ABSTRACT
A novel, simple capsule phase microextraction combined with high‐pressure liquid chromatography coupled with ultraviolet detection analytical method was developed for the simultaneous determination of organic monomers that can be released from dental resins [bisphenol‐A (BPA), bisphenol A glycerolatedimethacrylate (Bis‐GMA), triethylene glycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA)] in cola drinks. The critical parameters that affect the extraction, such as the selection of the capsule type and size, extraction time, type, and volume of elution solvent, stirring rate during the extraction, and desorption time, were optimized. The method was subsequently validated in terms of selectivity, linearity, accuracy, precision, and robustness and showed satisfactory results. The limits of detection and quantitation were equal to 0.06 and 0.2 ng/µL, respectively, and the recoveries ranged between 93.3% and 106.9%. The method was employed in the determination of monomers released from cola drinks. For this purpose, cola refreshments were incubated with dental resins for 24 h and 7 days, and the proposed method was successfully used for monitoring the release in cola samples.
Keywords: beverages, Bisphenol A, Capsule Phase Microextraction, green, microextraction
1. Introduction
Dental resins, being widely used in restorative dentistry due to their versatility and aesthetic quality, have dramatically improved patient care and treatment outcomes. However, the concern of leaching monomers out of these materials into the oral environment has elicited a wide interest in the issues of biocompatibility and safety [1]. In the area of dental resins, the detection of leached monomers is complicated due to the presence of the saliva matrix, gingival crevicular fluid, and other oral fluids [2].
Therefore, researchers have applied novel sample microextraction methods, which enhance sensitivity, selectivity, and speed in the determination of monomers. In particular, computer‐aided design/computer‐assisted manufacturing (CAD/CAM) materials represent a significant advancement in dental restorative materials. These materials are indicated for indirect restorations and are manufactured under conditions of high pressure and temperature, resulting in a matrix with a high degree of polymerization [3]. The advanced manufacturing process makes it highly unlikely that monomers will leach into the oral cavity compared with resin composites directly placed on dental tissues. The process of leaching monomers from resin‐based materials, directly or indirectly, is widely influenced by the biodegradation mechanisms mediated through saliva components, enzymes, and esterases. This has formed the basis of an extensive investigation within the dental research fraternity [4]. Although the release of monomers from resin composites applied directly is well understood, there is still a relative lack of information on the leaching of monomers from CAD/CAM materials, especially when in contact with beverages commonly ingested by patients. Monomer release has been under intensive interest for the past 20 years, and it was brought out that this phenomenon was significantly influenced by the consumption of different beverages [5].
However, this important issue remains poorly studied, particularly with regard to both resin composites and CAD/CAM materials in regard to dietary influence. For the determination of monomers, sample preparation is undoubtedly the most critical step in the analytical process. Traditional liquid–liquid extraction (LLE) has been sufficiently replaced by solid phase extraction (SPE), minimizing the use of hazardous organic solvents [6]. Recently, microextraction methods have been recognized as simple and effective sample preparation methods with advantages including low consumption of solvents, high preconcentration factors, and versatile analytical instrumentation compatibility [7, 8, 9, 10]. The use of microextraction techniques in the context of the analysis of monomers enables the extraction and quantitation of dentin monomers in ultralow‐volume oral fluids collected in an appropriate way. The efficient enrichment of target analytes counteracts the solution matrix interference effects, thus ensuring precise identification and quantification of monomer concentrations, which is of utmost importance for the evaluation of dental material biocompatibility and safety.
Among microextraction techniques [11], SPE using molecular imprinted polymers (MIPs) as sorbents [12, 13], solid phase microextraction (SPME) [14], stir bar sorptive extraction (SBSE) [14], dispersive liquid–liquid microextraction (DLLE), fabric phase extraction (FPSE) [15, 16] and capsule phase microextraction (CPME) [17, 18] have been developed as useful tools for the extraction and quantification of bisphenols and released monomers. These techniques exhibit many advantages, including increased sensitivity and selectivity, fast extraction kinetics, which can be favorable for short extraction times, increasing efficiency and reproducibility, and complying with the commands of Green Analytical Chemistry (GAC) [19]. CPME was introduced by Kabir and Furton in 2015 [20], and its analytical performance as an emerging technique used for the isolation of analytes from complex matrices has been proven [21, 22]. CPME uses a mechanically stable sorbent coating and eliminates the risk of breakage and reduces the cost of SPME. While CPME has already been explored for various analytes in food [18], environmental [22, 23], and biological samples [24], its application in the determination of dental monomers has not been explicitly evaluated yet. While previous studies have examined the levels of leaching monomers in saliva [2] and in alcoholic beverages [8], their extraction from common acidic drinks such as cola is a new and unexplored scenario.
