Fourier Transform Infrared Spectroscopy Analysis of Stains Incorporated in Fluoroaluminosilicate Glass Cements: An In Vitro Study

Sample preparation

GIC powder + GIC liquid (P+L)

P+L samples were dispensed according to the manufacturer's specifications, i.e., powder to liquid ratio of 3.6 g of powder for 1 mL of liquid, onto the mixing pad.

GIC powder + GIC liquid + Food stain (P+L+S)

The tip of a Williams graduated periodontal probe was dipped 0.5 mm into the stain, incorporated into the liquid, and stirred to obtain a homogenous mixture [Tables 1 and 2].

Manipulation of cement

Mixing of the cement was done using a mixing pad and an Agate spatula by the ‘Folding method Technique’. The samples were placed in the Teflon mould. As soon as the samples were set, Vaseline was applied to prevent craze of the material, and it was stored for 24 hours.

Preparation of samples for FTIR analysis

The samples from each group were ground using a mortar and pestle and diluted with KBr and compressed under 10 tons for 1 minute to produce pellets.

These pellets would then be analysed under FTIR spectroscopy.

FTIR analysis

The operational principle behind FTIR involves detecting energy absorption at specific wavelengths or wave numbers to investigate the chemical structure of the material being analysed. Instruments can emit a broad spectrum of infrared radiation. For this project, the mid-infrared region with wavelengths from 800 to 4000 cm⁻¹ was used.

The BRUKER ALPHA II was used to analyse the changes in the chemical structure of aged materials. FTIR spectroscopy offers insights into the vibration patterns of molecules through the assessment of optical absorption bands within the sample. These bands serve as unique markers or “fingerprints” for specific molecules, enabling the precise extraction of information about the sample under examination^[8] [Table 3].

Statistical analysis

Sample size calculation

Standard deviation (Mpa) = 4.0

Number of groups to be compared = 4

Margin of error = 9

At a 5% level of significance & 80% power of the test, the required sample size per group = 5

Therefore, total sample size = 20

This was calculated using the formula (sample size for one-way ANOVA)

$\frac{n=2[{(Z_{(1-\alpha /2)k}+Z_{(1-\beta )}]}^2·{\Sigma }^2}{d^2}$

$Z_{(1-\alpha /2)k}=2.64$

Z_(1-β) = 0.84 @ 80% power of test

∑ = standard deviation

d = margin of error

k = number of groups

Interpretation 1

Data from the FTIR Analysis of the Control group showed absorption bands at 3460, 2366, 1634, 1458, 1409, 1319, and 1071 cm⁻¹.

Carboxyl groups were observed at 1638 and 1319 cm⁻¹

The polyacrylate aluminium salt was characterised by the adsorption band at 1458 cm⁻¹.

The polyacrylate calcium salt was characterised by the adsorption band at 1409 cm⁻¹.

The following changes were noted by comparing Graphs 1 and 2. Data from Graphs 1–4 has been compiled and depicted in Graph 5. Data from Graph 1 and 2 has been compiled and depicted in Graph 6.

1. At 1450.57, a medium wave appearance depicting C-H bending indicates a methyl group has been incorporated.

2. At 1173.29, a strong wave appearance depicting C-O stretching indicates that an ester group has been incorporated.

3. At 1109.80, a strong wave appearance depicting C-O stretching indicates that aliphatic ether has been incorporated.

4. At 1078.39, a strong wave appearance depicting C-O stretching indicates a primary alcohol group has been incorporated.

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Graph 1:

FTIR spectra from the control group. FTIR: Fourier transform infrared spectroscopy.

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Graph 2:

FTIR spectra from G1 - brilliant blue. FTIR: Fourier transform infrared spectroscopy.

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Graph 3:

FTIR spectra from G2 - brilliant green. FTIR: Fourier transform infrared spectroscopy.

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Graph 4:

FTIR spectra from G3 - sunset yellow. FTIR: Fourier transform infrared spectroscopy.

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Graph 5:

FTIR spectra from the control, G1, G2, and G3, superimposed. FTIR: Fourier transform infrared spectroscopy.

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Graph 6:

FTIR spectra showing the comparison between the control group and G1. FTIR: Fourier transform infrared spectroscopy.

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Interpretation 2

1. A methyl group has been incorporated into the carboxyl peak.

The following changes were noted by comparing Graphs 1 and 3. Data from Graphs 1 and 3 has been compiled and depicted in Graph 7.

1. At 3452.4, a strong and broad wave appearance depicting O-H stretching indicates that an alcohol group containing an intermolecular bond is incorporated.

2. At 2980.96, a strong and broad wave appearance depicting N-H stretching indicates that an amine salt is incorporated.

3. At 970, a strong wave appearance depicting C=C bending indicates a disubstituted (trans) alkene is incorporated.

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Graph 7:

FTIR spectra showing the comparison between the control group and G2. FTIR: Fourier transform infrared spectroscopy.

