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Article

Influence of Quercetin and tt-Farnesol Enrichment on Physicochemical Properties of a Universal Adhesive System

by
Roberta Janaína Soares Mendes
1,
Regina Maria Puppin-Rontani
2 and
Aline Rogéria Freire de Castilho
3,*
1
Programa de Pós-Graduação em Odontologia, e Universidade Federal do Maranhão, São Luis 65080-805, Maranhão, Brazil
2
Departmento de Ciências da Saúde e Odontologia Infantil, Faculdade de Odontologia de Piracicaba, Universidade Estadual de Campinas, Piracicaba 13418-903, São Paulo, Brazil
3
Department of Pediatric Dentistry, Indiana University School of Dentistry, Indianapolis, IN 46202, USA
*
Author to whom correspondence should be addressed.
Adhesives 2025, 1(1), 3; https://doi.org/10.3390/adhesives1010003
Submission received: 5 September 2024 / Revised: 19 December 2024 / Accepted: 26 December 2024 / Published: 6 January 2025
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Abstract

:
This study investigated the impact of natural compound enrichment, specifically quercetin and trans, trans-farnesol (tt-farnesol), on the physicochemical properties of a universal adhesive system. A preliminary DPPH assay was conducted to determine the optimal concentrations of quercetin (0.24 mg/mL) and tt-farnesol (1.43 mg/mL) based on their radical scavenging abilities. These compounds were then incorporated into the adhesive system. Specimens (n = 5; 7 mm × 1 mm) of the adhesive system, both with and without the added compounds, were prepared and tested for water sorption, solubility, Knoop hardness, and softening percentage. Water sorption and solubility were measured after immersion in deionized water for 7 days, and Knoop hardness was measured before and after immersion in 75% ethanol. Softening percentage was calculated based on changes in hardness. Data on water sorption, solubility, and percentage of softening were submitted to the Student’s t-test (α = 5%) while Knoop hardness values were submitted to the Mann–Whitney test (α = 5%). Both quercetin and tt-farnesol exhibited important antioxidant activity (85.5% and 82%, respectively). Water sorption was similar for both groups (p > 0.05) but the experimental adhesive had a significantly higher solubility, lower hardness, and higher softening. The incorporation of quercetin and tt-farnesol into a universal adhesive system detrimentally affects its essential physicochemical properties, compromising its performance.

1. Introduction

Monomers play a pivotal role in forming a reticulated matrix in dentin bonding systems, ensuring chemical and physical stability at the resin–dentin interface [1,2]. This stability is critical for the longevity and resistance of restorations, as a stable interface can effectively inhibit bacterial penetration and subsequent secondary caries [2]. Establishing a lasting bond at the resin–dentin interface is critical to the long-term success of resin-based restorations, particularly in demanding oral conditions.
Despite advancements in adhesive dentistry, maintaining resin–dentin bonding durability remains a significant challenge. The intrinsic heterogeneity of dentin, coupled with its susceptibility to hydrolytic and enzymatic degradation, compromises the long-term stability of the hybrid layer [2,3]. Factors such as water absorption, enzymatic activity, and bacterial biofilm formation further exacerbate this degradation [4,5]. Current strategies to enhance bonding durability have focused on mitigating these factors. In this context, crosslinking agents that strengthen collagen fibrils and MMP inhibitors that suppress enzymatic degradation have demonstrated efficacy in preserving hybrid layer integrity and improving the bond’s longevity [3,4,5,6].
Given these challenges, there is growing interest in incorporating natural compounds with pharmacological properties into adhesive systems. Natural compounds such as quercetin and trans,trans-farnesol (tt-farnesol) have emerged as promising candidates due to their multifunctional properties [7,8,9,10]. These compounds not only provide crosslinking capabilities but also exhibit additional benefits such as antimicrobial activity and enzymatic modulation [7,8,9,10].
Quercetin (C15H10O7), a flavonoid commonly found in propolis and other natural sources, has shown potential as a crosslinking agent for dentin collagen [7,8]. It effectively modulates the activity of matrix metalloproteinases, particularly MMP-2 and MMP-9, which are known to degrade the collagen matrix at the resin–dentin interface [3,8]. Quercetin’s ability to form crosslinking bonds with collagen fibrils reduces water channel formation, thereby enhancing collagenase resistance and ensuring the stability of the hybrid layer [3]. Furthermore, its hydrophobic nature facilitates better adhesive penetration into moist dentin while minimizing the detrimental effects of water sorption, which is a common cause of bond degradation [3,4].
Similarly, trans,trans-farnesol (tt-farnesol; C15H26O), another compound extracted from propolis, exhibits a wide range of pharmacological properties. In addition to its antimicrobial activity, tt-farnesol has been shown to disrupt biofilm formation, which is critical in preventing secondary caries [10]. Its combination with quercetin is hypothesized to synergistically enhance the resin–dentin interface by fortifying it against biochemical and structural degradation, particularly in high-risk areas such as marginal interfaces prone to biofilm accumulation.
Considering the critical role of bonding in the success of resin-based composite restorations, developing innovative strategies to mitigate collagen degradation at the resin–dentin interface is imperative. The exploration of natural compounds with inherent pharmacological properties presents a promising avenue for enhancing adhesive systems. These compounds could not only address the limitations of conventional materials but also provide safer and more effective alternatives for improving resin–dentin bonding longevity.
This study aims to evaluate a self-etching adhesive system doped with quercetin and tt-farnesol, focusing on its physicochemical properties, including water sorption, solubility, microhardness, and softening percentage. It is hypothesized that doping the adhesive system with these natural compounds will preserve its initial physicochemical properties, providing a durable and resilient resin–dentin bond.