This work aims to present a novel CPME method combined with high‐pressure liquid chromatography coupled to UV (HPLC–UV) for the simultaneous determination of organic monomers bisphenol‐A (BPA), bisphenol A glycerolate dimethacrylate (Bis‐GMA), triethylene glycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA) that can be released from dental resins in cola‐type beverages. The study systematically optimizes the key extraction parameters of the CPME method to improve efficiency and usability for target analytes in a complex matrix such as cola beverages, adding practical and scientific value to the field of analytical chemistry and dental material safety.
2. Experimental
2.1. Chemicals and Reagents
HPLC‐grade water, methanol (MeOH), isopropanol (IPA), ethanol (EtOH), and acetonitrile (ACN) were obtained from Rotisolv (Karlsruhe, Germany). BPA, TEGDMA, UDMA, and Bis‐GMA analytical standards (99% purity) were purchased from Sigma‐Aldrich‐LLC (Taufkirchen, Germany). Stock solutions at a concentration of 50 ng/µL for BPA, Bis‐GMA, TEGDMA, and UDMA were prepared by dissolving the respective monomers in MeOH. Working standards, ranging from 0.2 to 5 ng/µL, were freshly prepared every three days by diluting the stock solutions with water and kept at 4°C.
2.2. Dental Resins
The tested dental materials were Vita Enamic, Katana Avencia, and Clearfil Majesty Posterior PLT. Vita Enamic is a hybrid CAD/CAM material based on a high‐temperature and high‐pressure polymerized resin‐infiltrated ceramic network. Such a structure combines the resiliency of resin with the strength of ceramics, making this material suitable for indirect restorations. Katana Avencia has a unique manufacturing method in which nano‐sized fillers are densely compressed into a block and uniformly impregnated with resin monomers. The resin is then heat‐polymerized, yielding a material with superior mechanical strength and excellent gloss retention. Clearfil Majesty Posterior PLT is a direct resin composite clinically used for direct resin restorations, offering high polishability and wear resistance for posterior applications. Their detailed characteristics have been reported in a previous study of our group [8]. A 2 mm2 or circular‐shaped piece of CAD/CAM blocks was cut with a water‐cooled, diamond‐coated, low speed band saw. A 1.5 mm diamond rotary tool was used to drill a central hole in each block. Specimens made of all four materials were immersed in 10 mL of cola beverage for 24 h and 7 days at room temperature to simulate exposure scenarios [1], using a silk thread to suspend them in airtight glass containers.
2.3. Instrumentation
The chromatographic separation was performed on a PerfectSil Target ODS‐3 (250 mm × 4.6 mm, 5 µm) analytical column (MZ AnalysenTechnik, Mainz, Germany) at room temperature. ACN:water (70:30, v/v) was used as the mobile phase, which was pumped in isocratic mode at a flow rate of 1 mL/min. The high‐pressure liquid chromatography (HPLC)‐UV analysis was conducted on the LC‐10 AD pump (Solid HPLC ultrafast measurement method, 2010, Shimadzu Corporation, Kyoto, Japan). Injection of the samples was through a Rheodyne 7125 injection valve with a loop volume of 20 mL (Rheodyne, Cotati, CA, USA), and detection was performed using an SSI 500 UV–Vis detector (SSI, State College, PA, USA) at 220 nm. For sample preparation, a Gerhardt RMH Magnetic Stirrer (Gerhardt, Königswinter, Germany) was used. The mobile phase was initially degassed, prior to chromatographic analysis, by sonication with an ultrasonic bath Transonic 460/H (35 kHz, 170 W, Elma, Germany). Before injection into the chromatographic system, samples were filtered using disposable Q‐Max RR syringe filters (0.22 µm nylon membrane; Frisenette ApS, Knebel, Denmark). Solvent evaporation was performed under a nitrogen stream using a ReactiVap 9‐port evaporator model 18780 (Pierce, Rockford, IL, USA).
