Nano-structured coatings in dentistry: surface analysis comparison of zinc oxide-coated stainless steel and zirconia crowns for pediatric use

INTRODUCTION

Green synthesis techniques for producing nano- particles have gained traction, as they focus on mini­mizing hazardous chemical use, reducing waste, and enhancing the efficiency of nano-particle production, while maintaining low costs and high yields. Metal oxides, including magnesium oxide, titanium oxide, copper oxide, and zinc oxide (ZnO), are often synthesized for their antimicrobial properties, and are recognized for their environmental stability, safety, and multifunctionality, making them suitable for bio-medical applica-tions, especially in dentistry [1, 2]. In pediatric dental medicine, managing dental caries is a significant con-cern, particularly when it comes to treating primary teeth. Stainless steel crowns are commonly used to re-store decayed primary molars due to their strength, affordability, and durability against chewing forces. However, the inert nature of stainless steel can lead to bio-film formation, which increases the risk of secondary complications, such as periodontal disease and recurrent caries. This situation highlights the need for restorative materials, which not only restore dental function, but also offer additional protective benefits [3].
The field of nano-dentistry merges nano-technology with dental science, facilitating the development of innovative materials with various clinical applications. Treating pediatric patients can be complex due to their heightened sensitivity and anxiety, making it essential to choose materials, which not only restore function but also ensure long-term oral health with minimal discomfort [4].
The development of ZnO nano-particles coatings on stainless steel crowns (SSCs) marks an important ad-vancement in pediatric dentistry due to ZnO’s antimicrobial properties, bio-compatibility, and ability to gen-erate reactive oxygen species (ROS) for additional protection. These coatings reduce bacterial bond, lowering the risks of caries and periodontal disease, which are crucial for young patients, who may struggle with oral hygiene. ZnO-coated crowns offer improved oral health outcomes and can potentially reduce the need for frequent dental interventions [5].
As a semi-conductor, ZnO exhibits optical prope­rties, which are beneficial in medical applications, includ-ing coatings. The small particle size of ZnO nano- particles significantly increases their surface area, enhancing molecular interactions, and contributing to du-rability of the coatings. This characteristic is particularly valuable in pediatric dentistry, where long-lasting restoration materials are essential for sustaining dental health. Despite the inherent advantages of stainless steel crowns, they remain susceptible to bio-film formation and related complications. Therefore, applying ZnO nano-particle coatings to SSCs represents a promising solution to these challenges.
Currently, no literature provide comprehensive insights on the implications of ZnO-coated dental materi-als, specifically for pediatric patients. Understanding these long-term effects is crucial, as ZnO coatings hold potential benefits, such as sustained antimicrobial protection and enhanced durability of restorations, which are particularly advantageous in children with developing oral health. By leveraging the unique properties of ZnO nano-particles, researchers can develop nano- structured coatings for SSCs and zirconia crowns, which enhance the durability of dental restorations and extend crowns’ longevity. This innovative approach marks a sig­nificant step forward in addressing issues related to the durability of pediatric dental materials.
The current comparative study focused on evaluating the surface characteristics of SSCs and zirconia crowns coated with ZnO nano-particles, specifically targeting pediatric applications. A sol-gel method was employed to ensure uniform distribution of nano-particles across all crown surfaces. Atomic force microscopy (AFM) was used to analyze the resulting surfaces and measure roughness, providing insights into the uniformity and potential effectiveness of the coatings in pediatric dentistry.

OBJECTIVES

The aims of the study were to evaluate and compare the surface roughness of ZnO nano-particles-coated stainless steel and zirconia crowns using AFM; to assess the surface uniformity and roughness in both crown types; and to analyze the surface texture variations and assess AFM scans’ peaks and valleys in both crown types for pediatric dental applications.