2. Materials and Methods

For the physicochemical characterization, the natural compounds quercetin and tt-farnesol, alongside a universal adhesive system (EG; Single Bond Universal, 3M ESPE, Sumaré, SP, Brazil), were employed. The adhesive system without compounds was used as a control (CG). Details of the compounds and chemical composition of the adhesive system are shown in Table 1 while Figure 1 outlines the design of the study.

2.1. DPPH Radical Scavenging Assay

To determine the compound concentrations to be incorporated into the adhesive system in a preliminary study, the DPPH (2,2-diphenyl-1-picrilhydrazil; Sigma-Aldrich, St. Louis, MO, USA) assay was performed. DPPH is a simple and fast assay that turns purple color into yellow when free radicals are neutralized by the antioxidants [11]. The antioxidant activity of the compounds quercetin and tt-farnesol was evaluated by capturing the DPPH radical. First, an ethanolic solution of DPPH at 0.6 mM was prepared. Then, stock solutions of the quercetin and tt-farnesol at 5 mg/mL were prepared by dissolving the compounds in 20% DMSO (dimethyl sulfoxide; Sigma-Aldrich) [12]. Aliquots of 100 μL of DPPH were placed in a microplate of 96 wells, in triplicate. Wells with DPPH and ascorbic acid (5 mg/mL; Sigma-Aldrich) and 20% DMSO were used as control.
The compound concentration ranged from 5 mg/mL to 0.24 mg/mL for each antioxidant assay. The absorbance was measured at 517 nm, microplate reader ELISA (Eon™, Bio-Tek Instruments, Wibrijk, Belgium) after 30 min of the addition of DPPH. The ability to reduce the DPPH radical was calculated by means of mean and standard deviation, to calculate the concentration capable of inhibiting 50% of the DPPH. The evaluation of the percentage of antioxidant activity was made using the following equation [13]:
% Antioxidant Activity = [(Ac − Aa]/Ac] × 100