2.4. CPME Fabrication and Extraction Protocol
The fabrication and characterization of the capsules was carried out in the Department of Chemistry and Biochemistry at Florida International University, Miami, Florida, USA. In brief, it has already been reported elsewhere [18]. In brief, Membrana Accurel porous capillary membranes were obtained from 3M Inc. (St. Paul, MN, USA). Cylindrical magnetic rods (1/4ʺ × 1/16ʺ) were purchased from K&J Magnetics Inc. (Pipersville, PA, USA).
CPME devices were created according to the following steps: (a) preparing and cleaning Accurel S6/2 porous polypropylene membranes; (b) preparing the sol solution for the surface coating and the monolithic bed; (c) creation of surface coating and monolithic bed of sol–gel sorbents on the walls of the polypropylene capillary and inside the lumen; (d) ageing, conditioning, and cleaning of the CPME devices. CPME devices were fabricated in different sizes, such as 1 cm and 2 cm. During the device fabrication process, first, Accurel polypropylene S6/2 porous membranes were cut. The capillary membranes were then cleaned with methylene chloride for 30 min under sonication and subsequently air‐dried at room temperature for 30 min. Subsequently, cylindrical magnets were inserted into empty polypropylene capsules of appropriate size, and then, one empty capsule and the capsule containing the magnet were fused together at the ends by impulse heat sealing. Both capsules were connected together by their ends. Afterwards, the sol–gel sorbent coating process was carried out. Sol solution for sol–gel sorbents was prepared by the sequential addition of tetramethyl orthosilicate (TMOS), polymer/ligand, IPA, hydrochloric acid catalyst, and deionized water at a molar ratio of 1:0.2:30:0.04:8, respectively, in a 50 mL reaction container. The mixture was kept at room temperature for 12 h so that acidic hydrolysis of the sol–gel precursors could be completed. Subsequently, the sol solution was centrifuged and the supernatant fraction was transferred into a wide‐mouth glass reaction vessel. Subsequently, NH4OH (1 M) was added to the solution in droplets at a molar ratio between TMOS and NH4OH of 1:0.10 with continuous stirring. A total of 30 units of the CPME devices were submerged in the sol solution, and then the reaction vessel containing the submerged CPME devices was sonicated for 5 min to remove air bubbles from the system. The gelation of the sol solution occurred in 1 h under the imposed reaction conditions. The sol solution formed a solid monolithic bed within the lumen of the capsules and a mesh‐like network on the surface of the porous polypropylene capsules and inside the pores of its thick walls. The CPME devices were then subjected to aging and thermal conditioning at 50°C for 24 h. The CPME devices were subsequently cleaned by brushing the sol–gel sorbent from their outer surface and rinsing with a mixture of MeOH:methylene chloride (50:50, v/v) under sonication for 30 min. The monolithic bed of the sol–gel sorbents was disintegrated into fine microparticles during sonication. The CPME devices were then dried in an oven at 50°C and were then ready for extraction.
During extraction, the capsule was handled with tweezers to prevent contamination. Before the extraction, the CPME media were immersed in 2 mL of ACN:MeOH (50:50, v/v) for 5 min for activation. Then, it was immersed in 2 mL of deionized water to remove organic solvents. For the extraction, the CPME media was transferred into a 4 mL screw‐capped glass tube vial containing 200 µL of sample diluted in a final volume of 3 mL with deionized water. The extraction was carried out within 45 min, under 350 rpm of stirring, as presented in Figure 1. Then, the CPME media was removed from the vial, rinsed with deionized water, and left for 2 min to air dry. In a further step, it was inserted in a clean vial containing 1 mL MeOH:ACN (50:50, v/v) for the elution of the retained analytes for 10 min under stirring at 350 rpm. The obtained extract was evaporated under a nitrogen stream and reconstituted in 200 µL of the mobile phase (ACN:H2O [70:30, v/v]) prior to the injection in the chromatographic system. The CPME media was cleaned with a mixture of ACN:MeOH, 50:50 v/v, dried, and stored for further use.
