Through complete adhesive impregnation of the dentin substrates, long lasting bonds are achieved in dental restorative procedures. The development of a homogenous hybrid layer is what maintains the stability of the bonded interface35. When a tooth is prepared with a dental bur, it forms a smear layer of debris in the dentin. This layer can then be treated or removed using an adhesive system, resulting in a hybrid layer made of collagen fibers, resin bonds, a dentin surface structure, and an intertubular structure. An adhesive layer develops over the hybrid layer, and resin tags are inserted into the tubules underneath the hybrid layer36.
A new hybrid material combining gold nanoparticles and polymers was created using photo-induced gold reduction along with current dental bonding agents. These metallic nanoparticles acted as contrast agents, enabling the clear characterization of adhesive and hybrid layers that were previously difficult to distinguish in samples lacking nanoparticles. There is potential to explore additional contrast agents to better identify dental bonding materials beneath layers of restorative materials in the future13.
Gold nanoparticles offer versatility because they can be tailored with specific functionalities on their surface. In biomedical research, gold nanoparticles have shown promise in reducing collagen pore size and inhibiting collagenase biodegradation by creating multiple cross-links within the collagen structure. Several studies have demonstrated the effectiveness of crosslinking type I collagen and gold nanoparticles in biomedical applications37. Type I collagen plays a vital role as a primary component and structural foundation in the organic matrix of dentin. Therefore, finding ways to decrease or avoid the breakdown of the network of collagen is crucial for improving the stability of bonds in dental contexts38.
Our study findings indicated that when a (diode, Er,Cr: YSGG) laser beam is applied after the bonding system, as observed in groups A0 and D0, there is increase in the penetration depth. This increase is attributed to the hot stream generated by the laser system during irradiation. The gold nanoparticles absorb some of the laser light, converting it into thermal energy, which is then transferred to the surrounding bonding material due to the photothermal properties of the AuNPs, as noted by Mazen et al39. Furthermore, Franke et al40 explained how heat plays a role in enhancing the penetration depth of adhesive systems, consequently improving the bond strength. This localized heat can aid in the transformation of the adhesive, which is consistent with our study's findings.
Additionally, Maenosono et al28 reported that post-bonding laser irradiation increases the penetration depth and bond strength by inducing heat within the adhesive, creating a reinforced substrate that improves the dentin-adhesive bond strength. The absorption of heat by gold nanoparticles, along with the low viscosity of the primer, contributes to enhanced primer penetration depth, thereby augmenting the penetration of the bonding agent44.
In contrast, our study showed that when a (diode / Er,Cr: YSGG) laser beam was applied before the bonding agent, the penetration depth and bonding strength decrease. These findings are consistent with previous studies conducted by Ramos, Firat and Gurgan20,16,43. Furthermore, Souza17 suggested that the high heat generated by laser caused the solvent of adhesive systems to evaporate prematurely, hindering penetration into dentin tubules. Similarly, Chen44and Ramos20 noted that laser systems operating at different power levels could create granular layers on the dentin surface, disrupting the bonding process and resulting in reduced penetration depth.
The hardness, which measures a material's resistance to indentation, is directly linked to its vulnerability to deformation and fracture. Understanding micromechanical properties such as the microhardness of irradiated human teeth is essential for improving clinical approaches for treating tooth fractures and predicting susceptibility to fracture45.
The average hardness determined in our study falls within previously reported values35, typically ranging from 50 to 57. Notably, both the diode and Er,Cr:YSGG laser irradiation increased the hardness compared to that of the control group, with the degree of increase depending on the laser's power output. This increase in hardness could be attributed to changes in the mineral content and composition of irradiated dental hard tissues46, which is supported by the established correlation between the mechanical properties and the mineral content of the tooth structure. Additionally, the irradiated dentin exhibited heightened hardness, likely due to processes such as melting, charring, and recrystallization, despite some incident energy being dispersed or reflected off the targeted dentinal surface47.
Previous research has explored the impact of lasers on surface roughness48. However, our research aimed to compare the impact of two types of lasers on surface roughness. The results revealed significant disparities in surface roughness among the tested groups, with notably greater values observed in group C1 than in group A1. In our investigation, the surface roughness of the control group ranged from (0.09 ± 0.03), which differs from findings reported in other studies (e.g., 1.45 ± 0.03). Following Er,Cr:YSGG laser treatment, there was a reduction in surface roughness, consistent with the observations of Hossain et al. (2002), who noted an increase in surface roughness after Er,Cr:YSGG irradiation49.
The application of a diode laser led to an increase in surface roughness in group C1 (0.26 ± 0.03). This outcome may be attributed to the accumulation of heat at the surface due to the low absorption of infrared radiation. This thermal effect could result in the formation of microspaces as a consequence of the loss of carbonate, water, and organic matrix from the surface irradiated by laser50,51.
The energy density of laser is calculated by multiplying the power density by the total duration of laser irradiation52. Therefore, increasing the power of diode laser could increase the energy density. Consequently, the heightened energy density could lead to a greater thermal load from diode laser irradiation. This increased energy density might explain the notable increase in surface roughness observed after diode laser irradiation relative to that of the control group.
In our study, we discovered that the Er,Cr:YSGG laser produced less surface roughness than did the diode laser. However, it is important to note that differences in surface roughness between laser systems may differ among various studies due to factors such as laser device properties, energy density, and evaluation techniques53.
Ana et al.54 reported that using Er,Cr:YSGG laser on root surfaces led to cleaner canals due to their photo-vaporization property, which effectively breaks down dental tissues 55,56. Similarly, Mohamed et al.57 supported our findings by noting that Er,Cr:YSGG laser caused less roughening than other tested lasers, making them preferable for dental treatment.