TYPE Original Research PUBLISHED 23 May 2023 DOI 10.3389/fchem.2023.1176661 Spiropyran-based chromic OPEN ACCESS hydrogels for CO2 absorption and EDITED BY Luis Manuel Frutos, University of Alcalá, Spain detection REVIEWED BY Priya Ranjan Sahoo, Arnau Marco, Gonzalo Guirado, Rosa María Sebastián* and University at Buffalo, United States Junji Zhang, Jordi Hernando* East China University of Science and Departament de Química, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, Spain Technology, China *CORRESPONDENCE Rosa María Sebastián, rosamaria.sebastian@uab.cat Jordi Hernando, By enabling rapid, cost-effective, user-friendly and in situ detection of carbon jordi.hernando@uab.cat dioxide, colorimetric CO2 sensors are of relevance for a variety of fields. However, RECEIVED 28 February 2023 it still remains a challenge the development of optical chemosensors for CO2 that ACCEPTED 10 May 2023 combine high sensitivity, selectivity and reusability with facile integration into solid PUBLISHED 23 May 2023 materials. Herein we pursued this goal by preparing hydrogels functionalized with CITATION spiropyrans, a well-known class of molecular switches that undergo different Marco A, Guirado G, Sebastián RM and Hernando J (2023), Spiropyran-based color changes upon application of light and acid stimuli. By varying the nature of chromic hydrogels for CO2 absorption the substituents of the spiropyran core, different acidochromic responses are and detection. obtained in aqueous media that allow discriminating CO2 from other acid gases Front. Chem. 11:1176661. doi: 10.3389/fchem.2023.1176661 (e.g., HCl). Interestingly, this behavior can be transferred to functional solid materials by synthesizing polymerizable spiropyran derivatives, which are used COPYRIGHT © 2023 Marco, Guirado, Sebastián and to prepare hydrogels. These materials preserve the acidochromic properties of the Hernando. This is an open-access article incorporated spiropyrans, thus leading to selective, reversible and quantifiable distributed under the terms of the color changes upon exposure to different CO2 amounts. In addition, CO2 Creative Commons Attribution License (CC BY). The use, distribution or desorption and, therefore, recovery of the initial state of the chemosensor is reproduction in other forums is favored by irradiation with visible light. This makes spiropyran-based chromic permitted, provided the original author(s) hydrogels promising systems for the colorimetric monitorization of carbon and the copyright owner(s) are credited and that the original publication in this dioxide in a diversity of applications. journal is cited, in accordance with accepted academic practice. No use, KEYWORDS distribution or reproduction is permitted which does not comply with these terms. spiropyran, hydrogel, CO2 detection, photochromism, acidochromism 1 Introduction Spiropyrans (SPs) are amongst the most preferred molecular switches for the preparation of smart materials (Berkovic et al., 2000; Klajn, 2014; Kortekaas and Browne, 2019; Rad et al., 2022). This is mainly due to their extremely versatile switching behavior, as SPs are sensitive to a large variety of external stimuli. On the one hand, they are well known to reversibly photoconvert from their colorless ring-open spiranic form (Sp) to their colored ring-closed merocyanine isomer (Mc) via C-O bond light-induced cleavage and C=C isomerization, a behavior that can also be promoted thermally, mechanically and by varying solvent polarity (Kortekaas and Browne, 2019). In addition, SPs also respond to protonation, metal complexation and redox-induced electrodimerization processes (Kumar et al., 2021; Santiago et al., 2021). As a result, they can be interconverted between different states with clearly distinguishable physico-chemical properties, a feature that can be exploited in a diversity of areas ranging from information storage and processing (Berkovic et al., 2000; Andréasson and Pischel, 2015; Xia et al., 2022) to (bio)imaging (Zhu et al., 2006; Zhu et al., 2014; Qi et al., 2017), polymer actuators (Florea et al., 2012; Li et al., 2020; Pandurangan et al., 2021), drug delivery (Jia et al., 2018; Fagan et al., 2021) and Frontiers in Chemistry 01 frontiersin.org Marco et al. 10.3389/fchem.2023.1176661 photocontrollable biological systems (Sakata et al., 2005; Andersson nitrospiropyrans in solution (Kortekaas et al., 2018; Kortekaas and et al., 2008; Bautista-Barrufet et al., 2014). Another relevant area of Browne, 2019). application of SPs is in (bio)sensing, where their multiresponsive To address these issues on route to the development of self- chromic and fluorochromic properties can be utilized to optically standing solid materials that could directly sense gaseous CO2 report on mechanical damage in polymer materials (Li et al., 2018; selectively, herein we propose an alternative strategy that relies Chen et al., 2021), temperature (Pietsch et al., 2011; Julià-López on two main design principles (Figure 1). First, acidochromic SPs et al., 2016), UV exposure (Leinen et al., 2020) and a plethora of will be incorporated into polymer hydrogels, which in contact with chemical analytes (e.g., metal ions, vapors, pH) (Quin et al., 2015; CO2 should intrinsically generate carbonic acid without the need of Prakash et al., 2016; Sahoo et al., 2018; Ali et al., 2020; Mandal et al., additional liquid media. As a result, the initial equilibrium mixture 2022). The latter is the interest of this work, where we present the of their colorless Sp and colored Mc isomers should convert into the development of novel SP-based materials for colorimetric CO2 corresponding protonated Mc form (McH), which exhibits a sensing. different measurable absorption. In this way, colorimetric CO2 Optical CO2 detection has been proposed as an alternative to detection could take place directly on the gas-gel interface. current commercial CO2 sensors based on IR spectroscopy, Second, separate hydrogels will be prepared that contain SPs with potentiometry and GC-MS, which might be instrumentally different acid-base properties–i.e., distinct pKa values for their complex, expensive and nonselective (Guo et al., 2012). By protonated McH state (pKa (McH)). Consequently, this should contrast, colorimetric and fluorometric methods for CO2 allow us discriminating the acidochromic signals measured for monitorization can provide smaller sizes, potentially low prices CO2 relative to other gases of differential acidity. and the capability for in situ analysis in a wide spectrum of applications (Guo et al., 2012; Xu et al., 2013; Xia et al., 2015; Lu et al., 2016)—e.g., greenhouse gas emission control (Shamsuzzaman 2 Results and discussion et al., 2021), food preservation (Puligundla et al., 2012), beverage fermentation (Shen et al., 2004) and work health and safety (Allen 2.1 Synthesis and acidochromic response of et al., 2016). However, it still remains a challenge the development of water-soluble spiropyrans optical chemosensors for CO2 that combine high sensitivity and selectivity with facile integration into solid materials (Chatterjee and To accomplish selective colorimetric CO2 sensing with SP- Sen, 2015; Guo et al., 2019). Herein we envisage to reach this based hydrogels, we aim to take