FIGURE 1.
CPME extraction device consisting of a 4 mL screw‐capped vial, and a CPME capsule immersed into the sample and placed onto the stirrer.
2.5. Optimization of the CPME Method
Four different CPME capsules, namely sol–gel Carbowax 20 M (sol–gel CW 20 M), sol–gel poly(tetrahydrofuran) (sol–gel PTHF), sol–gel hybrid structure containing polyimide (PCAP) and polydimethylsiloxane (PDMS) (PCAP‐PDMS‐PCAP), and sol–gel poly(ethylene glycol) (sol–gel PEG), were evaluated. Optimization experiments were carried out using the one‐factor‐at‐a‐time approach [25]. All critical parameters, including (a) CPME media type and size (1–2 cm), (b) elution solvent type (IPA, EtOH, MeOH, ACN, MeOH:ACN (50:50, v/v), (c) extraction time (10–60 min), (d) stirring rate during extraction (350, 750 rpm), (e) elution time (1–5 min), and (f) volume of elution solvent (1–2 mL), were optimized using spiked cola samples at a concentration level of 0.5 ng/µL.
2.6. Method Validation
Linearity, accuracy, precision, and selectivity were assessed to validate the CPME‐HPLC‐DAD method. Linearity was analyzed in triplicate, and the entire working range of linearity, 0.2–5 ng/µL, was defined; the limits of detection (LODs) and quantification (LOQs) were determined using a signal‐to‐noise (S/N) ratio of 3.3 for LOD and 10 for LOQ according to the following equation: LOQ = 10 S/N [15]. Accuracy and precision were studied with 200 µL of real samples spiked in triplicate at three different concentrations (0.2, 2, and 5 ng/µL), subjected to the FPSE pretreatment presented above in Section 2.4. Relative recoveries (R%) were used to express accuracy as mean [concentration found/added concentration] × 100. In intraday assays, precision was expressed as relative standard deviation (RSD%) upon replicate measurements of the spiked samples. Within‐day precision (repeatability) was assessed on five replicates (n = 5), and between‐day (intermediate) precision on triplicate analyses spread over four subsequent days (n = 3 × 4).
3. Results and Discussion
3.1. Optimization Results
The effectiveness of the CPME media with different sol–gel coating sorbents was examined. Each experiment was carried out in triplicate (n = 3). Sol–gel PTHF‐coated CPME membranes, which are medium‐polar, showed the highest recovery rates, and therefore they were selected as optimum for further testing. The size of the optimum membrane was also evaluated, and as is presented in Figure 2, the increase in size results in an increase in the extraction recovery rates.
FIGURE 2.
Impact of (a) CPME type and (b) size on the extraction recoveries of BPA, TEG‐DMA, UDMA, and BisGMA.
Considering the recovery rates for the different sorbents tested, the results are in accordance with the literature [26], showing that the patented CPME sol–gel sorbents have been reported to be stable across a wide pH range of 1–13, indicating robustness under various conditions. In this respect, the low pH value of the matrix (pH ∼2.5) did not affect the performance of the microextraction device.
The differences in the polarity of the target analytes were considered, and the efficacy of different solvents was evaluated for the elution of the target analytes, including MeOH, IPA, EtOH, ACN, and MeOH:ACN (50:50, v/v). As is shown in Figure 3, the mixture of MeOH:ACN (50:50, v/v) resulted in higher extraction recoveries and was selected for further experiments.
FIGURE 3.
Impact of elution solvent type on the extraction recoveries of BPA, TEG‐DMA, UDMA, and BisGMA.