2.2. Specimen Preparation

The DPPH assay determined the concentrations of quercetin (0.24 mg/mL) and tt-farnesol (1.43 mg/mL) to be incorporated into the adhesive system based on their antioxidant activity.
Briefly, five discs (7 mm × 1 mm) of the adhesive system containing (EG) or not (CG) quercetin and tt-farnesol at concentrations of 0.24 mg/mL and 1.43 mg/mL, respectively, were prepared for each group/assay using an elastomer mold (Silicone Printing Material by Condensation, Profile, Vigodent-Coltene, Rio de Janeiro, RJ, Brazil) [14]. The compounds were manually added to the adhesive system, mechanically shaken for 20 s, gently air-dried for 5 s for evaporation of the solvent, and then light-cured for 10 s using a Bluephase LED curing unit (Ivoclar Vivadent, Schaan, Liechtenstein) with a light output of 900 mW/cm2 according to the manufacturer. Power density of the light curing unit was monitored with a radiometer (Hilux Dental Curing Light Meter, Benlioglu Dental Inc., Demetron, Ankara, Turkey) before curing the specimens. For the specimen preparation, approximately 3 drops (90 μL) of the adhesive system were inserted in the elastomeric matrix with a piece of dental floss embedded in the material to handle the discs in the water.

2.3. Water Sorption and Solubility

Immediately after preparation, the specimens (n = 5) were stored individually in an organizer with a partition and kept in a desiccator with silica gel at 37 °C for 22 h. After that, the silica was changed, and specimens remained in the desiccator for another 2 h at 37 °C. After 24 h in the desiccator, each specimen was weighed in an analytical balance (BEL Engineering®, Piracicaba, São Paulo, Brazil) with an accuracy of 0.001 g. This weighing cycle was maintained and repeated until obtaining a constant mass (m1) of each disc referring to each group, by means of a variation ≤ 0.001 in the 24 h interval. The mass m1 was obtained after 13 days of weighing.
After obtaining the m1 mass, the diameter of specimens was measured, two measurements of diameter to remove the average later, and the thickness of the test body using a digital caliper (Mitutoyo, Tokyo, Japan) with an accuracy of 0.01 mm. From these values, the volume of the cylinder in mm3 was calculated. After checking the volume of each disc, they were immersed individually in an organizer with a partition and in each breakdown containing 4.66 mL of deionized water at 37 °C for 7 days, the decision of the final volume of water was calculated [14]. After 7 days, the water discs were removed and dried on absorbent paper for one minute then each specimen was weighed to obtain the m2. Specimens were placed again in a desiccator containing silica in gel, and the procedures described to obtain the m1 mass were repeated to obtain a constant mass m3. The constant m3 mass of specimens was taken after 10 days of weighing. The values in mg/mm3 of water sorption (Wsp) and solubility (Wsl) were calculated based on the following Equations (1) and (2).
Wsp = (m2 − m3)/V
Wsl = (m1 − m3)/V

2.4. Hardness and Softening Percentage

To evaluate softening in solvent, indirectly, the specimens (n = 5) of each control (CG) and experimental group (EG) were submitted to the Knoop microhardness evaluation test after 24 h (KHN1) of preparation of the specimens and after 7 days of these specimens being immersed in 75% ethanol [15] to evaluate the percentage of softening (KHN2). The specimens used were prepared as described before and were stored in the dark at 37 °C for 24 h. After storage time, the surface hardness was measured using a Future Tech FM-100 microhardness tester (Shimadzu, Tokyo, Japan) at a load of 25 gf for 10 s. Knoop hardness values were recorded. Then, the specimen was stored in 4 mL of 75% ethanol solution, protected from light, at 37 °C for 7 days. Five readings were taken for each specimen and the percentage of softening (% Softening) was calculated as follows: Equation (3).
% Softening = 100 − [(KHN2 × 100)/KHN1].

2.5. Statistical Analysis

The statistical data were analyzed for normality using the Shapiro–Wilk, Kolmogorov–Smirnov, and Anderson–Darling tests. The Shapiro–Wilk test is commonly employed to evaluate normality in small to moderate sample sizes and tests the null hypothesis that the data follow a normal distribution. The Kolmogorov–Smirnov test compares the sample distribution to a reference distribution, while the Anderson–Darling test gives more weight to the tails of the distribution and is more sensitive to deviations from normality.
To assess homogeneity of variance, Levene’s test and variance ratio tests were performed. Levene’s test checks the assumption of equal variances across groups by comparing the variances of different groups and is more robust to departures from normality than other tests. The variance ratio test evaluates the ratio of variances between two groups to assess their relative variability.
The Student’s t-test was used to evaluate sorption, solubility, and to compare the degree of softening of the specimens after immersion in 75% ethanol for 7 days. The Student’s t-test compares the means of two independent groups to determine if there is a statistically significant difference between them, assuming the data are normally distributed and the variances are equal.
For hardness analysis, which did not meet the assumptions of normality, the non-parametric Mann–Whitney U test was used. This test compares the ranks of values between two independent groups and is appropriate when data are not normally distributed.
All statistical analyses were performed using Jamovi 2.2.5 (Patreon, San Francisco, CA, USA). A significance level of α = 0.05 was used to determine statistical significance.