MATERIAL AND METHODS

This experimental, randomized in vitro study was conducted using a sample size of 20 (n = 10 crowns per group) to evaluate the surface characteristics of ZnO-coated stainless steel and zirconia crowns. The study obtained IHEC approval from the institutional human ethical committee (approval No. IHEC/SDC/FACULTY/22/PEDO/114). It focused on determining the physical and chemical properties of the nano-structured coatings applied to each material, using AFM to measure the surface roughness and topographical features.
ZnO-coated stainless steel crowns were prepared with a nano-structured layer of ZnO, applied through a specialized coating process, i.e., sol-gel method. The coating utilized a ZnO precursor solution, with concen-tration and source specifically chosen for optimal adhesion and surface coverage. A suitable solvent, such as ethanol or water, facilitated the application of coating, ensuring an even distribution. To characterize the surface, AFM was employed for detailed topographical analysis, allowing for precise examination of the surface structure and quality of the ZnO layer.
According to a study by Kachoei et al. [6], the effect size (Cohen’s d) for ZnO coating on stainless steel sur-faces, such as wires or crowns, was 3.94, indicating a very large effect in reducing frictional forces. This high-lighted a significant improvement in surface properties, particularly friction reduction, due to surface coating with ZnO nano-particles on stainless steel orthodontic wires, suggesting potential benefits for stainless steel crowns [6].
Based on this study, a priori sample size analysis was conducted using G*Power software. With an effect size of 1.53, a significance level () of 0.05, and a power of 0.90, it was determined that a sample size of 9 crowns per group would be sufficient to detect statistically significant differences between the two investi-gated groups. The large effect size indicated that a smaller sample size was sufficient to confidently achieve the study’s objectives. A one-tailed t-test was used to compare the means of the two independent groups, ensuring strong statistical power and reliable conclusions.
To prepare ZnO nano-particles coating, a sol-gel solution was created by dissolving zinc acetate in ethanol, followed by gradual addition of deionized water and a few drops of citric acid, which served to enhance the coating process. This mixture was stirred continuously until it formed a homogeneous solution. Stainless steel crowns were then immersed in the prepared ZnO sol-gel for 30 minutes, ensuring comprehensive coverage of their surfaces. After coating application, the crowns were allowed to dry at room temperature for 24 hours. Subsequently, they underwent calcination at a temperature of 500°C for 2 hours, resulting in the formation of the desired ZnO coating. Surface characteristics of the coated stainless steel crowns were thoroughly analyzed using AFM. Scans were conducted over a defined area, typically to assess the surface features and roughness values of coatings accurately, and an image size of 10 µm was captured in a downward scan direction with a time of 1.2 s per line. The measurement was done in air, utilizing a Stat 0.2 LAuD cantilever in static force mode. Z-axis raw data spanned 21.8 µm, with a deflection range of 33.7 mV in raw data, and –2.98 V to 4.28 V in mean fit. The setup included a SH_B scan head, 0.0% laser working point, and a deflection offset, all managed by soft-ware version 3.8.8.13 and controller firmware 3.8.8.13. Feedback was set to free-running mode, with a standard PID algo­rithm and an error range of 20 V, while amplifier con-trol maintained a constant drive. Gain settings included P-Gain at 6000, I-Gain at 5700, D-Gain at 4000, and secon­dary gain values at I-Gain2 of 1000.
An independent t-test was used for data analysis, comparing mean surface characteristics between the two groups: ZnO-coated stainless steel crowns and zirconia crowns. A one-tailed t-test with a significance level of 0.05 was conducted, assuming normality and equal variances across the groups. Based on a power analysis with a large effect size of 1.53 and a power of 90%, a minimum sample size of 9 cases was required. Statistical analysis aimed to determine if there was a significant difference in surface properties between the two crown materials, providing insights into their potentials for pediatric applications.

