Curcumin has potential application as a colorimetric sensor and can be used, for example, to monitor the freshness of foods, although some of its physicochemical properties, such as solubility and stability, limit its use. In the present work, a natural curcumin extract was incorporated into organic‒inorganic hybrid silica matrices produced by the sol-gel method using ammonium fluoride as the main catalyst. A series of pH sensors based on the encapsulation of curcumin were prepared using a series of organosilanes (methytriethoxysilane, octyltriethoxysilane, octadecyltrimethoxysilane and 3-(aminopropyltriethoxysilane)) and were used in the synthesis of hybrid silicas. The effectiveness of the employed sol-gel route and the properties of the encapsulated materials were evaluated by a set of complementary analytical techniques, namely, infrared spectroscopy, 29Si solid-state nuclear magnetic resonance spectroscopy, nitrogen porosimetry, dynamic light scattering, confocal laser scanning microscopy and diffuse reflectance UV‒Vis spectroscopy. The immobilization of compounds in a solid matrix can cause a shift in the maximum absorption bands in the ultraviolet‒visible region as a result of interactions between the organic molecules and the solid support. The incorporation of curcumin into different silica matrices improved the performance of the sensors to up to 3.5 times greater than that obtained by the free compound when interacting with ammonia vapor.
Research Article
Ammonium fluoride-catalyzed sol-gel route applied for curcumin-based pH sensors
https://doi.org/10.21203/rs.3.rs-4999552/v1
This work is licensed under a CC BY 4.0 License
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sol-gel
pH sensors
hybrid silica
curcumin
ammonium fluoride catalyst
Several natural extracts have been used as sensors for monitoring the quality and safety of packaged products [1], highlighting the use of anthocyanins, betalains, carotenoids, tannins and curcumin [2], and several compounds present in essential oils, such as cinnamaldehyde, the main constituent of cinnamon essential oil, and the aromatic monoterpenes carvacrol and thymol, which are extracted from various plants, have also been used as sensors [3]. Applications of these compounds include pH environment monitoring, antimicrobial or antioxidant action, moisture loss limitation, just to mention a few.
In this context, the potential of curcumin, the main compound extracted from Curcuma rhizomes, was investigated by several authors [4–7], who reported the halochromic property of this compound when interacting with alkaline vapors, suggesting its use, for example, for monitoring the freshness of foods with high protein contents.
Despite its vast potential applications, curcumin has several disadvantages that limit its use, such as low solubility in water at acidic or neutral pH, a high decomposition rate in alkaline media, and photodegradation in organic solvents [8]. Many efforts have been reported in the literature to ensure the stability and increase the bioavailability of curcumin. For example, curcumin can be encapsulated in liposomes and chitosan [9]; coordinated with macromolecules, such as gelatin and cyclodextrins [8]. Cross-linked citric acid has been employed to insolubilize polyvinyl alcohol nanofibers which material was employed to evaluated curcuma´s halochromic properties. Citric acid acts as the catalyst, converting curcumin into the keto tautomeric form, which favors its encapsulation and decrease in the color intensity.[10] pH sensors based on curcumin and thermoplastic-polyurethane electrospun-fibers have been designed visible color sensor. Such systems are based on the chemical variation from enol to di-keto form. Such material could be employed in the development of smart diagnostic clothing for cystic fibrosis patients who require continuous sweat pH monitoring.[11] Electrospun nanofiber halochromic pH sensors has been also proposed combining curcumin, chitosan and polyethylene oxide. Such curcumin loaded nanofiber allowed to visualize real time monitoring of chicken spoilage.[12]
Encapsulation of curcumin within silica particles has been carried out for different goals and application. For instance, silica has been encapsulated in commercial nanosilica (Ludox® HS-30 colloidal silica solution). In a simulated gastric digest mean, over 80% of the encapsulated curcumin was retained, while after two hours of incubation in a simulated intestinal environment, 60% of the encapsulated curcumin has been released.[13] Other studies involving curcumin-encapsulated silica has evaluated its citotoxicity and potentiality for anticancer application.[14] Other applications have dealt with food industry [15] and in the development of systems against leaching and protect against degradation [16].