3. Results

The analysis of the data on the antioxidant activity was carried out descriptively. The DPPH assay revealed noteworthy antioxidant activity for both quercetin and tt-farnesol, prompting their incorporation into the adhesive system at minimal concentrations (Figure 2). The highest value of antioxidant activity was observed for quercetin followed by tt-farnesol. Quercetin presented an average of 85.8% of free radical DPPH absorption. The compounds quercetin and tt-farnesol showed a percentage of DPPH radical capture greater than 50% at different concentrations (Table 2). Based on the DPPH radical scavenging activity, the quercetin and the tt-farnesol showed strong scavenging activity and were selected to be included in the adhesive system at their lowest concentrations of 0.24 mg/mL and 1.43 mg/mL, respectively.
Subsequent evaluation demonstrated similar water sorption between groups but significantly higher solubility in the experimental group (p < 0.05). Water sorption and solubility results are summarized in Table 3.
As shown in Figure 2, the experimental group displayed lower hardness and increased softening compared to the control.

4. Discussion

Efforts to augment adhesive restorations face challenges [2], and the exploration of natural products stands out as a promising avenue [5,7,10]. Several attempts are being made to improve the antimicrobial activity and bonding of the adhesive systems, but their physicochemical properties could be compromised [16,17]. The study’s adhesive system, labeled “Universal,” represents an advanced generation in dental adhesive systems, ensuring a stable interface between dental substrates and composite resin [1]. Incorporating quercetin and tt-farnesol aimed to enhance bonding by preventing hydrolytic degradation [18,19]; however, the hypothesis was not supported. The compounds adversely impacted the adhesive system’s physicochemical properties, particularly solubility and softening percentage, possibly due to delayed curing during specimen preparation as the experimental adhesive system required more photoactivation time (40 s in total) than recommended by the manufacturer which means that the longer the curing, the worse the permeability of the bonded interfaces [20]. Although it was not a concentration-dependent pattern, the delayed curing was also reported by Leyva Del Rio et al. (2020) [10], which added tt-farnesol to a universal adhesive system to evaluate its degree of conversion. Interestingly, studies indicate that high concentrations of quercetin might impede the adhesive system’s strength and polymerization, a concern echoed in our findings [17,21]. Moreover, the adhesive system’s composition, rich in solvents and hydrophilic monomers [22], might have affected polymerization reactions, influencing crosslinking density and thus material durability [15]. The presence of DMSO as a solvent could influence wettability and compound solubility but may also compromise physicochemical properties, indicating a delicate balance in solvent concentrations for optimal performance [22,23].