RESULTS

The mean roughness (Sa) was significantly lower for SSC (0.1485 ± 0.003 μm) compared with zirconia (7.189 ± 0.11861 μm), indicating that zirconia crowns had considerably rougher surfaces. This difference in roughness was consistently observed across other metrics, such as the root mean square roughness (Sq), where SSC showed a mean value of 0.2065 ± 0.003 μm and zirconia of 8.086 ± 0.02921 μm. The heightened surface roughness of zirconia indicated a more textured and variable surface, which may affect plaque accu-mulation and bacterial adhesion in a clinical setting. Further differences were noted in the maximum peak-to-valley height (Sy), where SSC demonstrated a relatively low value of 4.4030 ± 0.00298 μm compared with 21.792 ± 0.034 μm for zirconia. This greater peak-to-valley height on zirconia showed its pronounced surface undulations. Similar trends were observed in the maximum peak height (Sp), with SSC measuring at 2.2948 ± 0.00218 μm and zirconia at 10.8945 ± 0.02634 μm. These elevated peaks and deeper valleys on zirconia suggested a more prominent texture that could influence wear patterns and material longevity when used in crowns (Figures 1 and 2).
The mean spacing of surface features (Sm) in femtometers (fm) further highlighted differences between the materials, with SSC at –29.029 ± 0.00193 fm and zirconia at –5.9899 ± 0 fm. The closer spacing in SSC suggested a more uniform surface, which might provide better integration in pediatric applications by mini-mizing plaque retention. Additional roughness parameters measured at nano-meter scale (nm), included the arithmetic average roughness (Ra) and root mean square roughness along a line (Rq). SSC exhibited values of 149.95 ± 0.03028 nm for Ra and 187.34 ± 0.03028 nm for Rq, both of which were lower than zirconia’s Ra of 299.41 ± 0.08857 nm and Rq of 330.03 ± 0.127 nm. These measurements reinforced zirconia’s compara-tively rough surface profile.
The maximum peak-to-valley height along a line (Ry) and maximum valley depth along a line (Rv) were employed as additional indicators of surface variability. SSC showed a higher peak-to-valley height at 1045.3070 ± 0.030592 nm compared with zirconia’s 909.95 ± 0.20069 nm, while zirconia’s maximum valley depth was less pronounced than SSC’s (–539.57 ± 0.2006 nm for zirconia vs. –601.03 ± 2.538 nm for SSC). The mean roughness depth along a line (Rm) also highlighted a deeper surface texture on SSC, with SSC showing –29.106 ± 0.73711 fm compared with zirconia’s –9.1407 ± 0.01785 fm.
In Table 1, the comparative analysis of surface roughness parameters for ZnO nano-particle-coated stain-less steel and zirconia crowns revealed significant diffe­rences in surface properties, as evidenced by high F- and t-values with p-values less than 0.001 across all para­meters. Zirconia crowns exhibited substantially rougher surfaces with higher peak measurements (i.e., Ra, Rq, Ry, and Rp), while ZnO-coated stainless steel crowns demonstrated a smoother profile. These differences were consistent under both equal and unequal variance assumptions, underscoring the robustness of the results. The findings highlighted the distinct surface charac-teristics of each material, which could influence their clinical performance and suitability in pediatric dentis-try.
The effect size analysis shown in Table 2 revealed significant differences in the surface characteristics be-tween the two coatings, especially in Rv, Ry, and Rm parameters. Higher values of Cohen’s d and Hedges’ g for Rv (1.80) and Rm (0.52) parameters indicated substantial differences, suggesting that the ZnO coating im-pacted surface roughness and texture more effectively than zirconia in certain measures. Glass’s delta further supported these findings, highlighting distinct effects for Ry and Rp. Overall, the results implied that nano-structured ZnO coatings could enhance specific surface properties, potentially improving the clinical performance of crowns in pediatric dentistry.
Figure 3 illustrated a granular surface texture on a ZnO nano-particle-coated stainless steel crown, en-hancing antimicrobial properties by increasing surface area and promoting bacterial cell interactions. The uniform particle distribution ensured effective coverage, minimizing uncoated areas where bacteria might adhere. The 50 µm scale bar provided a close-up view, highlighting particle arrangement and surface roughness. The central crosshairs likely indicated measurement points for particle size or surface depth, offering insights into the coating’s thickness and uni-formity.
Figure 4 showed a uniform distribution of ZnO nano-particles on a broader scale, with a 500 µm scale bar offering a zoomed-out view to assess coating con-sistency across the crown surface. This broader perspective helped identify any clusters or aggregation, essen-tial for consistent antimicrobial action. The central crosshairs showed points for measuring coating thickness or surface texture. The uniform, textured surface enhanced bactericidal effects by promoting ROS generation upon microbial contact, providing a strong antimicrobial barrier on the crown.
Figure 3 demonstrated a rough, textured zirconia surface with micrometer-scale topographic variations, possibly due to a ZnO nano-particle coating. The central cross-hair with colored lines (red and blue) likely marked measurement points. The 50 µm scale bar revealed small surface structures, suggesting that the ZnO nano-particles created a rough surface.
Figure 5 displayed a uniform diagonal stripe across the zirconia crown’s surface, possibly indicating a denser or aligned application of ZnO coating in that area. The central cross-hair with colored lines marked a measurement point, potentially for assessing surface roughness, height, or other characteristics. The micrometer- scale features indicated the uniformity and coverage level of the ZnO coating on the zirconia substrate.
Figure 6 illustrated AFM scans of a ZnO nano- particle-coated stainless steel crown, showing its surface topography and roughness. The Z-axis scan and de-flection raw data highlighted surface texture variations, with color gradients indicating peaks and valleys. The mean fit graphs provided smoothed surface measurements, while the 3D visualization depicted uniform distribution of nano-particles. These analyses assessed the coating’s quality, which influenced various proper-ties, e.g., surface roughness, and potential impact on clinical properties, such as bio-compatibility and dura-bility. Figure 2 showed the surface characteristics of a ZnO nano-particle-coated zirconia crown using AFM. It re-vealed uniform coating distribution, nano-scale height variations, and surface roughness through Z-axis and deflection scans. The 3D view displayed ridges and grooves, indicating textural features, which are important for adhesion and antimicrobial properties. Additionally, the mean fit graphs confirmed coating integrity, showing minimal defects and a stable surface profile appropriate for clinical applications.