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Interpretation 3

1. No changes have been made in the carboxyl group.

The following changes were noted by comparing Graphs 1 and 4. Data from Graphs 1 and 4 has been compiled and depicted in Graph 8.

1. At 2965.05, an NH stretching indicating an amine salt has been incorporated.

2. At 1463.75, a C-H bending indicating a methylene group has been incorporated.

3. At 1458.15, a C-H bending indicating a methyl group was incorporated.

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Graph 8:

FTIR spectra showing the comparison between the control group and G3. FTIR: Fourier transform infrared spectroscopy.

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Interpretation 4

1. A methyl group has been added to the carboxyl group.

Final interpretation

To interpret the FTIR graph, the IR Spectra table from Sigma Aldrich was used.

Functional groups were identified beforehand for the cement to aid in the ease of graph interpretation.

The identified functional groups have been mentioned under INTERPRETATION 1.

A peak in the corresponding wavelength indicates its presence.

G1 & G3 showed higher modifications in the FTIR Analysis spectra corresponding to a carboxyl group.

G2 showed fewer modifications in the FTIR analysis spectra corresponding to a carboxyl group.

A lesser number of modifications indicates that there is no change in the active functional group (carboxyl group).

Chemistry and setting

GIC shares fundamental chemical properties, with slight variations in powder composition and particle size tailored to achieve specific functions. The consistency of mixed GICs depends on the particle size distribution and the desired P/L ratio and can range from low viscosity to very high viscosity.^[9]

Glass composition

The glass composition in GIC varies according to each manufacturer, but it always contains the key elements - silica, alumina, and fluoride. The pivotal factor governing the chemical reaction with polyacrylic acid lies in the proportion between alumina and silica. Strontium, Barium, and other higher atomic number metal oxides are added to the glass to increase the radiopacity of the set material. Depending upon the overall composition and raw materials, the silica glass is melted at temperatures ranging from 1100°C and 1500°C.^[9–11]

Liquid composition

Originally, aqueous solutions of polyacrylic acid (around 40% to 50%) were used, but such liquids had limited longevity due to gelation properties and were highly viscous in nature. Currently, the liquids are copolymers of itaconic, maleic, or tricarboxylic acids.

Tartaric acid is an additive that governs the pace in the GIC liquid, which allows a wider range of glasses to be used, enhances handling characteristics, reduces viscosity, prolongs the available working duration, increases the longevity before gelling of liquid occurs, increases working time, and increases the curing process.^{[9, 12, 13]}

Setting reaction

When GIC powder and liquid are combined, the acid prompts the dissolution of glass, liberating calcium, aluminium, sodium, and fluoride ions. Water serves as the medium for this reaction. Initially, calcium ions crosslink the polyacrylic acid chains; however, within 24 hours, aluminium ions replace the calcium ions. The involvement of sodium and fluoride ions in the cement's crosslinking process is minimal. While some sodium ions can displace hydrogen ions from carboxylic groups, fluoride ions disperse within the crosslinked (base) phase of the solidified cement. As time progresses, the crosslinked phase undergoes hydration, maturing in the process. A silica-rich gel forms on the surface of glass beads, coating the remaining undissolved portion. Consequently, the solidified cement comprises insoluble glass particles encased in a silica gel coating, embedded within an amorphous matrix containing calcium, hydrated aluminium poly-salt, and fluoride.

Figure 1 illustrates the structure of set GIC restoration.^{[9, 10, 14]}

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Figure 1:

The diagram depicting the organisation of GIC. Unreacted glass particles, depicted as solid blue particles, are enveloped in a gel formed by the leaching of aluminium and calcium ions from the glass due to the polyacrylic acid's attack. Calcium and aluminium ions react with the carboxyl groups of polyacrylic acid, creating poly salts and establishing a cross-linked structure. The carboxyl group undergoes a reaction with the calcium in enamel and dentin. GIC: Glass ionomer cement. COO−: Carboxylate ion, Ca++: Calcium ion, Al++: Aluminium ion.

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Mechanism of action

Glass ionomers adhere to the tooth structure through the chelation process involving the carboxyl groups within the polyacrylic acids and the calcium present in the enamel and dentin's apatite.^[9]

The diagrammatic representation shows the importance of the carboxylic group in the bonding of the tooth structure. According to the FTIR spectra, all the groups showed an increase in correspondence to the carboxylic group spectra (1638 and 1319 cm⁻¹), which leads to the assumption that there shouldn't be any changes affecting the bonding strength of the cement to the tooth structure [Figure 1].

Clinical relevance

Utilising stained glass ionomer cement in disabled or constantly supervised patients could benefit the caretakers by aiding in the easy monitoring of restoration integrity.

This approach has the potential to improve the patients' oral hygiene as the restoration can be easily identified.

Easy evaluation and identification of restoration and its margins during recall appointments.

Limitations

This study has evaluated only the change in the active functional group of stains incorporated in the fluoroaluminosilicate glass cement by FTIR analysis. Further studies are required to study and evaluate the bonding strength and mechanical properties of the cement.