The sol-gel method is frequently used to produce silica-based materials, enabling the incorporation of compounds within siloxane networks, including pH sensors. Several routes can be used, including synthesis using fluoride ions as the catalyst for the hydrolysis and condensation reactions of molecular precursors, typically silicon alkoxides [17]. Although its conditions are less investigated in the literature, it has the great advantage of increasing the reaction speed compared to other routes [18]. In the present work, a sol-gel synthesis using ammonium fluoride and ammonium hydroxide as reaction catalysts was investigated for the encapsulation of curcumin in organic‒inorganic hybrid silicas aiming to produce colorimetric sensors for ammonia detection.
2.1 Chemicals
Tetraethoxysilane (TEOS, Sigma Aldrich, 98%), 3-(aminopropyltriethoxysilane) (Sigma Aldrich, 98%), methyltriethoxysilane (Sigma Aldrich, 99%), octyltriethoxysilane (Dow Corning, 96%), octadecyltrimethoxysilane (Sigma Aldrich, 90%), ammonium hydroxide (nuclear, NH4OH 28–30%), ammonium fluoride (Sigma Aldrich, 98%), ethanol (nuclear, 95%) and curcumin (from Curcuma longa, Sigma Aldrich, 65%) were used. All chemicals were used as received.
2.2 Synthesis of pH sensors
The sol‒gel route using ammonium hydroxide and ammonium fluoride as the catalysts was employed to prepare curcumin-encapsulated silica-based organic–inorganic hybrid materials following a procedure adapted from the literature [17]. A typical TEOS/organosilane molar ratio of 9 was used to prepare these sensors. In this approach, 18.5 mmol of TEOS and 2.1 mmol of a selected organosilane were added to 10 mL of a curcumin ethanolic solution (1 mol.% based on alkoxide precursors) and mixed under magnetic stirring for 2 minutes. Then, a water/ethanol mixture containing the catalysts was added, and the obtained solution was stirred at room temperature until gel formation. The resulting solution was prepared with 6.4 mL of deionized water, 10 mL of ethanol and 337 µL of catalyst stock solution previously prepared by dissolving 1.852 g of ammonium fluoride in 100 mL of deionized water, followed by the addition of 20.5 g of ammonium hydroxide. The gelification time lasted ca. 20 min. The resulting materials were then dried at 50°C for 48 hours in an oven and manually milled to obtain powder samples.
Control samples were prepared following the same procedure without the addition of curcumin. In addition, nonhybrid materials with TEOS as the only alkoxide precursor were also prepared. Table 1 shows the employed labels for the different resulting materials.
| Hybrid silica labels | Organosilane compound | |
|---|---|---|
| Control | Sensor | |
| C0 | C0_C | - |
| C1 | C1_C | Methytriethoxylsilane |
| C8 | C8_C | Octyltriethoxysilane |
| C18 | C18_C | Octadecyltrimethoxysilane |
| APTES | APTES_C | 3-(aminopropyltriethoxysilane) |
2.3 Characterization of the samples
Structural analysis of the powder samples was conducted by means of a Bruker ALPHA-P Fourier transform infrared (FT-IR) spectrometer instrument equipped with an attenuated total reflectance (ATR) accessory and zinc selenide (ZnSe) crystal. 29Si solid-state nuclear magnetic resonance (NMR) spectroscopy was carried out on a DD2 500/54AR Agilent instrument. Measurements were performed at 99.3 MHz for 29Si employing cross-polarization and magic angle spinning (CP MAS) to improve the spectral quality. The textural analysis was evaluated by nitrogen adsorption-desorption isotherms that were measured at − 196°C in a TriStar II 3020 Micromeritics after degassing (10− 2 bar) the samples at 120°C for 48 h, and the Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area. The particle size was determined via dynamic light scattering (DLS) in an ethanol dispersion at room temperature using a Nano-S90 Zetasizer (Malvern).
The optical properties of the solid sensors were evaluated by a Carry 100 Varian ultraviolet‒visible (UV‒Vis) spectrophotometer with a diffuse reflectance accessory. The spectra were acquired in reflectance mode in the wavelength range from 360 nm to 800 nm. Morphological characterization was performed using confocal laser scanning microscopy (FV1000 Olympus) with an excitation laser line of 473 nm. Images were collected along the z-axis and processed by FluoView software.