The immersion of adhesive systems in 100% ethanol, commonly used to assess softening percentage, leads to severe degradation due to the abundance of hydrophilic molecules and the scarcity of inorganic particles in their composition [22]. Additionally, resin composites and adhesive systems generally experience reduced hardness following immersion in 100% ethanol [15]. In this study, immersion in 100% ethanol did not allow the assessment of crosslinking density or hardness due to the loss of specimens post-degradation. The use of 75% ethanol, on the other hand, should trigger the release of uncured monomers, resulting in the dissolution of linear polymers [22]. The incorporation of compounds into the adhesive system likely decreased the material’s degree of cure, leading to softer chain polymers and higher solubility values significantly impacting the chemical degradation during the reticulation reaction [24]. Clinically, this could compromise bonding longevity. Moreover, the substances’ molecular weight probably influenced crosslinking density, as higher molecular weights tend to correlate with lower crosslink densities.
Contemporary adhesive solvents like water, acetone, and ethanol are directly related to their evaporation rates, replacing water within the adhesive system, and promoting solvent binding [22]. Dimethyl sulfoxide (DMSO), classified as a class 3 solvent, exhibits amphiphilic characteristics and lower human health risks [25]. DMSO’s selection as a solvent for quercetin and tt-farnesol was due to its role in dental adhesive systems, increasing spaces between fibrils, potentially enhancing monomer penetration as collagen fibrils require hydration, and DMSO is water-miscible [26]. This feature suggests DMSO’s interaction with water, likely improving wettability properties [25] and facilitating better compound solubility in the universal adhesive system due to its amphiphilic nature. Gotti et al. noted that adhesive systems incorporating pure quercetin without a solvent appeared less homogeneous, potentially affecting the antioxidant’s solubility within the adhesive system [17].
Solvents dissolve monomers, aiding their penetration into the tooth structure, facilitating infiltration into the exposed collagen network with adhesive monomers [22]. Studies suggest that high concentrations of DMSO pretreatment could improve immediate and long-term bond strength [27,28]. However, the quantity of DMSO may significantly impact adhesive system physicochemical properties [22]. Salim Al-Ani et al. (2019) [27] demonstrated that high DMSO content could adversely affect both chemical and mechanical properties, regardless of monomer composition. Incorporating 5% or more of DMSO increased monomer conversion slightly, albeit an increased degree of conversion does not always imply improved polymer quality; higher crosslinking often enhances resistance to hydrolytic degradation compared to lower crosslinking densities [25,29].
The preliminary findings suggest that incorporating natural compounds like quercetin and tt-farnesol into a universal adhesive system could detrimentally impact material durability. As water sorption and solubility significantly influence restoration performance, further preclinical studies are necessary to gain deeper insights into natural compound-based adhesive systems. The primary limitation of this study was the degradation of the adhesive system following the incorporation of tt-farnesol and quercetin, which impeded the assessment of crucial physicochemical properties, including the bonding of adhesive systems to dentin through the microtensile bond strength test. This study tested only one concentration of each compound, underscoring the need for further exploration of tt-farnesol and quercetin associations to enhance adhesive restoration durability. Comprehensive understanding necessitates analyzing the degree of conversion and short- and long-term compound release from adhesive systems to comprehend the behavior and longevity of resin–dentin interfaces in natural products-based adhesive systems.