The volume of the elution solvent was also optimized, and the use of 1 and 2 mL MeOH:ACN (50:50, v/v) was considered. According to the results presented in Figure 4a, the increase in the volume of the elution solvent did not result in a dramatic increase in the extraction rates, and as a result, 1 mL of MeOH:ACN (50:50, v/v) was selected as the optimum elution solvent volume for the rest of the experiments. For the optimization of the extraction time, 5–60 min were tested, and the extraction recoveries increased with the increase of the extraction time. No significant differences in the extraction rates were shown between 45 and 60 min of extraction, indicating that an equilibrium is reached (Figure 4b). As a result, 45 min of extraction time was the optimum value selected. The 45‐min of extraction time is typical for such analytes, as the extraction kinetics offer a balance between efficiency and practicality [2]. The elution time was evaluated considering the extraction recoveries of the elution time within 1 and 5 min, and the results indicated that 1 min of elution under 350 rpm is enough for the elution of the target analytes (Figure 4c). Finally, the stirring rate during the extraction was evaluated after testing the effects of 350 and 750 rpm of stirring rate, and no significant differences were observed (Figure 4d). Thus, 350 rpm was selected as the optimum value.
FIGURE 4.
Impact of (a) elution solvent volume, (b) extraction time, (c) elution time, and (d) extraction stirring rate on the extraction recoveries of BPA, TEG‐DMA, UDMA, and BisGMA.
The optimized CPME protocol involved the use of a PTHF capsule of 2 cm size. The extraction was carried out within 45 min, under 350 rpm of stirring, and 1 mL of MeOH:ACN (50:50, v/v) was used for the elution of the target analytes within 1 min under 350 rpm of stirring.
3.2. Method Validation Results
Linearity studies covered the entire working range of 0.2–5 ng/µL and were performed in triplicate (n = 3). Calibration curves were constructed by plotting the peak area for each concentration level and were linear, as is presented in Table 1. The LOQs and LODs were calculated to be 0.2 ng/µL and 0.06 ng/µL, respectively.
TABLE 1.
Linearity data.
| Analyte | Calibration curve | Linear range (ng/µL) | r 2 | LOD (ng/µL) | LOQ (ng/µL) |
|---|---|---|---|---|---|
| BPA | y = 0.0693x + 0.018 | 0.2–5 | 0.9965 | 0.06 | 0.2 |
| TEGDMA | y = 0.0123x + 0.0243 | 0.2–5 | 0.9955 | 0.06 | 0.2 |
| UDMA | y = 0.0198x + 0.0036 | 0.2–5 | 0.9975 | 0.06 | 0.2 |
| Bis‐GMA | y = 0,0226x + 0.0002 | 0.2–5 | 0.9923 | 0.06 | 0.2 |
Accuracy and precision were assessed by means of relative percentage of recovery at three concentration levels (0.2, 1, and 5 ng/µL), and are presented in Table 2. The RSD% of the within‐day (n = 5) and between‐day assays (n = 3 × 4) was lower than 11.1 and 12.2, respectively. The relative recovery values ranged between 93.3 and 106.9%, indicating that the proposed method shows good accuracy.
TABLE 2.
Within‐day and between‐day accuracy and precision results.
| Within‐day accuracy and precision results (n = 5) in spiked cola samples | |||||||
|---|---|---|---|---|---|---|---|
| Added 0.2 ng/µL | Added 2.0 ng/µL | Added 5.0 ng/µL | |||||
| Analyte | Recovery% | RSD % | Recovery% | RSD % | Recovery% | RSD% | |
| BPA | 102.5 | 6.3 | 103.6 | 1.5 | 99.3 | 1.4 | |
| TEG‐DMA | 103.5 | 5.6 | 99.6 | 3.8 | 94.6 | 5.0 | |
| UDMA | 102.6 | 11.1 | 97.4 | 4.9 | 98.3 | 2.8 | |
| BisGMA | 104.4 | 3.7 | 106.9 | 6.7 | 105.1 | 2.1 | |
| Between day accuracy and precision results (n = 4 days × 3 replicates) in spiked cola samples | |||||||
| BPA | 101.4 | 4.2 | 101.5 | 2.0 | 99.4 | 1.0 | |
| TEG‐DMA | 97.4 | 7.7 | 95.0 | 6.5 | 93.3 | 2.7 | |
| UDMA | 93.8 | 12.2 | 95.4 | 6.0 | 96.5 | 2.6 | |
| BisGMA | 94.6 | 3.8 | 100.2 | 7.2 | 102.9 | 2.4 | |
To assess selectivity, five cola samples were extracted using the optimized CPME protocol and analyzed by HPLC–UV. No interferences were monitored within the chromatographic window, as is presented in Figure 5a. A characteristic chromatogram of a standard mixture of BPA, TEGD‐MΑ, UDMA, and Bis‐GMA at a 5 ng/µL concentration level is presented in Figure 5b. BPA eluted at 4 min, TEGDMA at 5.6 min, UDMA at 7.8 min, and Bis‐GMA at 9.6 min.
