increase in the particle size and the reduction in the band gap [34]. The overall absorbances of ZnO-PAAS are greater than the individual absorbances of pristine ZnO, ZnO-SHMP, and ZnO-SDBS. This finding indicates that the addition of dispersant PAAS enhances the absorptivity of ZnO. Therefore, in subsequent studies, these materials can be used to improve the antibacterial ability of ZnO under light conditions or in other fields, such as improving the photocatalytic degradation of pollutants [41]. The band gap widths of the four samples were calculated via the Tauc method [42], with the band gap energies of pristine ZnO, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS being 3.23 eV, 3.18 eV, 3.16 eV, and 3.12 eV, respectively. The reduction in bandgap energy facilitates the excitation of photoinduced charge carriers by low energy photons, thereby enabling the generation of reactive oxygen species (ROS) even under weak indoor light illumination. This capability holds promise for practical applications in inhibiting bacterial growth [43].

Fig. 6
figure 6

UV‒Vis spectra (a) and calculated band gaps (b) of pristine ZnO, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS

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3.7 Dispersibility and antibacterial properties of the synthesized ZnO

The zeta potential serves as a quantification of the interparticle repulsion or attraction strength, and a higher absolute value of the zeta potential indicates enhanced stability and dispersion within the system. For example, Soumitra et al. [44] demonstrated that their ZnO composite exhibited a significantly high zeta potential value, resulting in minimal particle agglomeration and superior dispersion. Figure 7(a) shows the zeta potential diagram for all four samples, clearly indicating that the three samples with added dispersants possess substantially higher absolute zeta potentials than pristine ZnO without added dispersants. Furthermore, particle size was also determined (as illustrated in Fig. 7(b)). The average particle sizes of pristine ZnO, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS were measured to be 300.0 nm, 336.1 nm, 281.3 nm, and 351.8 nm, respectively; these values were slightly lower than those obtained from TEM analysis. Despite the variation in particle diameters resulting from different measurement techniques employed, the discrepancy between the two methods remained consistent regarding the variation in particle size. Notably, both ZnO-SDBS and ZnO-PAAS with dispersants exhibited marginally larger average particle sizes compared to pristine ZnO without dispersants.

Fig. 7
figure 7

Zeta potential (a) and particle size distributions (b) of the ZnO samples

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The dispersion of the ZnO samples was analyzed by measuring the sedimentation rate and absorbance. Figure 8(a) shows that the sedimentation rate of commercially available ZnO reached nearly 100% after 0.5 days of sedimentation. The settling rates of the four synthesized ZnO sample solutions increased as the settling period progressed. Additionally, the three samples prepared with dispersant addition, namely, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS, demonstrated lower settling rates than did the samples without dispersant addition. Hence, the dispersibility of commercially available ZnO is significantly inferior to that of synthesized ZnO, and the dispersibility of the three samples with added dispersants is also superior to that of pristine ZnO. Among them, ZnO-PAAS has the highest dispersibility. Compared with other dispersants, PAAS possesses a substantial number of carboxylic acid groups and features elongated molecular chains. Consequently, the electric double layer formed upon adsorption onto the ZnO surface exhibited enhanced electrostatic repulsion effects. Additionally, the steric hindrance caused by its lengthy molecular chain surpasses that of smaller molecules, thereby resulting in superior dispersion stability for ZnO [16].

By combining the calculation of the sedimentation rate with absorbance, a more precise and compelling representation of the dispersion magnitude of each sample may be achieved, as the subjective influence of the experimenter’s observation is taken into account. Figure 8(b) shows graphs depicting the changes in absorbance over time for five samples at a wavelength of 400 nm. As the absorbance increased, the concentration of ZnO also increased, indicating a well-dispersed powder. In contrast, the absorbance of commercially available ZnO was consistently lower than the absorbances of the four sets of synthesized samples at all time intervals. Figure 8(c) shows a picture of each sample taken after 30 days of placement. It is evident that commercially available ZnO and pristine ZnO have fully settled, whereas ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS have maintained a certain level of dispersion. To summarize, the absorbance of pristine ZnO was less than that of ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS. Furthermore, the overall absorbance of ZnO-PAAS was the highest. Thus, it can be concluded that, compared with commercially available ZnO, synthesized ZnO exhibited superior dispersibility. Furthermore, the samples prepared with the addition of dispersants, namely, ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS, all demonstrated better dispersibility than pristine ZnO. Notably, ZnO-PAAS demonstrated the highest level of dispersibility among all the samples in terms of sedimentation rate, absorbance, and visual observation.