advantage of the tunable objective by means of hydrogels functionalized with SP dyes. acidochromic behavior of spiropyrans depending on their To date the use of SPs for optical CO2 detection has been scarce substituents (Kortekaas et al., 2018; Berton et al., 2020; and mainly circumscribed to liquid solutions and suspensions. On Wimberger et al., 2021a; Berton et al., 2021; Wimberger et al., the one hand, the well-known interaction between carbon dioxide 2021b). As aqueous media saturated with CO2 present pH ~ 4 at and amidines in protic solvents has been used to induce color ambient conditions (Haghi et al., 2017), this means that large changes in SPs upon CO2 exposure in solution, either by variation of SP protonation and, therefore, maximal color changes incorporating amidine groups to their structure (Darwish et al., in our hydrogels when exposed to CO2 will only take place for 2011) or as external additives (Darwish et al., 2010; Boyd et al., 2013; spiropyran derivatives with pKa (McH) ~ 5–6. By contrast, if SP Shin et al., 2022). In a different approach, a mechanochromic SP was derivatives are used with pKa (McH) < 4.5, minor or no color anchored in polyacrylate nanoparticles with pending amino groups, changes should be observed in the hydrogels upon CO2 absorption, which swelled and changed their color when treated with CO2 in whereas a distinct acidochromic response would be triggered by liquid suspension (Li et al., 2016). However, no attempts to transfer other acid gases that are more soluble (and stronger) in water–e.g., these strategies to solid materials have been described, which is an HCl, formic acid and acetic acid. essential requirement to eventually accomplish sensing device To test this hypothesis, we decided to investigate the fabrication. To our knowledge, only two precedents of the acidochomic response in aqueous solution of three different incorporation of SPs in solid matrices for CO2 detection can be water-soluble SPs (SP1-SP3, Scheme 1A). SP1 and SP2 were found in the literature. In both reports, nitrospiropyran derivatives chosen because of their clearly different pKa (McH) values that were attached to organic polymer nanofibers, which underwent a should lead to distinct CO2-induced acidochromic responses: pKa yellow-to-red color variation when in contact with aqueous CO2 that (McH) ~ 4.7 and 6.3 in the dark for SP1 and SP2, respectively was ascribed to SP protonation (Shang et al., 2017; Dekhordi et al., (Wimberger et al., 2021a). In addition, SP1 resembles the 2021). But even in these cases, immersion in water (or previous nitrospiropyran dyes previously anchored to polymer fibers for wetting for prolonged times) of the SP-functionalized polymers was CO2 detection in aqueous media (Shang et al., 2017; Dekhordi needed for CO2 detection, as it requires carbonic acid formation. In et al., 2021). As for SP3, it is a novel SP derivative that was addition, no proof was given about the sensing selectivity of these designed to further increase pKa (McH) relative to SP2. For this materials for CO2 relative to other gaseous or dissolved acids (e.g., purpose, two electro-donating methoxy groups were introduced to HCl), which are also known to promote the acidochromic response the 6 and 8 positions of the benzopyran moiety of the compound, of nitrospiropyrans within polymer matrices (Nam et al., 2014; which should further destabilize the negative charge of the phenolate Genovese et al., 2015; Genovese et al., 2017; Rad et al., 2020; Guo moiety in the Mc isomer and, therefore, decrease the acidity of its et al., 2021; Xie et al., 2022). In those cases, the formation of a yellow protonated McH form. protonated SP species was instead observed upon acid exposure, For the synthesis of SP3, we followed the classical route for SP which is in better agreement with the well-established behavior of preparation, which required condensation of 2- Frontiers in Chemistry 02 frontiersin.org Marco et al. 10.3389/fchem.2023.1176661 FIGURE 1 Strategy devised in this work for the sensing of gaseous CO2 with SP-based hydrogels. CO2-induced protonation of the initial Sp-Mc equilibrium mixture should only take place for hydrogels functionalized with SPs exhibiting high pKa (McH). By contrast, other stronger and more water-soluble acid gases such as HCl should induce McH formation for all acidochromic SPs. SCHEME 1 (A) Structures of water-soluble acidochromic SP1-SP3. (B) Synthesis of SP3. Frontiers in Chemistry 03 frontiersin.org Marco et al. 10.3389/fchem.2023.1176661 choline hydroxide (ChOH) yielded the target spiropyran SP3 in a 24% overall yield. The UV-vis absorption spectra of SP1-SP3 were initially measured both in water at neutral pH (pH = 7, Figure 2) and in a polar nonaqueous solvent (MeOH, see Supplementary Figure S10). In both media, several different absorption bands were registered both in the UV and visible regions. As previously described for similar compounds (Kortekaas et al., 2018; Wimberger et al., 2021a), these bands can be mainly attributed to the coexistence of two different species in equilibrium: (a) their initial spiranic colorless Sp state, which shows maximum absorption at λmax ~ 300 (SP2) and 340 nm (SP1, SP3); and (b) their open Mc isomer exhibiting a longer conjugation path, which results in visible light absorption at λmax ~ 380 and 510 nm (SP1), 380 and 530 nm (SP2), and 410 and 590 nm (SP3). As a result, both the aqueous and MeOH solutions of SP1-SP3 presented pink-red (SP1, SP2) and blue (SP3) coloration. As expected, the intensity of their coloration at equal concentration–i.e., the absorbance ratio between Mc and Sp bands–increased in water at pH = 7 relative to MeOH. This can be ascribed to the larger stabilization of the charged phenolate moiety of Mc in aqueous media relative to organic solvents, which displaces the Sp-Mc equilibrium towards the formation of the colored ring-open isomer (Wimberger et al., 2021a). The absorption spectra of SPs can be even further complicated by their photo- and acidochromic properties. Thus, irradiation of aqueous solutions of SP1 and SP2 with both UV and visible light in water resulted in reversible solution decoloration due to Mc → Sp photoconversion for both compounds (Wimberger et al., 2021a) (see Supplementary Figures S11A, B). Surprisingly, such effect was not observed for SP3 neither via steady-state nor time-resolved absorption measurements, which suggests that it is a nonphotoswitchable spiropyran (see Supplementary Figure S11C). A similar result was reported for other SP derivatives bearing strong mesomeric electron-donating groups (Balmond et al., 2016), which we tentatively attribute to the occurrence of photoinduced intramolecular charge transfer processes between the electron- FIGURE 2 poor indolium and electron-rich benzopyran rings upon (A) UV-vis absorption spectra of SP1 (c = 1.7 10−5 M) in water at irradiation that efficiently compete with photoisomerization. Of pH = 7 and after addition of HCl(g) and CO2(g) until saturation. (B,C) more interest for our work is the acidochromic behavior of SP1- UV-vis absorption of SP2 (c = 4.7 10−5 M) and SP3 (c = 3.6 10−5 M) in water at pH = 7, pH = 9 and after addition of HCl(g) and CO2(g) SP3. In the case of SP1, no protonation effects should be expected in until saturation. In all the cases, HCl(g) or CO2(g) flows were bubbled water at pH = 7 due to its low pKa (McH) ~ 4.7 (Wimberger et al., through the solutions until no more absorption changes were 2021a). By contrast, as pKa (McH) ~ 6.3 for SP2 (Wimberger et al., observed. 