FIGURE 5.
(a) Chromatogram of a cola sample extracted using CPME, (b) Characteristic chromatogram of a standard mixture of BPA, TEGDMS, UDMA, and Bis‐GMA at 5 ng/µL concentration level.
The performance characteristics of the CPME‐HPLC–UV method were reported in other works reported in the literature on the determination of dental monomers in beverages and require less solvent, compared to the method proposed by Nikolaou et al. [8]. Furthermore, the high recovery rates of the developed method indicate that the design of the CPME capsule increases the surface area‐to‐volume ratio due to the use of micron‐sized sorbent particles, providing high extraction efficiency compared to SPME [27]. The developed CPME‐HPLC–UV has been successfully applied in the analysis of cola samples for the determination of monomers, and the results have already been published [1].
3.3. Evaluation of the Green Character, Applicability, and Reusability
The reusability of the CPME media was tested, and it can be used up to eight times without a significant loss in the extraction performance (up to 10% of loss). The surface chemistry of the CPME capsule partially loses selectivity, reducing partitioning efficiency after some uses in a complex matrix, such as cola drinks. CPME has excellent scalability potential as it could be applied in industrial, clinical, and high‐throughput applications. Its reusability and minimal reagent use make it highly suitable for transitioning it from analytical research laboratories to high‐throughput environments. Owing to its simple design, it could be applied for multi‐sample extraction steps, while it could also be integrated into automated extraction systems.
The green character of the method was assessed using the ComplexGAPI metric tool, and the green color indicates the high compliance of the CPME‐HPLC–UV method with the set requirements, while the yellow and red colors indicate medium and low compliance, respectively. The green color of ComplexGapi indicates optimal environmental performance, yellow represents moderate environmental impact, while red highlights areas with considerable environmental effect. This color‐coded pictogram allows quick visual interpretation of the environmental impact of the method. Overall, the proposed method could be characterized as green according to Figure 6a. The blue applicability grade index (BAGI) was also used to evaluate the applicability of the method, and the score value of 65 demonstrates good method performance (Figure 6b). Overall, CPME operates with the use of very low amounts of solvents, and as a result, it dramatically decreases the generation of hazardous waste. In this respect, CPME minimizes the exposure of analysts, lowering toxicity and occupational hazards.
FIGURE 6.
Evaluation of the method's (a) greenness using the ComplexGAPI metric tool and (b) method's applicability using the BAGI metric tool.
4. Conclusions
In this work, CPME‐HPLC–UV was applied for the first time for the selective extraction of dental resins from cola‐type beverages. The method was optimized and validated, resulting in an accurate, sensitive, and precise analytical protocol. Complex‐GAPI and BAGI were used to assess the green character and the applicability of the method. The CPME media can be effectively used up to eight times, showing that the method proposes a cost‐effective sample preparation protocol that is in accordance with the demands of GAC. Overall, the proposed method combines a fast extraction protocol that requires low solvent consumption and can be successfully applied for the monitoring of leaching of dental resins to cola‐type beverages.
Author Contributions
Magdalini Vlavitsi: data curation, formal analysis, investigation, methodology, validation. Natasa P. Kalogiouri: formal analysis, investigation, methodology, validation, visualization, writing – original draft. Petros Mourouzis: conceptualization, project administration, investigation, methodology, supervision, writing – review and editing. Abuzar Kabir: conceptualization, data curation, investigation, methodology, project administration, supervision, visualization. Kenneth Furton: supervision, project administration. Victoria F. Samanidou: conceptualization, investigation, methodology, project administration, supervision, visualization, writing – review and editing.
Ethics Statement
This study followed all applicable international and domestic ethical regulations.
Conflicts of Interest
The authors declare no conflicts of interest.
Data Availability Statement
The authors confirm that the data supporting the findings of this study will be made available on request.
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Data Availability Statement
The authors confirm that the data supporting the findings of this study will be made available on request.