5. Conclusions

The incorporation of quercetin and tt-farnesol into a universal adhesive system compromised essential physicochemical properties, including solubility and hardness. Further optimization is required to ensure the longevity and performance of adhesive systems enriched with natural compounds.

Author Contributions

R.M.P.-R. and A.R.F.d.C. were responsible for the conceptualization of the study, methodology, writing—review and editing, supervision, and project administration. A.R.F.d.C. was responsible for funding acquisition. R.J.S.M. undertook the literature review, formal analysis, investigation, data curation, and drafting of the text. All authors have read and agreed to the published version of the manuscript.

Funding

This study was partially funded by the Coordination for the Improvement of Higher Education Personnel—Brazil (CAPES), process no. 88887.514653/2020-00, Financial Code 001 and the Foundation for Support to Research and Scientific and Technological Development of Maranhão (FAPEMA), process no. 149227/2021.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the manuscript, further inquiries can be directed to the corresponding author.

Acknowledgments

We acknowledge the financial support provided by CAPES and FAPEMA, which made this study possible.

Conflicts of Interest

The authors state that there are no conflicts of interest to declare.

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Adhesives 01 00003 g001
Figure 1. Flowchart of the experimental design used in this study. (A)—Preliminary study: Natural compounds that were submitted to the DPPH assay; (B)—Physicochemical characterization: Groups and assays. Source: Authors.
Figure 1. Flowchart of the experimental design used in this study. (A)—Preliminary study: Natural compounds that were submitted to the DPPH assay; (B)—Physicochemical characterization: Groups and assays. Source: Authors.
Adhesives 01 00003 g001
Adhesives 01 00003 g002
Figure 2. Hardness and softening percentage of control (CG) and experimental (EG) groups.
Figure 2. Hardness and softening percentage of control (CG) and experimental (EG) groups.
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Table 1. Name, composition, CAS registry number, molecular weight, brand, and batch of compounds and adhesive system used in the study.
Table 1. Name, composition, CAS registry number, molecular weight, brand, and batch of compounds and adhesive system used in the study.
Product NameCompositionCAS NumberMolecular Weight Molecular FormulaBrandBatch
Quercetin *-849061-97-8302.24C15H10O7Sigma-
Aldrich		LRAB7760
Trans,trans-farnesol *-106-28-5222.37C15H26OSigma-
Aldrich		BCCC6806
Single Bond Universal **2-hydroxyethyl methacrylate, bisphenol A dimethacrylate dimethacrylate dimethyl (BisGMA), 2-propenoic acid, 2-methyl-, reaction products with 1,10- decanediol and phosphorus oxide (P2O5), ethanol, water, silica treated from silane, acrylic copolymer and itaconic acid, caphorquinone, dimethylaminobenzoat (-4), 2-dimethylaminoethyl methacrylate, 2,6-di-terc-butyl-p-cresol---3M™ ESPE4126641269 4127841279 41282
* Sigma Aldrich (www.sigma-aldrich.com). ** Material Safety Data Sheet information.
Table 2. Analysis of DPPH radical absorption percentage according to the concentration of each compound that presented antioxidant activity.
Table 2. Analysis of DPPH radical absorption percentage according to the concentration of each compound that presented antioxidant activity.
CompoundsConcentration% of DPPH Absorption
MeanQuercetin0.98885.8
tt-farnesol0.98836.3
MedianQuercetin0.38888.0
tt-farnesol0.38820.6
Standard deviationQuercetin1.043.51
tt-farnesol1.0444.3
MinimumQuercetin0.24480.2
tt-farnesol0.244−5.30
MaximumQuercetin2.6288.3
tt-farnesol2.6285.2
25th percentileQuercetin0.25884.7
tt-farnesol0.258−0.920
50th percentileQuercetin0.38888.0
tt-farnesol0.38820.6
75th percentileQuercetin1.4388.1
tt-farnesol1.4382.0
Table 3. Results of the evaluation of water sorption and solubility (μg/mm3) of the control (CG) and experimental (EG) groups.
Table 3. Results of the evaluation of water sorption and solubility (μg/mm3) of the control (CG) and experimental (EG) groups.
GroupMean (SD)
SorptionCG132.3 (18.04) a
EG173 (12.7) a
SolubilityCG34.4 (9.02) b
EG163 (11.8) a
CG—Control group. EG—Experimental group. Statistical differences between groups of materials are expressed by different superscript letters in columns according to Shapiro–Wilk, Levene’s test, and Student’s t-test.
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MDPI and ACS Style

Mendes, R.J.S.; Puppin-Rontani, R.M.; de Castilho, A.R.F. Influence of Quercetin and tt-Farnesol Enrichment on Physicochemical Properties of a Universal Adhesive System. Adhesives 2025, 1, 3. https://doi.org/10.3390/adhesives1010003

AMA Style

Mendes RJS, Puppin-Rontani RM, de Castilho ARF. Influence of Quercetin and tt-Farnesol Enrichment on Physicochemical Properties of a Universal Adhesive System. Adhesives. 2025; 1(1):3. https://doi.org/10.3390/adhesives1010003

Chicago/Turabian Style

Mendes, Roberta Janaína Soares, Regina Maria Puppin-Rontani, and Aline Rogéria Freire de Castilho. 2025. "Influence of Quercetin and tt-Farnesol Enrichment on Physicochemical Properties of a Universal Adhesive System" Adhesives 1, no. 1: 3. https://doi.org/10.3390/adhesives1010003

APA Style

Mendes, R. J. S., Puppin-Rontani, R. M., & de Castilho, A. R. F. (2025). Influence of Quercetin and tt-Farnesol Enrichment on Physicochemical Properties of a Universal Adhesive System. Adhesives, 1(1), 3. https://doi.org/10.3390/adhesives1010003

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