DISCUSSION

Nano-technology, involving manipulating materials at the nano-scale (1-100 nm), has revolutionized vari-ous fields, including dentistry. At this scale, materials exhibit unique physical and chemical properties, such as increased surface area, enhanced strength, and greater durability, making them effective in numerous appli-cations. Zinc, a vital trace mineral present in the muscle, bone, skin, and tooth’s hard tissues, plays a significant role in dental innovations. ZnO NPs appear as a white, odorless powder with a molecular weight of 81.38 g/ mol, and according to FDA, they are generally recognized as safe (GRAS), affirming their suitability for medical use [7]. ZnO NPs exhibit unique optical, magnetic, morphological, electrical, catalytic, mechanical, and photo-chemical properties, making them valuable in dentistry. These characteristics can be tailored by adjusting particle size, doping with additional compounds, or modify­ing synthesis methods. Smaller particle sizes tend to enhance these properties, making ZnO NPs especially useful in applications, where durability, mechanical resilience, and antimicrobial effects are needed, such as pediatric dentistry. In this study investigating ZnO NPs coating on stainless steel and zirconia crowns, revealed notable differences in coating morphology. Stain-less steel compared with zirconia, displays a more consistent distribution of nano-particles, likely due to its surface properties, which support an even coating adherence. Such homogeneity in ZnO coatings is crucial for pedia­tric dental crowns, where resistance to wear and maintenance stability under chewing forces can be improved, enhancing crown performance and longevity [8, 9].
ZnO coatings are well-known for their antibacterial properties, which are particularly important in pediat-ric dentistry for preventing caries and gingival inflammation. While this study emphasized the analysis of surface characteristics rather than directly assessing antimicrobial efficacy, the established antibacterial profile of ZnO coatings could enhance the clinical application of both stainless steel and zirconia crowns. No-tably, the uniform coating on stainless steel crown may offer a more effective barrier against bacteria. Future research can quantify this effect through micro-biological testing to further validate the role of ZnO-coated crowns in maintaining oral health [10, 11].
A relevant theory in this context is the “Trojan horse effect”, which posits that the acidic lysosomal envi-ronment facilitates nano-particle degradation, leading to the conversion of core metals into ions. This process releases toxic substances, which disrupt cellular reproduction [12]. Additionally, ZnO exhibits antimicrobial activity by altering the micro-environment surrounding microbes, generating ROS and increasing the solubility of nano-particles. These mechanisms can interact with the –SH groups of microbial enzymes, causing orga-nelle dysfunction and denaturing proteins, which ultimately damage DNA and interfere with the micro-organisms’ replication process [13, 14].
Another possible antimicrobial action involves the release of hydrogen peroxide (H2O2) and displacement of magnesium ions, both of which can disrupt bacterial metabolism. The enhanced antibacterial effect is also attributed to the increased surface-to-volume ratio of nano- particles, facilitating greater interactions with microbial cells [15].
The anti-inflammatory properties of ZnO NPs can be beneficial when applied as a coating on steel crowns in children. ZnO’s ability to reduce inflammation can help minimize gum irritation and inflammation around dental crowns, especially in pediatric patients, who are more sensitive to oral discomfort. Additionally, ZnO NPs can offer antimicrobial effects, further enhancing the therapeutic properties by preventing bacterial growth around the crown. This makes ZnO-coated steel crowns an excellent option for improving oral health, reducing inflammation, and promoting healing in children [16]. Similarly, stainless steel wires and orthodontic brackets coated with chitosan NPs or ZnO NPs, reduce the friction between orthodontic brackets and stainless steel wire, thus enhancing the anchorage