2.4 Response to ammonia vapor
The performance of the encapsulated curcumin sensors was evaluated according to Cichero and Santos [18] with some modifications. In the present study, approximately 100 mg of the obtained samples was placed in a closed vessel with a total volume of 250 cm3 and kept in contact with ammonia vapor generated by ammonium hydroxide solution (28–30% w/v, 10 mL) for 5 minutes. The analysis was performed by ultraviolet‒visible (UV‒Vis) spectroscopy with a diffuse reflectance accessory, and the data were collected in the wavelength range between 360 nm and 800 nm using Cary WinUV Color® software in the CIELAB color space (CIE 1976 L∗a∗b∗) employing the CIE D65 illuminant. The color differences (ΔE*ab) of each sample measured by the Euclidean distance between the color coordinates before and immediately after interaction with ammonia were calculated according to Eq. 1 [19].
𝛥𝐸∗𝑎𝑏 = [𝛥𝐿∗2 + 𝛥𝑎∗2 + 𝛥𝑏∗2]1/2 (1)
The reversibility of the powder sensors was also investigated by acquiring color coordinates after 24 and 48 h of contact with ammonia vapor. The samples were allowed to desorb ammonia at room temperature. The ΔE*ab was calculated based on the initial color of each sample before it came into contact with the alkaline vapor.
Sol-gel synthesis employing fluoride ion as the catalyst affords the advantage of increasing the reaction speed compared to other routes [18]. The reactions follow the SN2 type mechanism, where the F − ion acts as a nucleophile, attacking the silicon alkoxide, with subsequent release of water or alcohol, when in an acidic medium. The generated species becomes quite susceptible to nucleophilic attack by a water molecule due to the strongly withdrawing inductive effect of fluoride, leading to a five-coordinated transition state. The hydrolysis reaction is completed after catalyst regeneration. The condensation reaction goes through a hexacoordinated transition state.
3.1 Structural characterization of hybrid silica-based materials
The produced silicas were first evaluated by infrared spectroscopy, as shown in Fig. 1, and the assignments of the main bands identified in the spectra are summarized in Table 2.
| Wavenumber (cm− 1) | Assignment | Wavenumber (cm− 1) | Assignment |
|---|---|---|---|
| 3400 | ν O-H (H2O, SiOH) | 1270 | δ H-C-H (Si-CH3) |
| 2960 | νas C-H (CH3) | 1300 − 1000 | νas Si-O-(Si) |
| 2918 | νas C-H (CH2) | 950 | ν Si-OH e ν Si-O− |
| 2850 | νs C-H (CH2) | 837 | ρ CH3, ν Si-C |
| 1640 | δ H-O-H | 800 | νs Si-O-Si |
| 1550–1640 | δ H-N-H | 560 | ν Si-O, δ O-Si-O |
| 1465 | δ H-C-H |
The spectral region between 1300 cm− 1 and 850 cm− 1 represents the bands of greatest interest for structural evaluation of silica-based materials. In the case of asymmetric stretching of siloxane groups, it is possible to decompose this band into longitudinal optical (LO4 and LO6) and transverse optical (TO4 and TO6) components, which are divided into doublets due to the presence of four- and six-member rings in the silica network, respectively.
To quantify the previous analyses, it was necessary to perform a deconvolution of the bands following the Gaussian adjustment model after normalizing the spectra. The profiles found are presented in Fig. 2, where the stretching (Si-O−) and Si-OH were integrated as a single band, which was identified as ν Si-Od, as its area can indicate the degree of hydrophilicity of the sample according to Eq. 2 [20].
The values of the integrals of the longitudinal and transverse vibrational modes of the bands related to the asymmetric stretching of siloxane make it possible to determine the percentages of six- and four-member cyclic arrangements according to Eq. 3. The results are listed in Table 3.
Table 3 Deconvolution of the 850–1300 cm-1 spectral region of the FTIR spectra of all the samples: wavenumber (cm-1), integrated area of components (A), and variance (r²).