2021a), a fraction of their molecules must be protonated to form the corresponding McH state in neutral aqueous media. This was easily proven by base addition until pH = 9, which resulted in a decrement of the absorption at ~400–500 nm ascribed to McH (Figure 2B) - hydroxybenzaldehyde 4 with indolium 5 (Scheme 1B, see i.e., it led to full deprotonation of the system, which was converted Supplementary Figures S1–S9). While the latter was obtained as into a pure equilibrium mixture of the Sp and Mc isomers of SP2. previously reported (Shi et al., 2011), a new synthetic route had to be Interestingly, a similar result was observed for SP3 upon basification devised to prepare 4. It started with the 2-bromination of (Figure 2C). Clearly, this result suggests that the pKa (McH) of commercially available 3,5-dimetoxybenzaldehyde (1) with SP3 lies closer to that of SP2 and, therefore, that it gives rise to a N-bromosuccinimide (NBS), which was followed by protection of mixture of Sp, Mc and McH molecules upon dissolution in water the aldehyde group as a cyclic acetal. The resulting intermediate 3 at pH = 7. was reacted with n-BuLi and B(OBu)3 to yield a boronic ester that To get a deeper insight on the acidochromic behavior of SP1- could then be oxidized to the corresponding alcohol in the presence SP3, the pH-induced variation of its UV-vis absorption spectra in of hydrogen peroxide. Further acid treatment for aldehyde water was carefully monitored in the dark (Figure 3A and deprotection furnished the desired benzaldehyde 4. Finally, Supplementary Figure S12). From this data, the pKa (McH) of condensation of this compound with indolium 5 mediated by new compound SP3 was determined to be 6.17, while the values Frontiers in Chemistry 04 frontiersin.org Marco et al. 10.3389/fchem.2023.1176661 FIGURE 3 (A) Variation of the UV-vis absorption of SP3 in water with pH. Measurements were conducted by dissolving SP3 (c = 3.6 10−5 M) in independent buffer solutions of pH = 1.13, 1.59, 2.13, 2.64, 3.08, 3.61, 4.13, 4.72, 5.32, 5.78, 6.39, 6.84, 7.18, 7.56, 8.08, 8.69, 8.81, 9.57, 10.23 and 10.71. (B) pH-induced variation of the absorbance of SP1-SP3 in aqueous media at λ = 505 (SP1), 425 (SP2) and 585 nm (SP3). Symbols correspond to experimental data, while lines are fits to Eq. 1 for pKa (McH) determination (see Section 4.4). found for SP1 (pKa (McH) = 4.63) and SP2 (pKa (McH) = 6.23) 500 nm fully disappeared, while intense absorption bands raised reproduced those already described in the literature (Wimberger with λmax ~ 430 nm (Figure 2). As a result, all the solutions turned et al., 2021a) (Figure 3B and Supplementary Table S1). Although from pink-red (SP1-SP2) or blue (SP3) to yellow (SP1-SP2) or they characterize the acid-base behavior of SP1-SP3 in the dark in orange (SP3), which is in agreement with the complete protonation aqueous media, it must be noted that the experimentally determined of SP molecules to yield McH - i.e., high acid media were produced pKa (McH) values are not the intrinsic pKa constants of the McH by HCl dissolution whose pH must be even significantly lower than state of SP1-SP3 - i.e., the acidity constants of the pure McH-Mc the pKa (McH) of SP1. On the other hand, different acidochromic ionization process (Berton et al., 2020; Wimberger et al., 2021a). responses were obtained when the aqueous solutions of SP1-SP3 This is due to the fact that, upon deprotonation of McH, its were saturated with CO2(g). For SP2 and SP3 with larger pKa (McH), conjugate base Mc establishes a further thermal equilibrium with absorption changes similar to those observed with HCl(g) were the spiranic Sp state that also affects the acid-base process (see registered - i.e., their nearly full protonation to produce the Supplementary Scheme S1). By proper analysis of this equilibrium corresponding yellow- (SP2) and orange-colored (SP3) McH by 1H NMR or UV-vis spectroscopy, the intrinsic acidity constants molecules. However, this was not the case for SP1 having a more of McH can also be determined (Berton et al., 2020; Wimberger acidic McH state. For this compound, the McH absorption spectrum et al., 2021a), which in our case were found to be pKa,intrinsic (McH) = was not retrieved after CO2 exposure, which indicates that 4.82, 7.03 and 6.66 for SP1, SP2 and SP3, respectively (see insufficiently acid pH values were reached as to promote large SP Supplementary Table S1). protonation. Instead, other CO2-induced colorimetric changes were According to our results, the newly synthesized SP3 presents observed for SP1 in water: a variation of the relative intensity of the similar pKa (McH) and pKa,intrinsic (McH) values to SP2 and we Sp and Mc absorption bands, which suggests a change in the could not make its McH form even less acidic. However, two composition of the Sp-Mc mixture after CO2 dissolution and can important differences were observed for the acidochromic be related with the previous reports on CO2 detection using polymer behavior of SP3 relative to SP2 that could be of interest for fibers functionalized with similar nitrospiropyrans (Shang et al., future works. First, when dissolved in aqueous media of variable 2017; Dekhordi et al., 2021). Overall, these results validate our pH, SP3 reaches its thermal Sp-Mc-McH equilibrium state very strategy of exploiting SPs of contrasting pKa (McH) values to rapidly at room temperature (<1 min), while it takes much longer discriminate the acidochromic responses caused by different gases. for SP2 (up to 15 min at pH ~ 6). This indicates that the thermal ring-opening and ring-closing processes are significantly faster for SP3. Second, the acidochromic properties of SP3 are not affected by 2.2 Synthesis and acidochromic response of light irradiation, as the photoinduced isomerization of its McH form spiropyran-based hydrogels is also inhibited. This is in contrast with SP1 and SP2, whose McH state photoisomerizes upon irradiation to yield a more acidic cis- To transfer the acidochromic behavior of SPs to solid materials McH form (see Supplementary Figures S11D, F and Supplementary that could be applied to CO2 detection directly from the gas phase, Scheme S1). we devised the utilization of SP-loaded hydrogels (HGs). Because of Because of the differences in pKa (McH) between SP1 and SP2- the similar acidochromic properties of SP2 and SP3, only the use of SP3, their aqueous solutions should differently respond to acid gases SP2 was considered for the preparation of HGs with high pKa of distinct solubility and strength in water. On the one hand, when (McH), as it is more synthetically accessible than SP3. In addition, HCl(g) was bubbled to water solutions of SP1-SP3, similar changes SP1-based HGs were also targeted as low pKa (McH) materials. In all were observed in all the cases: the typical Mc absorption band at λ > these cases, we decided to covalently anchor SPs to the hydrogel Frontiers in Chemistry 05 frontiersin.org Marco et al. 