control and root resorption risk [17]. The ZnO nano-particles-coated orthodontic appliances minimize bacterial adhesion and enamel de-mineralization due to its antimicrobial and re-mineralization potential [18]. Tiwari et al. [20] demonstrated that ZnO NPs in dental composites pre-vent bacterial growth and bio-film formation, enhancing the durability of dental materials. Rajasekar et al. [21] found that β-chitosan-derived ZnO NPs effectively target cariogenic bacteria, offering a promising strategy for preventing and treating dental caries. These findings highlight the potential of ZnO NPs to improve oral health by reducing infections and improving the performance of dental applications. The findings highlight the potential of β-Ch-ZnO-NPs as a promising therapeutic strategy for the prevention and treatment of dental caries by targeting microbial metabolism and enhancing oral health. ZnO NPs have the potential to be widely used in dental applications to improve treatment outcomes, including increased strength of materials and reduced bacterial count around dental appliances [19].
Surface roughness measurements revealed that ZnO-coated stainless steel has a lower roughness profile compared with zirconia, indicating that stainless steel may provide a smoother and more uniform ZnO coat-ing. This smooth surface can be beneficial in reducing bacterial adhesion, improving esthetic appeal, and comfort of crowns. Additionally, the enhanced proliferation of osteoblast cells observed on ZnO-coated sur-faces after 5 and 10 days, along with the antimicrobial properties against S. aureus, suggest that ZnO coatings can promote oral health by mitigating infection risks. This is particularly relevant for stainless steel crowns, as the smoother coating can lead to better bio-compatibility, supporting the long-term stability and effective-ness of pediatric crowns, while also addressing the challenges posed by children’s high masticatory activities.
Studies by Rajasekar et al. [21] and Kulshrestha et al. [22] further confirm the potential of ZnO nano-particles to inhibit S. mutans bio-film formation on dental implant surfaces, reinforcing the value of such coatings in enhancing the durability and safety of stainless steel crowns in pediatric dentistry.
The findings of a study on ZnO-coated stainless steel crowns, particularly the continuous and uniform ZnO layer’s contribution to bacterial resistance, supported the relevance of ZnO nano-particles in bio-medical ap-plications, such as dentistry [23]. The cyto-toxicity testing on human gingival fibroblast cells further reinforced ZnO’s suitability for oral applications. Despite exhibiting some cyto-toxic effects at higher concentrations, ZnO nano-particles showed controlled bio-compatibility and safety at doses viable in clinical use. This dose-dependent cyto-toxicity, combined with stainless steel’s ability to maintain a uniform ZnO layer, aligned with the goal of creating durable, antimicrobial crowns, which reduce bacterial risks around crown margins. Therefore, ZnO’s bio-compatibility and antimicrobial properties, validated in this cyto-toxicity study, support its potential for safe and effective pediatric dental crown applications [24].
A research investigating ZnO nano-particles (ZnO-NPs) conjugated with clinically-approved antibiotics, highlighted ZnO’s role in amplifying drug efficacy against resistant bacterial strains, presenting a valuable approach amid global concerns about antibiotic resistance. These findings are particularly important for den-tistry, where ZnO-NPs’ strong antimicrobial properties can benefit dental materials by reducing bacterial ad-hesion and preventing bio-film formation around the crowns. The demonstrated effectiveness of ZnO-NPs in combating multidrug-resistant bacteria, aligned with the potential for ZnO-coated stainless steel and zirconia crowns to provide enhanced clinical outcomes in dentistry, especially for pediatric patients. This research emphasizes the importance of ZnO-NPs in creating more dura-ble, antimicrobial dental materials, indicating the need for inter-disciplinary collaboration to bridge research advances with clinical applications in nano-dentistry [25].