| C0 | C1 | C8 | C18 | APTES | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Control | Sensor | Control | Sensor | Control | Sensor | Control | Sensor | Control | Sensor | |||||
| LO6 | 1224 | 1222 | 1216 | 1213 | 1221 | 1220 | 1217 | 1218 | 1216 | 1220 | ||||
| A | 7.4 | 3.8 | 5.8 | 9.4 | 5.4 | 3.8 | 8.0 | 5.4 | 6.3 | 4.0 | ||||
| LO4 | 1162 | 1149 | 1142 | 1139 | 1161 | 1153 | 1147 | 1148 | 1155 | 1161 | ||||
| A | 30.5 | 48.2 | 47.1 | 45.2 | 34.2 | 42.6 | 39.3 | 42.5 | 28.4 | 28.0 | ||||
| TO4 | 1078 | 1073 | 1069 | 1071 | 1077 | 1075 | 1074 | 1074 | 1073 | 1073 | ||||
| A | 69.5 | 52.4 | 50.7 | 49.5 | 69.7 | 59.5 | 55.1 | 52.4 | 57.7 | 62.1 | ||||
| TO6 | 1042 | 1040 | 1036 | 1038 | 1039 | 1038 | 1046 | 1042 | 1038 | 1039 | ||||
| A | 11.9 | 14.2 | 17.3 | 16.9 | 15.2 | 16.7 | 12.2 | 16.6 | 25.3 | 26.0 | ||||
| ν Si-Od | 957 | 956 | 951 | 952 | 951 | 952 | 952 | 954 | 954 | 952 | ||||
| A | 11.5 | 13.4 | 12.8 | 13.1 | 12.2 | 12.7 | 7.9 | 10.3 | 19.2 | 18.6 | ||||
| r² | 0.9997 | 0.9992 | 0.9999 | 0.9998 | 0.9997 | 0.9998 | 0.9998 | 0.9992 | 0.9996 | 0.9996 | ||||
| %(SiO)6 | 16.2 | 15.2 | 19.1 | 21.7 | 16.6 | 16.7 | 17.6 | 18.9 | 26.8 | 25.0 | ||||
| %Si-Od | 8.8 | 10.2 | 9.6 | 9.7 | 8.9 | 9.4 | 6.5 | 8.1 | 14.0 | 13.4 | ||||
Four-membered arrangements are found in greater proportions in all the samples, and their formation is reported to be thermodynamically favorable [21]. With the addition of organoalkoxysilanes to the reaction, the percentage of six-membered siloxane units is expected to increase as a consequence of better accommodation of longer chains within the network [25]. For the evaluated samples, the proportion of these arrangements was significantly affected only by the APTES hybrid silica. The same was observed for the groups responsible for the increase in hydrophilicity in the silica network (%Si-Od) due to the interaction of silanol with the amino groups of the organosilane during synthesis.
When curcumin was incorporated into the matrices, a small disturbance in the formed silica network was observed, demonstrating that the condensation reactions were not significantly affected by the presence of these compounds at the studied concentrations.
The structural characteristics obtained from the FTIR spectra were correlated with the NMR 29Si spectra to better evaluate the role of the sol-gel route. The spectra corresponding to the silica matrices, as well as the sensor encapsulated in nonhybrid silica, are shown in Fig. 3, from which it was possible to identify the trifunctional (Ti) and tetrafunctional (Qi) silicon species indicated in Table 4, which generate signals with different chemical shifts in NMR spectra according to the degree of condensation in the molecules [26].
The presence of trifunctional species in the hybrid silicas confirmed the maintenance of the Si-C covalent bond after the hydrolysis of alkoxysilanes.
| Sample | Chemical shift (ppm) | |||||
|---|---|---|---|---|---|---|
| T1 | T2 | T3 | Q2 | Q3 | Q4 | |
| C0 | - | - | - | - | -102.6 | -112.9 |
| C1 | - | -54.5 | -63.3 | - | -102.2 | -112.0 |
| C8 | - | -56.9 | -64.9 | -92.1 | -101.5 | -110.9 |
| C18 | -56.0 | -65.8 | -70.3 | -94.0 | -103.4 | -113.8 |
| APTES | - | -68.2 | -75.2 | - | -109.5 | -118.5 |
| C0_C | - | - | - | -92.6 | -101.4 | -111.3 |
3.2 Textural analysis
Another important technique used in the characterization of hybrid silicas was nitrogen porosimetry. The N2 adsorption-desorption isotherms presented in Fig. 4 (a) reveal similarities between the samples. According to the IUPAC [27] classification, C0, C1, C8 and APTES silicas correspond to type IV(a), which defines them as mesoporous materials, while C18 is presented as a combination of type II and IV(a) isotherms; therefore, these silicas are essentially nonporous or macroporous solids containing mesopores on their surface. The hysteresis presented by the materials appears as a result of capillary condensation, and their shapes provide information about the pore structure. In the case of the sample produced only from TEOS, type H1 hysteresis indicates the existence of uniform mesopores, with a narrow range of size distributions. Hybrid silicas, on the other hand, present nonparallel lines for the adsorption and desorption phenomena as a result of the heterogeneity in the pores. Solids containing methyl, octyl and 3-aminopropylsilane groups exhibit H2(a) hysteresis due to pore blockage, while silica containing octadecylsilane exhibits H3 hysteresis due to macropores that are not completely filled during condensation.