10.3389/fchem.2023.1176661 SCHEME 2 Synthesis of SP-loaded acrylamide hydrogels HG1 and HG2 using spiropyran-acrylate conjugates SP1 and SP2. acrylamide HGs (Gombert et al., 2020), scanning electron microscopy analysis of HG1 and HG2 after freeze-drying revealed their hollow pore structure (Supplementary Figure S15A). As for their mechanical properties, they were characterized by dynamic mechanical analysis (DMA), which provided very similar values of storage modulus (G′) and loss modulus (G″) for both types of gels: G’ ~ 700 Pa and G” ~ 15 Pa (Supplementary Figure S15B). The large G”/G’ ratio determined (~50) is consistent with the chemical gel-like nature of HG1 and HG2, whose covalent cross-linking polymer phase confers a predominant elastic behavior over a rather large linear viscoelastic range (up to 10% strain; Supplementary Figure S15C). Thanks to the low spiropyran loading, HG1 and HG2 were sufficiently translucent as to be characterized by UV-vis absorption spectroscopy in transmission mode. Figure 4A shows the UV-vis absorption spectra of freshly prepared HG1 and HG2 at room temperature, which resemble those measured for SP1 and SP2 in water at pH = 7. In both cases, we observed the presence of a clear absorption band at λmax ~ 520 (HG1) and 550 nm (HG2), which can be unambiguously ascribed to the fraction of spiropyran molecules in the hydrogels that are in their Mc state. As a result, both materials FIGURE 4 presented a pink coloration similar to that of diluted aqueous (A) UV-vis absorption spectra of freshly prepared HG1 and solutions of SP1 and SP2 at neutral pH (Figure 4B). In addition, HG2 under ambient conditions. (B) Photographs of pieces of the HG1 and HG2 preserved the photochromic properties of their hydrogels measured in (A). constituting spiropyrans, as they nearly turned colorless upon irradiation with green light due to reversible light-induced Mc → Sp conversion (see Supplementary Figure S16). However, this behavior was observed to be lost after storage of the hydrogels matrix, thus aiming to prevent their migration and loss during for several weeks under ambient conditions, whose color evolved to manipulation and use. For this, we followed reported procedures for lighter pink. Most probably, this was due to SP hydrolysis inside the the incorporation of SPs in acrylamide HGs (Li et al., 2020), which hydrogels, which is known to take place in neutral aqueous media required the previous synthesis of two spiropyran-acrylate for the unprotonated Mc form (Hammarson et al., 2013; Berton conjugates: SP4 (He et al., 2017) and SP5 (Li et al., 2020), which et al., 2020; Wimberger et al., 2021a). present the same spiropyran dyes as SP1 and SP2, respectively Next, we evaluated the acidochromic behavior of HG1 and (Scheme 2 and Supplementary Figures S13, S14). These HG2 when immersed in aqueous buffers of different pH. Similar compounds were then added to DMSO:water 4:1 solutions of responses to those registered for SP1 and SP2 in free solution were acrylamide and bisacrylamide monomers, which were subjected registered for the hydrogels, which were mainly characterized by the to radical polymerization in glass containers. The resulting gels emergence of the typical McH absorption band at λmax ~ 430 nm were subsequently treated with MeOH to remove unreacted species upon acidification (Figure 5A and Supplementary Figure S17) - and immersed in water to obtain the target hydrogels HG1 (SP4) and i.e., they turned yellow at sufficiently acid pH values. Importantly, HG2 (SP5) with a thickness of ~ 5 mm, a swelling ratio of ca. 27 and we could use this data to determine the pKa (McH) in the dark of the a SP content of ~ 1.2 10−6 wt.% (Scheme 2). As reported for other spiropyrans loaded in HG1 and HG2: pKa (McH) = 4.40 and 5.75, Frontiers in Chemistry 06 frontiersin.org Marco et al. 10.3389/fchem.2023.1176661 FIGURE 5 (A) Variation of the UV-vis absorption of HG2. Measurements were conducted by immersing HG2 in independent buffer solutions of pH = 3.96, 4.45, 4.87, 5.53, 6.01, 6.57, 7.04, 7.46, 8.12 and 8.62. (B) pH-induced variation of the absorbance of HG1 and HG2 in aqueous media at λ = 520 (HG1) and 430 nm (HG2). Symbols correspond to experimental data, while lines are fits to Eq. 1 for pKa (McH) determination (see Section 4.4). respectively (Figure 5B). These values are slightly lower than those different acids. In addition, they demonstrate that SP-based found for SP1 and SP2 in water, which must be attributed to the hydrogels permit direct colorimetric detection of gaseous CO2 absence of the negatively charged sulfonate group for the without the need of any prior treatment, thus enhancing the spiropyrans anchored to the hydrogels. In particular, it has been measurement conditions described in previous reports based on demonstrated for a series of N-substituted SPs that such group polymer fibers functionalized with nitrospiropyran derivatives stabilizes the nearby positive charge of the indolium ring of Mc and (Shang et al., 2017; Dekhordi et al., 2021). Moreover, it must also McH, thus increasing pKa (McH) in the dark (Wimberger et al., be noted that, in opposition to these precedents, the colorimetric 2021a). In spite of this, the SP acidity constants of the two hydrogels response to CO2 of our hydrogels does agree with the reported prepared still lay on the desired ranges to achieve selective CO2 behavior of SPs upon protonation in aqueous solution. detection: (a) pKa (McH) < 4.5 for HG1, which should make it nearly Encouraged by these positive results, we further investigated the insensitive to CO2 exposure, and (b) pKa (McH) ~ 5-6 for HG2, chromic detection of CO2 with HG1 and HG2. For this, their UV-vis which should allow for CO2-induced colorimetry with optimal absorption spectra were measured upon exposure to controlled sensitivity. Actually, as the pH detection window of acidochromic amounts of CO2 (from 0 to 100 mL at a regular flux of compounds typically lies ~ 1.5 units around their pKa value 5 mL min-1). Importantly, no differences were observed when (Steinegger et al., 2020), HG2 should be sensitive to changes these experiments were conducted under ambient light or in the within the pH ~ 4.25–7.25 interval. This essentially matches the dark, thus indicating that the acidochromic responses of the gels are dynamic range needed for CO2 determination in water at room not affected by low intensity illumination. This, in combination with temperature and atmospheric pressure, which spans from pH ~ the fact that the spiropyran molecules confined in HG1 and HG2 are 4.0 to 7.0 (Haghi et al., 2017). not exposed to other chemical stimuli that could promote color changes (e.g., metal ions, solvent variation), should make their chromic behavior essentially report on CO2 interaction–i.e., on 2.3 CO2 absorption and detection with CO2 absorption into the gels and subsequent acidification of spiropyran-based hydrogels their aqueous liquid phase. In fact, steady spectral variations were registered for HG2 in these experiments that are compatible with To demonstrate that the distinct acidochromic properties of CO2-induced continuous spiropyran protonation: the absorption HG1 and HG2 allow selective detection of CO2, we first investigated band of Mc at λmax ~ 550 