Fig. 8
figure 8

Sedimentation rate (a), absorbance (b) of each sample at a wavelength of 400 nm, and dispersion of each sample (c) after 30 days

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Figure 9 shows the antibacterial impact of various concentrations of ZnO on E. coli and S. aureus. Within a specific concentration range, increasing the concentration of each sample leads to a more pronounced antibacterial effect [45]. Compared with those of commercially available ZnO and pristine ZnO, the antibacterial properties of ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS were greater against both E. coli and S. aureus. All three ZnO samples prepared with dispersant, when present at a concentration of 5 g/L, had antibacterial rates greater than 99% against both E. coli and S. aureus. Furthermore, to examine the potential antibacterial properties of the dispersant, an antibacterial assay was conducted, and the results are presented in Fig. 10. These findings suggest that both SHMP and PAAS do not demonstrate any antibacterial activity against E. coli or S. aureus, whereas SDBS has a mild antibacterial effect, albeit inferior to the antibacterial efficacy of the corresponding ZnO. Thus, the inherent antibacterial activity of ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS originates from ZnO rather than from dispersants.

The antibacterial action of the synthesized ZnO nanoparticles was found to be influenced by their structure, shape, particle size, and other factors, as determined via TEM and zeta potential analysis. Shankar et al. [46] synthesized ZnO nanoparticles of various morphologies and sizes using different zinc salts and hydrolyzing agents. These nanoparticles exhibited distinct antibacterial properties against both E. coli and S. aureus. The addition of dispersant in this study results in a distinct surface morphology for ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS, which differs significantly from that of pristine ZnO. This disparity in surface morphology may explain why the antibacterial effect of the three samples is superior to that of pristine ZnO [25]. Based on XPS analysis, both ZnO-SDBS and ZnO-PAAS exhibited a higher density of surface hydroxyl groups, leading to an increased generation of ROS. The elevated ROS production contributed to the enhanced antibacterial efficacy of ZnO, thereby elucidating the superior antibacterial performance observed for ZnO-SDBS and ZnO-PAAS. The particle size of ZnO-SDBS was the smallest, but it did not exhibit the best antibacterial activity against both bacteria. Furthermore, the ZnO-SHMP compound has the highest zeta potential; however, it is not ideal for effectively combating bacterial growth. Assessing the antibacterial characteristics of a sample on the basis of a single factor is evidently insufficient in terms of rigor. Further analysis is needed to determine the decisive factors for the antibacterial effect and the link between these factors.

In general, a higher level of dispersion results in greater antibacterial performance [47]. ZnO-PAAS exhibited the best dispersion in this study, and it also had the most effective antibacterial activity against E. coli across all the concentration gradients. Furthermore, when paired with TGA analysis, ZnO-PAAS exhibited the most significant rate of weight loss. However, it still managed to reach the highest antibacterial rate, indicating its superior antibacterial performance. The antibacterial action of ZnO-PAAS on S. aureus was slightly inferior to that of ZnO-SHMP at each concentration gradient. Hence, our investigation demonstrated a definite positive association between dispersibility and antibacterial efficacy. Nevertheless, dispersibility is merely one of the crucial elements that impacts antibacterial effectiveness. This study unequivocally demonstrated that ZnO-PAAS has a remarkable ability to disperse, hence significantly improving its antibacterial effectiveness. It can be concluded that the antibacterial activity of ZnO increases in proportion to its degree of dispersibility.

Fig. 9
figure 9

The antibacterial rates of different ZnO samples vary with different concentrations

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Fig. 10
figure 10

Antibacterial activity of SHMP, SDBS, and PAAS

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3.8 Antibacterial mechanism of the synthesized ZnO

Previous studies have categorized the antibacterial mechanisms of ZnO nanoparticles into three main types: direct interaction of ZnO with bacteria, zinc ion dissolution, and generation of ROS [48]. The process of zinc ion dissolution occurs due to the presence of a substantial concentration of Zn2+ ions, which are very poisonous to bacteria, resulting in their death [49]. The antibacterial mechanism of ROS involves the production of hydroxyl radicals (•OH) and superoxide radicals (•O2−) on the surface of ZnO nanoparticles [50], which leads to intracellular oxidative stress and ultimately results in necrosis. We conducted an experimental investigation to determine the presence of zinc ions and ROS in the antibacterial process of synthesized ZnO.