When analyzing the isotherms of the encapsulated materials illustrated in Fig. 4 (b), it is observed that they follow the same IUPAC classification as the corresponding silica, therefore presenting the same nature and characteristics as the pore network of their matrices. This shows that the reactions were governed mainly by the alkoxysilanes present, corroborating the conclusions drawn from the analysis of the FTIR spectra, from which little influence was found from the encapsulated compounds on the network structure of the obtained silica.
The shapes of the sensors' hysteresis are also similar to those of their respective matrices, with the exception of samples encapsulated in C0 and C1 silicas, where the hysteresis becomes a combination of H2(a) and H5, the latter being associated with a structure containing both open or partially blocked pores [27], which is reported less frequently in the literature but can be explained by the presence of hybrid components within the network.
Based on the isotherms, it is also possible to observe the differences in the N2 adsorption capacities of these materials. The surface areas, calculated by the BET method, and the average pore width, determined by the BJH method, are presented in Table 5.
The introduction of organic chains to the silica network generally results in an increase in surface area, which can be related to the increase in the percentage of cyclic arrangements of six siloxane units, which results in a more porous structure [28]. However, the surface area of C18 silica is an exception due to the obstruction of its pores by the longest chain in the series.
Although curcumin did not significantly affect the structure of the siloxane network, its incorporation led to changes in surface area or in the porosity of the obtained solids, thus demonstrating the influence of the compounds on the adsorption capacity of the resulting solids.
| Sample | SBET [m² g− 1] | Average pore width (Å) | |||
|---|---|---|---|---|---|
| Control | Sensor | Control | Sensor | ||
| C0 | 298.7 | 350.6 | 74.1 | 61.0 | |
| C1 | 530.8 | 551.2 | 45.3 | 49.7 | |
| C8 | 452.6 | 472.2 | 44.1 | 44.7 | |
| C18 | 187.8 | 211.1 | 126.3 | 118.7 | |
| APTES | 376.9 | 298.9 | 53.3 | 55.1 | |
3.3 Particle size evaluation
The dynamic light scattering (DLS) technique was employed to evaluate the size and distribution of the silica particles. The results are presented in Table 6, where it is observed that only the organoalkoxysilane with polar groups affected the average particle size, indicating a disadvantage in nucleation caused by the presence of APTES groups. High polydispersity indices denote a broadening in the particle size distribution, ranging from a moderate (0.1 ≤ PDI ≤ 0.4) to a broad (PDI > 0.4) polydispersity distribution [29].
The same evaluation procedure was used for the sensors containing curcumin, although a direct relationship could not be established with the values obtained from the matrices due to the light absorption and fluorescence presented by the encapsulated compound, which led to possible deviations in the measurements. However, similar to silicas, Table 6 shows that the largest particles among all encapsulates were obtained for sensors containing organic polar groups.
The polydispersity indices were also quite high, indicating heterogeneity in the particle size distribution, with sedimentation of particles with larger diameters occurring in some cases.
| Sample | Average particle size (nm) | Polydispersity index, PDI | |||
|---|---|---|---|---|---|
| Control | Sensor | Control | Sensor | ||
| C0 | 705 | 823 | 0.493 | 0.303 | |
| C1 | 756 | 716 | 0.465 | 0.347 | |
| C8 | 747 | 799 | 0.327 | 0.568 | |
| C18 | 720 | 742 | 0.241 | 0.328 | |
| APTES | 2265 | 1550 | 0.546 | 0.367 | |
3.4 Curcumin distribution characterization
Confocal laser scanning microscopy was used to evaluate the distribution of curcumin in the silica matrices. This technique is based on the fluorescence emitted by molecules when excited using a laser of suitable wavelength and provides a z-axis scan. Figure 5 shows the resulting projections of several focal planes for all the sensors, through which a relatively homogeneous distribution of curcumin can be verified across the entire length of the grains. However, the fluorescence intensities could not be compared to verify the contents of this compound in each matrix since the laser power varied between the samples. Different intensities can be observed only within the grains of the same sample, indicating that some agglomerations of curcumin occurred mainly in the sample encapsulated in hybrid silica containing APTES groups.