nm progressively disappeared, whereas the their colorimetric behavior when fully saturated with fluxes of two absorbance of McH at λmax ~ 430 nm concomitantly raised different acid gases: CO2 and HCl. As observed in Figure 6A, only (Figure 6B). By contrast, negligible absorption changes were HG2 was found to respond to CO2 exposure and suffer the expected measured for HG1 under the same conditions. This is clearly pink-to-yellow color change - i.e., the protonation of its spiropyran illustrated by Figure 6C, where the variation of the absorbance at moieties into the yellow-colored McH form. By contrast, the red- λmax (McH) with the flowed CO2 volume is plotted for both pink color of HG1 remained almost unaltered, which indicates that HG1 and HG2. Aside from the large differential response of both no (or minor) spiropyran protonation took place due to its lower hydrogels to CO2, two additional conclusions can be inferred from pKa (McH). As expected, a different situation was encountered upon this figure. First, it must be noted that CO2-induced color changes in treatment with a stronger and more water-soluble gas such as HCl. HG2 occurred very rapidly (less than 1 min), thus demonstrating the In this case, both HG1 and HG2 underwent the color variation capacity of our system to quickly sense CO2 concentration changes associated to McH formation, which demonstrates that lower in the surrounding atmosphere. However, saturation of the pH values were reached within the hydrogels (Figure 6A). colorimetric response required large CO2 volumes, probably Interestingly, these results prove the viability of our strategy to because of the time needed for the gas to diffuse throughout the discriminate between the acidochromic responses caused by whole system and reach all the embedded SP molecules (~ several Frontiers in Chemistry 07 frontiersin.org Marco et al. 10.3389/fchem.2023.1176661 FIGURE 7 (A) UV-vis absorption of HG2 measured at the following conditions: (a) after preparation; (b) after saturation with gaseous CO2; (c) after exposure of the CO2-saturated hydrogel to a mild flux of N2 for 40 min; and (d) after irradiation of the CO2-saturated hydrogel at 405 nm for 5 min (B) Variation of the absorbance of HG2 at λ = 430 nm when subjected to 5 consecutive cycles of CO2 saturation and N2-induced CO2 desorption. FIGURE 6 (A) Images of freshly prepared HG1 and HG2 before and after completed. Alternatively, we aimed to accelerate CO2 desorption by exposure to gaseous CO2 or HCl fluxes until no more changes in color were observed. (B) Variation of the UV-vis absorption spectrum of exposing HG2 to a mild N2 flux. As observed in Figure 7A, this also HG2 when subjected to different times of exposure to a regular allowed the absorbance of the initial CO2-free hydrogel to be flux of gaseous CO2 (5 mL min-1). (C) Variation of the absorbance of recovered, though it still required rather long times (~40 min). HG1 and HG2 at λ = 430 nm when subjected to different times of exposure to a regular flux of gaseous CO2 (5 mL min-1). Error bars As a result, nonnegligible loss of the water content of the show the standard errors of the mean for 5 replicates measured in hydrogel was observed to take place during CO2 desorption, different hydrogels. For each exposure time, the concentration of CO2 which resulted in a visible change in the rugosity and elasticity of absorbed within the gel was estimated to be 0.10, 0.15, 0.23, 0.70, 1.04, 1.44, 5.49, 116.34, 485.17 and 2023.29 mg L-1. the material that could not be fully recovered by subsequent immersion in water. Unfortunately, this situation was aggravated when HG2 was subjected to consecutive cycles of CO2 absorption (~20 min) and N2-induced CO2 desorption (~40 min), which had a minutes). Second, a rather linear response with CO2 exposure detrimental effect on the amplitude of the colorimetric response volume was found, which could be used to conduct quantitative obtained (Figure 7B). In spite of this, clear CO2-induced pink-to- CO2 measurements. Actually, based on our previous determination yellow color changes could still be registered for HG2 after 5 cycles of the pH-dependent variation of the absorption spectra of HG2, the of reuse. Surprisingly, we also observed that the recovery of CO2-induced color changes measured for this sample could be HG2 after CO2 detection could be largely accelerated upon correlated to the amount of CO2 dissolved within the gel irradiation with high intensity light-emitting diodes (LEDs). In (Supplementary Figure S18). According to this data, we could particular, illumination at 405 nm of the McH species in the estimate that the CO2 detection range of HG2 spans from 0 to hydrogel after CO2 absorption led to a complete loss of their 500 mg L-1 with a detection limit of 0.10 mg L-1. absorption signal at λmax ~ 430 nm after just 5 min. Importantly, Finally, we explored the reversibility of the CO2-promoted the Mc absorbance at λmax ~ 550 nm was simultaneously recovered colorimetric response of HG2. In a first attempt, we just exposed in a large extent, thus confirming that McH illumination did not just a CO2-saturated hydrogel to ambient air, which eventually resulted lead to cis-McH formation in our hydrogel by photoisomerization in CO2 release and the recovery of its initial UV-vis absorption but also to deprotonation to recover the initial Sp-Mc mixture. spectrum with no presence of McH absorbance. However, this Therefore, illumination allows faster recyclability of HG2 for process was observed to be slow, as it took about 120 min to be subsequent reuse. Frontiers in Chemistry 08 frontiersin.org Marco et al. 10.3389/fchem.2023.1176661 3 Conclusion each type of gel were analyzed independently to determine the average storage and loss moduli. In this work we reported a new strategy for exploiting the acidochromic properties of spiropyrans to accomplish selective colorimetric detection of gaseous CO2 with solid materials. For 4.2 Synthesis of spiropyran derivatives this, we first demonstrated that chemical tailoring of the acidity SP1-SP5 constant of SPs allows discriminating the color changes caused in aqueous media by CO2 from other acid gases. Thus, only SPs whose 4.2.1 Synthesis of SP1, SP2, SP4 and SP5 protonated merocyanine form shows a sufficiently high pKa in the Spiropyrans SP1 (Liu et al., 2019), SP2 (Shi et al., 2011), SP4 (He dark (pKa (McH) ~ 5–6) led to ample colorimetric responses when et al., 2017) and SP5 (Li et al., 2020) were prepared according to exposed to CO2 in water solution. By contrast, more water-soluble reported procedures. SP1 and SP2 were isolated as the and stronger acids such as HCl also triggered similar color variations corresponding triethylammonium salts. For all of these for SPs with lower pKa (McH) (pKa (McH) < 4.5). To transfer this compounds (and SP3), NMR characterization was performed in behavior to the solid state, we prepared hydrogels functionalized nonaqueous solvents, where they exist as the colorless spiranic form. with SPs of distinct acid-base properties, whose pKa (McH) values SP1: 1H NMR (360 MHz, CD3OD) δ = 8.09 (d, J = 2.8 Hz, 1H), 8.02 were proven to be essentially preserved inside these materials. As a (dd, J = 9.0, 2.7 Hz, 1H), 7.13 (td, J = 7.7, 1.3 Hz, 