This study investigated the mechanism of Zn2+ dissolution by selecting ZnO samples obtained from synthesized pristine ZnO and ZnO-PAAS. Among these samples, ZnO-PAAS demonstrated superior antibacterial properties compared with those of the other ZnO samples in this study. The antibacterial tests maintained a consistent mixing ratio of ZnO and the bacterial suspension (volume ratio of 1:1), and after 8 h of agitation followed by centrifugation, the release of Zn2+ into the supernatant was observed. According to ICP detection [51], the average concentrations of Zn2+ in pristine ZnO in liquid culture media for E. coli and S. aureus were 20.9 mg/L and 13.7 mg/L, respectively. The average concentrations of Zn2+ in ZnO-PAAS in liquid culture media for E. coli and S. aureus were 27.5 mg/L and 41.1 mg/L, respectively. The results indicated that both ZnO and ZnO-PAAS have the ability to release a significant quantity of Zn2+ throughout the antibacterial process. Zn2+ interacts with the bacterial cell walls and cell membranes of E. coli and S. aureus, resulting in the eventual death of the bacterium. Furthermore, the antibacterial properties of Zn2+ are anticipated to be directly correlated with its concentration, with a critical concentration being a determining factor [52]. Compared with pristine ZnO, ZnO-PAAS resulted in more Zn2+ release, indicating that the enhanced antibacterial action of ZnO-PAAS may be attributed to a relatively greater contribution from Zn2+ release.

ZnO exhibits exceptional photocatalytic characteristics as a semiconductor material. The above UV‒Vis spectra analysis demonstrated that ZnO exhibited a high level of light absorption ability following the introduction of the dispersant. Aftab et al. [53] reported that ZnO nanoparticles synthesized via a green approach had enhanced antibacterial capabilities when subjected to illumination. The current work examined the antibacterial effects of several ZnOs that were prepared via ultrasound-assisted synthesis and various dispersants. The objective of this study was to determine whether this ZnO exhibited improved antibacterial properties when exposed to light. Figure 11 shows the results of antibacterial performance experiments conducted on four different types of ZnO, both with and without exposure to light. The antibacterial rates were computed separately for the dark and light settings, with the blank group serving as a reference. The results demonstrated that the antibacterial effectiveness of each sample against E. coli and S. aureus was slightly greater in the presence of light than in the absence of light. One possible explanation is that the presence of light leads to the generation of ROS, which enhances the effectiveness of the antibacterial activity [54]. In this work, it can be concluded that Zn2+ plays a more vital role than does fluorescent light illumination in determining the antibacterial activity of prepared ZnO. However, the prepared ZnO shows antibacterial ability even under low light levels, which is crucial for the application of ZnO as an antibacterial additive in practical use. Compared to previously reported ZnO-based antibacterial materials (refer to Table 1), our sample exhibited exceptional antibacterial efficacy against both Gram-positive and Gram-negative bacteria. Additionally, the synthesis of our sample involved a straightforward procedure and utilized readily available raw materials, thereby highlighting its potential for large-scale applications.

Fig. 11
figure 11

Antibacterial activity of four kinds of ZnO against E. coli and S. aureus with and without illumination