3.5 Optical properties of the sensors
The immobilization of compounds in a solid matrix can cause a shift in the maximum absorption bands in the ultraviolet‒visible region as a result of interactions between the organic molecules and the solid support, such as the formation of hydrogen bonds, steric effects, surface acidity, and polarity of the medium [30]. Therefore, diffuse reflectance UV‒Vis spectroscopy was used to evaluate the behavior of curcumin encapsulated in silica and hybrid silicas. Figure 6 shows the results obtained after applying the Kubelka–Munk function (K/S) shown in Eq. 4, where the reflectance values (Rλ) obtained from the measurements were converted into values approximately proportional to the absorbance of the materials [31].
K and S are parameters related to absorption and light scattering, respectively.
The curcumin spectrum has a maximum absorption band in the visible region centered at 476 nm, attributed to the π-π* electronic transition [28]. After encapsulation in the silica matrix, hypsochromic shifts are observed, leading to an increase in the band gap, with the most pronounced effects being observed for matrices containing C18 and APTES groups. The results demonstrated the interaction of the compounds with the silica matrices in which they were immobilized.
3.6 Performance evaluation of the curcumin sensors
The performance of the encapsulated materials was evaluated in comparison to that of free curcumin. The color changes are shown in Fig. 7, as well as the reversibility of the sensors. Compared with those of free curcumin, the color variations in encapsulated curcumin samples are much clearer, although all the results demonstrate ΔE*ab values greater than 2, indicating that an inexperienced observer is able to perceive the change in color [19]. This difference is probably due to the greater diffusion of ammonia and water vapors through the pores of the samples, the hydrophilicity of the silica network and the high surface area allowing greater interaction with the indicator, although the sample encapsulated in a matrix bearing an octadecylsilane group exhibited the greatest color variation. However, this conclusion is supported by the fact that curcumin agglomerates inside the sensor grains containing 3-aminopropylsilane groups, as demonstrated by confocal laser scanning microscopy, resulting in a reduction in ΔE*ab compared to that of the other silica matrices.
Regarding the reversibility of the sensors, a large variation is noted in the first 24 hours of measurements, with most samples returning close to their original color, which in turn allows the sensors to be reused.
Sol-gel synthesis using ammonium fluoride as the catalyst has the great advantage of increasing the reaction speed compared to other routes explored in the literature. The evaluated procedure proved to be effective for the production of silica and hybrid silicas containing methyl, octyl, octadecyl and 3-aminopropylsilane groups, as shown by the asymmetric stretching band of siloxane in the FTIR spectra. The presence of trifunctional silicon species in hybrid silicas, demonstrated by NMR 29Si spectra, confirms this information. The introduction of these chains to the silica network also resulted in an increase in the specific area of the solid. The exception was the sample containing Group C18, which promoted pore blockage and a consequent reduction in nitrogen adsorption capacity.
When curcumin was incorporated into silica, a small disturbance in the percentage of cyclic siloxane arrangements was observed. Nitrogen porosimetry demonstrated that the pore structures remained similar to those of the respective matrices; however, there was significant variation in the porosities of these materials, with specific areas varying between 211.1 m²g− 1 and 551.2 m²g− 1. Furthermore, it was found that the encapsulation procedure in different matrices affects the maximum absorption bands in the ultraviolet‒visible region due to interactions established mainly with the aromatic rings of the compounds. The effectiveness of encapsulation was investigated by confocal laser scanning microscopy, through which it was found that curcumin was distributed relatively homogeneously in the grains, with the exception of those encapsulated in APTES hybrid silica, where agglomeration was observed inside the network.
The response of each sensor to changes in the pH of the medium also demonstrated a dependence on the matrix to which it was incorporated, where the encapsulation procedure improved the compound's performance, leading to a total variation in color up to 3.5 times greater than that obtained for the free compound.
The results presented confirmed the performance of curcumin encapsulated in silica and of hybrid silica containing methylsilane groups for ammonia detection, suggesting the potential for application in monitoring food freshness. This procedure is proposed as a fast and inexpensive approach for the development of natural pH sensors.
CRediT authorship contribution statement
P. C. Kazmarczak. Conceptualization, Methodology, Data curation, Writing - original draft. M. C. Cichero. Data curation. J. H. Z. dos Santos: Supervision, Writing - review & editing. Financing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments.
The authors thank CNPq (Project # 421181/2023-0) and FAPERGS (22/2551- 0000838-0) for financial support.
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No competing interests reported.