1H), 7.10–7.03 (m, 2H), result, differential colorimetric responses could be promoted for 6.84–6.75 (m, 2H), 6.70 (d, J = 7.8 Hz, 1H), 6.02 (d, J = 10.4 Hz, 1H), these hydrogels when treated with CO2 relative to HCl, thus allowing 3.43–3.26 (m, 2H), 3.20 (q, J = 7.3 Hz, 6H), 2.92–2.74 (m, 2H), 2.19–2.02 selective optical detection of carbon dioxide. In addition, we (m, 2H), 1.30 (t, J = 7.3 Hz, 9H), 1.27 (s, 3H), 1.18 (s, 3H) ppm. demonstrated that the CO2-induced color changes in the SP2: 1H NMR (360 MHz, CD3OD) δ = 7.04–6.91 (m, 4H), 6.83 hydrogels are fast, quantifiable and reversible, while they can be (d, J = 10.2 Hz, 1H), 6.75–6.64 (m, 2H), 6.58–6.50 (m, 2H), 5.70 (d, directly measured at the gas-solid interface without the need of J = 10.3 Hz, 1H), 3.32–3.08 (m, 2H), 2.97 (q, J = 7.2 Hz, 6H), immersion or pretreatment with water. In light of these properties, 2.83–2.64 (m, 2H), 2.11–1.90 (m, 2H), 1.17 (s, 3H), 1.16 (t, J = our spiropyran-based hydrogels appear as promising systems for the 7.3 Hz, 9H), 1.06 (s, 3H) ppm. colorimetric detection of CO2 in a wide range of applications. SP4: 1H NMR (300 MHz, CDCl3): δ = 8.11–7.97 (m, 2H), 7.23 (td, J1 = 7.7 Hz, J2 = 1.3 Hz, 1H), 7.12 (ddd, J1 = 7.3 Hz, J2 = 1.3 Hz, J3 = 0.5 Hz, 1H), 6.98–6.86 (m, 2H), 6.77 (dt, J1 = 8.3 Hz, J2 = 0.8 Hz, 4 Materials and methods 1H), 6.75–6.70 (m, 1H), 6.09 (dq, J1 = 1.6 Hz, J2 = 1.0 Hz, 1H), 5.90 (d, J = 10.3 Hz, 1H), 5.59 (qt, J = 1.6 Hz, 1H), 4.32 (dd, J1 = 6.6 Hz, 4.1 Materials and characterization methods J2 = 6.0 Hz, 2H), 3.58 (dt, J1 = 15.1 Hz, J2 = 6.6 Hz, 1H), 3.45 (dt, J1 = 15.1 Hz, J2 = 6.0 Hz, 1H), 1.94 (dd, J1 = 1.6 Hz, J2 = 1.0 Hz, 3H), 1.30 Commercial reagents were used as received. Anhydrous THF was (s, 3H), 1.19 (s, 3H) ppm. used after column drying in a solvent dispenser from Innovative SP5: 1H NMR (300 MHz, CDCl3): δ = 7.21 (tt, J1 = 7.7 Hz, J2 = technology (PureSolv-MD-2). Acetonitrile was dried with molecular 1.3 Hz, 1H), 7.17–7.04 (m, 3H), 6.92–6.82 (m, 3H), 6.71 (dd, J1 = sieves, 3 A beads, 4-8 mesh from Sigma-Aldrich. Reactions were 7.9 Hz, J2 = 2.6 Hz, 2H), 6.11 (dq, J1 = 1.6 Hz, J2 = 1.0 Hz, 1H), 5.72 monitored by analytical thin-layer chromatography (TLC), using (d, J = 10.1 Hz, 1H), 5.58 (qt, J = 1.6 Hz, 1H), 4.33 (t, J = 6.4 Hz, 2H), silica gel 60 F254 precoated aluminium plates (0.20 mm thickness), 3.69–3.58 (m, 1H), 3.43 (dt, J1 = 15.1 Hz, J2 = 6.2 Hz, 1H), 1.95 (s, and development was made using a UV lamp at 254 nm. Flash column 2H), 1.33 (s, 3H), 1.18 (s, 3H) ppm. chromatography was performed using silica gel (230–400 mesh). NMR spectra were recorded on Bruker DPX360 (360 MHz for 1H 4.2.2 Synthesis of SP3 NMR), Bruker Ascend 300 MHz (300 MHz for 1H NMR) and Bruker 4.2.2.1 Synthesis of 2-bromo-3,5- Ascend 400 MHz (400 MHz for 1H NMR) spectrometers, using the dimethoxybenzaldehyde (2) residual peaks of the deuterated solvents as a reference. IR-ATR spectra A mixture of 2.012 g of 3,5-dimethoxybenzaldehyde were recorded in a Bruker Tensor 27 Golden Gate spectrometer with a (12.11 mmol, 1) and 2.222 g of N-bromosuccinimide diamond tip. Mass spectra were recorded in a Bruker Esquire 3,000+ (12.48 mmol) in 20 mL of anhydrous acetonitrile was stirred at spectrometer using ESI and APCI. UV-Vis absorption spectra were room temperature overnight. 60 mL of aqueous NaOH were added recorded on an Agilent HP 8453 spectrophotometer using HPLC quality and the mixture was extracted with ethyl acetate (30 mL) three solvents and 1 cm quartz cuvettes. To photoisomerize compounds, LEDs times. The combined organic phases were dried over anhydrous at 405 nm and 532 nm and UV lamps at 312 nm and 365 nm were used. Na2SO4 and the solvent was removed under vacuum to yield 2.849 g pH was measured using a Hach sensION mm340 pHmeter. For CO2 of 2 (11.62 mmol; 96% yield) as a pale powder. Its 1H-NMR capture tests, a Bronkhorst EL-FLOW flow meter was used and spectrum was in accordance with bibliographical data (Liao et al., controlled by FlowDDE and FlowView software. Scanning electron 2012). 1H NMR (300 MHz, CDCl3): δ = 10.44 (s, 1H), 7.07 (d, J = microscopy analysis of the hydrogels was conducted with a FEI 2.8 Hz, 1H), 6.74 (d, J = 2.8 Hz, 1H), 3.94 (s, 3H), 3.88 (s, 3H) ppm. Quanta 650 ESEM microscope applying a 5 kV voltage. Prior to SEM imaging, hydrogels were freeze-dried and metalized with a layer 4.2.2.2 Synthesis of 2-(2-bromo-3,5-dimethoxyphenyl)- of about 5 nm of Pt. Dynamic mechanical analysis of HG1 and HG2 was 1,3-dioxolane (3) conducted by means of strain sweep measurements using a Discovery In a two necked bottom flask equipped with a Dean-Stark HR-2 rheometer at room temperature (frequency = 1 Hz). 5 samples of apparatus, a solution of 3.063 g of 2 (12.50 mmol), 6.6 mL of Frontiers in Chemistry 09 frontiersin.org Marco et al. 10.3389/fchem.2023.1176661 ethylene glycol (118.3 mmol) and 0.157 g of p-toluene sulfonic acid 2 mL of a 40% solution of choline hydroxide in water and were (0.82 mmol) in 120 mL of toluene was refluxed overnight. Next, the heated at 80°C for 1 h. The solvent was removed under vacuum and reaction was poured into 100 mL of a NaOH solution 3 M and the the residue was purified through flash column chromatography with mixture was extracted twice with 100 mL of CH2Cl2. Once the CHCl3:MeOH 9:1 to yield a red solid, which was dissolved in MeOH, organic phase was dried with anhydrous Na2SO4 and the solvent Et3N was added until the solution was clear and the solvent was removed under vacuum, purification was conducted through flash removed under vacuum to yield 0.383 g of the triethylammonium column chromatography with chloroform to yield 3.082 g of 3 salt of SP3 (0.70 mmol; 39% yield) as a dark solid. 1H NMR (300 MHz, (10.66 mmol; 85% yield). Its 1H-NMR spectrum was in DMSO-d6) δ = 7.11–7.01 (m, 2H), 6.89 (d, J = 10.2 Hz, 1H), 6.71 (td, J = accordance with bibliographical data (Huang et al., 2011). 1H 7.4, 0.9 Hz, 1H), 6.60 (d, J = 7.7 Hz, 1H), 6.42 (d, J = 2.8 Hz, 1H), 6.37 (d, NMR (300 MHz, CDCl3): δ = 6.82 (d, J = 2.8 Hz, 1H), 6.53 (d, J = 2.8 Hz, 1H), 5.73 (d, J = 10.2 Hz, 1H), 3.69 (s, 3H), 3.61 (s, 3H), J = 2.8 Hz, 1H), 6.15 (s, 1H), 4.21–4.14 (m, 2H), 4.14–4.06 (m, 2H), 3.25–3.08 (m, 2H), 2.85 (br, 6H), 2.45–2.33 (m, 2H), 1.83 (qt, J = 7.6 Hz, 3.90 (s, 3H), 3.85 (s, 3H) ppm. 2H), 1.16 (s, 3H), 1.12–1.01 (m, 12H) ppm; 13C NMR (75 MHz, DMSO-d6) δ = 153.15, 147.85, 147.59, 137.49, 136.49, 129.62, 4.2.2.3 Synthesis of 2-hydroxy-3,5- 127.77, 121.95, 121.40, 119.20, 118.53, 106.62, 104.11, 102.61, 101.45, dimethoxybenzaldehyde (4) 55.95, 55.82, 52.15, 49.73, 46.18, 42.83, 26.35, 25.44, 20.09, 10.21 ppm; A solution of 1.030 g of 3 (3.56 mmol) in 25 mL of anhydrous IR (ATR, cm-1): 2,966, 2,687, 2,363, 2,342, 1,587, 1,481, 1,393, 1,355, THF was purged with argon and cooled in an acetone bath at −78°C. 1,288, 1,240, 1,213, 1,198, 1,158, 1,109, 1,017, 981, 948, 827, 808, 746, 2.4 mL of a 1.6 M solution of n-BuLi (3.84 mmol) in hexane were 681, 655; HRMS (ESI) m/z: [M+H+Na]+ calcd for C23H27NNaO6S+ added and the solution was stirred for 5 min at −78°C. Then, 2 mL of 468.1451; found 468.1434. B(OBu)3 (7.41 mmol) were added, the cold bath was removed, and the solution was stirred for 1 h. Afterwards, 10 mL of water and 4 mL of 30% H2O2 were added, and