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Table 1 Comparison of antibacterial ability with previous reports
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Figure 12 shows the utilization of pristine ZnO and ZnO-PAAS to examine the electrical signals of •OH and •O2− via EPR. Figure 12(a-b) shows the lack of substantial •OH and •O2− signals observed in pristine ZnO and ZnO-PAAS in the absence of light. When illuminated for 10 min, as shown in Fig. 12(c), both pristine ZnO and ZnO-PAAS displayed signal peak intensity ratios of 1:2:2:1, which is characteristic of •OH signal curves [38]. Figure 12(d) displays the •O2− production curves of pristine ZnO and ZnO-PAAS after 10 min of illumination. A comparison of these curves with the standard •O2− signal curves revealed that the peak intensities of segments 1, 2, 4, and 6 are essentially identical, whereas the peak intensities of segments 3 and 5 are relatively lower [38]. As indicated by the circles in the figure, the peaks in the 3rd and 5th segments of the test curve are less prominent, yet they nevertheless conform to a conventional •O2− signal curve. After 10 min of illumination, pristine ZnO exhibited higher signal intensities of •OH and •O2− than did ZnO-PAAS in all ranges. This indicates that pristine ZnO generates a greater amount of ROS. Overall, the findings suggest that the antibacterial mechanism of the prepared samples is likely due to the combined impact of the direct contact mechanism, the solubility of Zn2+, and the generation of ROS. Nevertheless, the impact of each antibacterial mechanism on the ultimate antibacterial outcome varies among different ZnO samples. The antibacterial mechanism of synthesized ZnO samples in this work was proposed in Fig. 13.

Fig. 12
figure 12

The yields of pristine ZnO and ZnO-PAAS were measured for (a) •OH and (b) •O2− in darkness and for (c) •OH and (d) •O2− under illumination for 10 minutes

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Fig. 13
figure 13

Antibacterial mechanism diagram of synthesized ZnO

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3.9 Antibacterial paint applications of the synthesized ZnO

Figure 14 displays the antibacterial properties of the four paint samples: a control group with no paint, a commercially available waterborne paint with antibacterial properties, a commercially available waterborne paint without antibacterial activity and supplemented with commercial ZnO, and a commercially available waterborne coating without antibacterial activity and incorporated with ZnO-PAAS. Compared with the control paint, the antibacterial paint and commercially available antibacterial paint exhibited a certain degree of antibacterial activity against both E. coli and S. aureus. When ZnO-PAAS is added to paint that does not have antibacterial properties, the antibacterial activity significantly increases, resulting in an antibacterial rate of over 99%.

The effectiveness of antibacterial products relies on the deliberate and regulated release of active Zn2+ ions from the coated surface into the pathogenic environment, and the effectiveness of the antibacterial properties relies on the concentration of Zn2+ ions [60]. The ICP test results indicated earlier that the average contents of Zn2+ in ZnO-PAAS in the liquid culture media of E. coli and S. aureus were 27.5 mg/L and 41.1 mg/L, respectively. Thus, ZnO-PAAS has the ability to release a relatively high concentration of Zn2+ ions, resulting in highly significant antibacterial effectiveness. The aforementioned findings suggest that the use of ultrasonically assisted synthesis, along with the incorporation of dispersants, can be employed for the production of coatings and for antibacterial applications.

Fig. 14
figure 14

Antibacterial properties of the four different paints

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4 Conclusions

In summary, we prepared ZnO through a one-step method with ultrasonic assistance and the addition of dispersants. The synthesized ZnO exhibited a needle-like shape, crystal purity, defined crystal size, and distinct functional groups, as revealed by characterization. The addition of dispersants did not affect the crystal structure of ZnO but resulted in a reduction in the crystal size. The incorporation of dispersants on the surface of ZnO was revealed by weight loss at higher temperatures, and the light absorbance of ZnO increased with the addition of dispersants. The antibacterial action is partially determined by factors such as the shape and particle size of ZnO. ZnO-SHMP, ZnO-SDBS, and ZnO-PAAS all exhibited more than 99% suppression of E. coli and S. aureus, with ZnO-PAAS showing both the best dispersibility and antibacterial activity against E. coli and S. aureus, demonstrating that the antibacterial activity of ZnO was consistent with its dispersity. Dispersibility is an important indicator of antibacterial performance. The antibacterial mechanism of the produced ZnO involves a synergistic combination of the direct contact mechanism, the Zn2+ dissolution mechanism, and the ROS production mechanism. The Zn2+ dissolution mechanism is the most significant contributor to the antibacterial activity of ZnO. The presence of light can stimulate the generation of ROS, including •OH and •O2−, resulting in more effective antibacterial action than in situations without light. With the addition of ZnO-PAAS, the waterborne paint exhibited a notable antibacterial effect, suggesting the potential application of ZnO in paint and other areas. The synthesis, antibacterial mechanism, and application of ZnO as a paint additive in this study provide an important reference for the rational design and development of stable and effective inorganic antibacterial materials.