ZnO coatings on stainless steel and zirconia crowns can optimize their performance based on patient needs. Reducing surface roughness in SSCs minimizes bacte­rial adhesion sites, which is crucial in pediatric dentistry for preventing plaque accumulation and bio-film formation. This smoother surface enhances oral hygiene maintenance, lowers the risk of recurrent caries and gum irritation, and supports long-term dental health of young patients. The smoother ZnO-coated stainless steel reduces plaque buildup and improves es-thetics, making it a suitable option for younger children where hygiene and appearance are the priorities. In contrast, ZnO- coated zirconia, with its added durability, better suits cases requiring high wear resistance, offering extended lifespan and reduced risk of secondary decay. This allows clinicians to choose crowns tailored to each child’s needs, balancing esthetics, hygiene, and durability [26].
This study offers valuable findings on the surface characteristics of ZnO-coated stainless steel and zirconia crowns, yet it has limitations. One primary drawback is the lack of in vivo testing, which would reveal how these materials interact with oral environment and perform under actual physiological conditions. Moreover, expanding future research to evaluate ZnO coating stability over time, should include in vitro aging tests, wear resistance testing, and surface roughness analysis to simulate oral conditions and monitor structural integri-ty. Assessing antimicrobial efficacy, chemical composition, and cyto-toxicity helps ensure sustained bio-compatibility and safety. Additional tests, such as microbial adhesion assessment, bond strength testing, and environmental scanning electron microscopy (ESEM), can provide insights into physical changes, while ion release monitoring detects potential leaching of Zn2+, collectively giving a comprehensive view on coating durability. Future studies should focus on the cyto-toxicity of ZnO nano-particles, particularly their potential irritant effects on oral tissues at higher concentrations, emphasizing the need for strict control in pediatric applications. Moreover, potential research should investigate the long-term release and accumulation of Zn2+ ions from these nano-particles, assessing the safety of low levels while understanding the risks associated with excessive ion release, especially in young patients with developing immune systems. Additional investigations can explore and compare the properties of different types of nano-particles, including their efficacy, cy-to-toxicity, and overall safety profiles, particularly in randomized controlled trials. Such studies would provide deeper insights into the comparative benefits of ZnO coatings and other nano-particles in pediatric dentistry, offering clinicians comprehensive, evidence-based data for selecting the most effective and safe materials for young patients [27].

CONCLUSIONS

This study emphasizes that ZnO-NPs coatings on stainless steel crown and zirconia crowns can enhance their application in pediatric dentistry by improving surface properties and surface characteristics. ZnO-coated SSCs exhibited significantly smoother surfaces than zirconia, suggestive of reducing plaque accumulation and bio-film formation risks, essential for long-term oral health in children. These findings suggest that ZnO coat-ings on SSCs result in changes to surface properties, which could provide an advanced solution for pediatric restorative care. Future research should focus on in vivo studies to further assess the long-term clinical effi-cacy of these coatings in pediatric populations. Also, further studies should assess antimicrobial efficacy against common pediatric oral pathogens, and to understand the coatings stability, bio-compatibility, dura-bility, and potential cumulative effects of ZnO over extended periods to determine the full scope of its efficacy and safety in pediatric dentistry.

DISCLOSURES

1. Institutional review board statement: This study was approved by the Ethics Committee of the Saveetha Dental College & Hospital, SIMATS University, Chennai, Tamil Nadu, India (approval number: IHEC/SDC/FACULTY/22/PEDO/114).
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: None.

References