the solution was stirred for 4.3 Synthesis of SP-based hydrogels 30 min. The reaction was quenched in a cold bath by addition of a HG1-HG2 solution of 10.076 g of Na2S2O3 in 15 mL of water, and then the crude was then extracted with Et2O, the organic phase was dried The synthesis of spiropyran-based hydrogels was carried with anhydrous Na2SO4, and the solvent removed under vacuum. according to a reported procedure (Li et al., 2020). Hydrogels The resulting oil was redissolved with 40 mL of acetone and 40 mL were prepared by dissolving 142.2 mg (2 mmols) of acrylamide, of HCl 1M, and the solution was stirred for 2 h. The crude was first 12.4 mg (80 μmol) of N,N-methylenebisacrylamide, 1.2 μmol of extracted with AcOEt. Then, the organic phase was extracted with each polymerizable spiropyran (SP4 or SP5) and 9.1 mg NaOH 1M and concentrated HCl was added to the resulting (40 μmol) of ammonium persulfate in 2.2 mL of a 4:1 DMSO/ aqueous phase in a cold bath until some turbulence appeared water mixture. Then, 9 μL (60 μmol) of (acid pH). Finally, the acid aqueous phase was extracted with tetramethylethylenediamine were added at once to start the CHCl3 and the solvent removed under vacuum. The solid polymerization at room temperature for 2 h. The resultant gel obtained was further purified through flash column was soaked in MeOH and then water overnight to obtain the chromatography with chloroform to yield 0.492 g of 4 final gel which was cut into small pieces for optical characterization. (2.70 mmol; 75% yield) as a yellow solid. Its 1H-NMR spectrum was in accordance with bibliographical data (Liao et al., 2012). 1H NMR (300 MHz, CDCl3): δ = 10.70 (s, 1H), 9.91 (s, 1H), 6.77 (d, J = 4.4 Characterization of the acidochromic 2.8 Hz, 1H), 6.61 (d, J = 2.8 Hz, 1H), 3.92 (s, 3H), 3.85 (s, 3H) ppm. properties of SP1-SP3 and HG1-HG2 4.2.2.4 Synthesis of 3-(2,3,3-trimethyl-3H-indol-1-ium-1- The pH-induced variation of the UV-vis absorption spectra of SP1- yl)propane-1-sulfonate (5) 3 were measured in buffer solutions using 1-cm quartz cuvettes. Buffer A solution of 0.992 g of 2,3,3-trimethylindolenine (6.23 mmol) solutions were prepared by adding HCl (35 wt.%) to a K3PO4 (100 mM) and 0.835 g of 1,3-propanesultone (6.84 mmol) in 5 mL of toluene solution while measuring the pH of the final sample. Then, 2 mL of the was refluxed under argon atmosphere for 12 h. The crude was cooled corresponding buffer were mixed with 20 μL of a stock solution of the in an ice bath and the precipitated was filtered and then dissolved in SP of interest (c ~ 10–3 M) in a 1-cm quartz cuvette; then, the resulting water. The resulting solution was extracted with diethyl ether twice mixture was left to equilibrate for different times before the final and the solvent removed under vacuum to yield 1.203 g of 5 absorption spectrum in the dark was registered (up to 2 h for SP1, (4.28 mmol; 69% yield). Its 1H-NMR spectrum was in 5–15 min for SP2 and < 1 min for SP3). To investigate the light effect on accordance with bibliographical data (Shi et al., 2011). 1H NMR the acidochromic behavior, irradiation at 405 nm for 5 min was (360 MHz, CDCl3): δ = 7.96 (d, J = 7.0 Hz, 1H), 7.62-7-50 (m, 3H), conducted before measuring the absorbance spectrum. 5.00 (t, J = 7.8 Hz, 2H), 3.08–3.02 (m, 5H), 2.40 (br, 2H), 1.61 (s, For HG1-2, pH-dependent UV-vis absorption measurements 6H) ppm. were performed in air using a custom-made metallic T-shaped cuvette where a piece of the gels was sandwiched between two 4.2.2.5 Synthesis of SP3 metal plates having a 0.5 cm-in-diameter opening to allow the light SP3 was prepared following a procedure reported for the beam to go through the sample. Before their measurement, the gels synthesis of other spiropyrans (Pargaonkar et al., 2018). 0.329 g were submerged in 5 mL of the buffer solution of interest in a glass of 4 (1.80 mmol) and 0.502 g of 5 (1.80 mmol) were dissolved in vial and left to equilibrate for 90 min. Frontiers in Chemistry 10 frontiersin.org Marco et al. 10.3389/fchem.2023.1176661 In order to determine the pKa (McH) values of SP1-SP3 and Author contributions HG1-HG2, their absorbance at selected wavelengths (505 nm for SP1, 425 nm for SP2, 585 nm for SP3, 520 nm for HG1, and 420 nm AM conducted the experimental work and analysis. GG, RS, and for HG2) were plotted against the pH of the buffer solutions used JH designed the experiments and supervised the work. All authors and the resulting plots were fitted to Equation 1 (Berton et al., 2020; listed have made a substantial, direct, and intellectual contribution Wimberger et al., 2021a). to the work and approved it for publication. Aac − Abas Aeq  Abas + (1) 1 + exppH−pKPa (McH) Funding In this equation, Aeq is the absorbance measured at each pH, Abas is the absorbance at the most basic pH, Aac is the absorbance at This work was supported by grants PID2019-106171RB-I00 the most acidic pH and P is a parameter of the fitting. (funded by MCIN/AEI/10.13039/501100011033 and ERDF–"A way of making Europe"), TED2021-130797B-IOO (funded by MCIN/AEI/10.13039/501100011033 and “NextGenerationEU”/ 4.5 Acid gas detection tests with HG1-HG2 PRTR) and 2021 SGR 00064 and 2021 SGR 00052 (funded by AGAUR, Generalitat de Catalunya). To investigate the colorimetric response of HG1-HG2 to different acid gases, pieces of the gels were introduced in closed vials where the gases of interest were injected. CO2(g) (>99.995% Acknowledgments purity) was introduced using to different ways: (a) in a controlled manner with a flow meter (5 mL min-1) for defined times, or (b) AM thanks the Generalitat de Catalunya (AGAUR) for his injecting a balloon filled with CO2. HCl(g) was introduced by predoctoral FI fellowship. bubbling a solution of concentrated HCl with N2. When full saturation of the hydrogels with CO2(g) or HCl(g) was desired, samples were left to equilibrate for 30 min so that gases could Conflict of interest completely diffuse inside the gels. In all the cases, UV-vis absorption measurements were conducted in air as described in The authors declare that the research was conducted in the Section 4.4. To revert back the acidochromic response from HG1- absence of any commercial or financial relationships that could be HG2, the gels were left in open air for about 120 min, exposed to a N2 construed as a potential conflict of interest. atmosphere in a closed vial for about 40 min, or irradiated with an LED at 405 nm (51 mW cm-2) for 5 min. For HG2, the CO2 concentration absorbed within the gel for each exposure time Publisher’s note was estimated from: (a) the pH dependence of the absorption spectra of HG2; (b) the acidity constants of CO2 (pKa1 = 6.36) All claims expressed in this article are solely those of the authors and and HCO3− (pKa2 = 10.25) in pure water at 25°C and atmospheric do not necessarily represent those of their affiliated organizations, or pressure and the autoionization constant of water at these those of the publisher, the editors and the reviewers. Any product that conditions (pKw = 14). may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher. Data availability statement Supplementary material The raw and processed data required to reproduce these findings is available in the CORA.RDR, Research Data Repository (https:// The Supplementary Material for this article can be found online dataverse.csuc.cat/): https://doi.org/10.34810/data727 (accessed on at: https://www.frontiersin.org/articles/10.3389/fchem.2023.1176661/ 16 May 2023). full#supplementary-material References Ali, A. A., Kharbash, R., and Kim, Y. (2020). Chemo- and biosensing applications of spiropyran Balmond, E. I., Tautges, B. K., Faulkner, A. L., Or, V. W., Hodur, B. M., Shaw, J. T., et al. and its derivatives - a review. Anal. Chim. Acta 1110, 199–223. doi:10.1016/j.aca.2020.01.057 (2016). Comparative evaluation of substituent effect on the photochromic properties of spiropyrans and spirooxazines. J. Org. Chem. 81, 8744–8758. doi:10.1021/acs.joc.6b01193 Allen, J. G., Macnaughton, P., Satish, U., Santanam, S., Vallarino, J., and Spengler, J. D. (2016). Associations of cognitive function scores with carbon dioxide, ventilation, and volatile organic Bautista-Barrufet, A., López-Gallego, F., Rojas-Cervellera, V., Rovira, C., Pericàs, M. compound exposures in office workers: A controlled exposure study of green and conventional A., Guisán, J. M., et al. (2014). Optical control of enzyme enantioselectivity in solid office environments. Environ. Health Perspect. 124, 805–812. doi:10.1289/ehp.1510037 phase. ACS Catal. 4, 1004–1009. doi:10.1021/cs401115s Andersson, J., Li, S., Lincoln, P., and Andréasson, J. (2008). Photoswitched DNA- Berkovic, G., Krongauz, V., and Weiss, V. (2000). Spiropyrans and spirooxazines for binding of a photochromic spiropyran. J. Am. Chem. Soc. 130, 11836–11837. doi:10. memories and switches. Chem. Rev. 100, 1741–1754. doi:10.1021/cr9800715 1021/ja801968f Berton, C., Busiello, D. M., Zamuner, S., Solari, E., Scopelliti, R., Fadaei-Tirani, F., Andréasson, J., and Pischel, U. (2015). Molecules with a sense of logic: A progress et al. (2020). Thermodynamics and kinetics of protonated merocyanine photoacids in report. Chem. Soc. Rev. 44, 1053–1069. doi:10.1039/C4CS00342J water. Chem. Sci. 11, 8457–8468. doi:10.1039/D0SC03152F Frontiers in Chemistry 11 frontiersin.org Marco et al. 10.3389/fchem.2023.1176661 Berton, C., Busiello, D. M., Zamuner, S., Scopelliti, R., Fadaei-Tirani, F., Severin, K., system in aqueous media. J. Electroanal. Chem. 903, 115845. doi:10.1016/j.jelechem. et al. (2021). Light-switchable buffers. Angew. Chem. Int. Ed. 60, 21737–21740. doi:10. 2021.115845 1002/anie.202109250 Leinen, M. B., Klein, P., Sebastian, F. L., Zorn, N. F., Adamczyk, S., Allard, S., et al. Boyd, A. R., Jessop, P. G., Dust, J. M., and Buncel, E. (2013). Switchable polarity (2020). Spiropyran-functionalized polymer–carbon nanotube hybrids for dynamic solvent (SPS) systems: Probing solvatoswitching with a spiropyran (SP)–merocyanine optical memory devices and UV Sensors. Adv. Electron. Mater. 6, 2000717. doi:10. (MC) photoswitch. Org. Biomol. Chem. 11, 6047–6055. doi:10.1039/c3ob41204k 1002/aelm.202000717 Chatterjee, C., and Sen, A. (2015). Sensitive colorimetric sensors for visual detection Li, M., Lei, L., Zhang, Q., and Zhu, S. (2016). CO2-breathing induced reversible of carbon dioxide and sulfur dioxide. J. Mater. Chem. A 3, 5642–5647. doi:10.1039/ activation of mechanophore within microgels. Macromol. Rapid Commun. 37, 957–962. c4ta06321j doi:10.1002/marc.201600119 Chen, Y., Mellot, G., van Luijk, D., Creton, C., and Sijbesma, R. P. (2021). Li, M., Zhang, Q., Zhou, Y.-N., and Zhu, S. (2018). Let spiropyran help polymers feel Mechanochemical tools for polymer materials. Chem. Soc. Rev. 50, 4100–4140. force. Prog. Polym. Sci. 79, 26–39. doi:10.1016/j.progpolymsci.2017.11.001 doi:10.1039/d0cs00940g Li, C., Iscen, A., Palmer, L. C., Schatz, G. C., and Stupp, S. I. (2020). Light-driven Darwish, T. A., Evans, R. A., James, M., Malic, M., Triani, G., and Hanley, T. L. (2010). expansion of spiropyran hydrogels. J. Am. Chem. Soc. 142, 8447–8453. doi:10.1021/jacs. CO2 triggering and controlling orthogonally multiresponsive photochromic systems. 0c02201 J. Am. Chem. Soc. 132, 10748–10755. doi:10.1021/ja1013322 Liao, Y.-R., Kuo, P.-C., Liang, J.-W., Shen, Y.-C., and Wu, T.-S. (2012). An efficient Darwish, T. A., Evans, R. A., James, M., and Hanley, T. L. (2011). total synthesis of benzocamphorin H and its anti-inflammatory activity. Tetrahedron Spiropyran–amidine: A molecular canary for visual detection of carbon dioxide gas. Lett. 53, 6202–6204. doi:10.1016/j.tetlet.2012.08.138 Chem. Eur. J. 17, 11399–11404. doi:10.1002/chem.201101723 Liu, J., Tang, W., Sheng, L., Du, Z., Zhang, T., Su, X., et al. (2019). Effects of Dekhordi, T. F., Shirin-Abadi, A. R., Karimipour, K., and Mahdavian, A. R. (2021). substituents on metastable-State photoacids: Design, synthesis, and evaluation of their CO2-electric potential-and photo-switchable-hydrophilicity membrane (x-SHM) as an photochemical properties. Chem. Asian J. 14, 438–445. doi:10.1002/asia.201801687 efficient color-changeable tool for oil/water separation. Polymer 212, 123250. doi:10. Lu, W., Xiao, P., Gu, J., Zhang, J., Huang, Y., Huang, Q., et al. (2016). Aggregation- 1016/j.polymer.2020.123250 induced emission of tetraphenylethylene-modified polyethyleneimine for highly Fagan, A., Bartkowski, M., and Giordani, S. (2021). Spiropyran-based drug delivery selective CO2 detection. Sensors Actuators B Chem. 228, 551–556. doi:10.1016/j.snb. systems. Front. Chem. 9, 720087. doi:10.3389/fchem.2021.720087 2016.01.096 Florea, L., Diamond, D., and Benito-López, F. (2012). Photo-responsive polymeric Mandal, M., Banik, D., Karak, A., Manna, S. K., and Mahapatra, A. K. (2022). structures based on spiropyran. Macromol. Mater. Eng. 297, 1148–1159. doi:10.1002/ Spiropyran−merocyanine based photochromic fluorescent probes: Design, mame.201200306 synthesis, and applications. ACS Omega 7, 36988–37007. doi:10.1021/acsomega. 2c04969 Genovese, M. E., Athanassiou, A., and Fragouli, D. (2015). Photoactivated acidochromic elastomeric films for on demand acidic vapor sensing. J. Mater. Nam, Y.-S., Yoo, I., Yarimaga, O., Park, I. S., Park, D.-H., Song, S., et al. (2014). Chem. A 3, 22441–22447. doi:10.1039/c5ta06118k Photochromic spiropyran-embedded PDMS for highly sensitive and tunable optochemical gas sensing. Chem. Commun. 50, 4251–4254. doi:10.1039/ Genovese, M. E., Caputo, G., Nanni, G., Setti, C., Bustreo, M., Perotto, G., et al. (2017). c4cc00567h Light responsive silk nanofibers: An optochemical platform for environmental applications. ACS Appl. Mater. Interfaces 9, 40707–40715. doi:10.1021/acsami.7b13372 Pandurangan, K., Barrett, R., Diamond, D., and McCaul, M. (2021). Fluidic platforms incorporating photo-responsive soft-polymers based on spiropyran: From green Gombert, Y., Roncoroni, F., Sánchez-Ferrer, A., and Spencer, N. D. (2020). The synthesis to precision flow control. Front. Mater. 7, 615021. doi:10.3389/fmats.2020. hierarchical bulk molecular structure of poly(acrylamide) hydrogels: Beyond the fishing 615021 net. Soft Matter 16, 9789–9798. doi:10.1039/d0sm01536a Pargaonkar, J. G., Patil, S. K., and Vajekar, S. N. (2018). Greener route for the Guo, Z., Song, N. R., Moon, J. H., Kim, M., Jun, E. J., Choi, J., et al. (2012). A synthesis of photo- and thermochromic spiropyrans using a highly efficient, reusable, benzobisimidazolium-based fluorescent and colorimetric chemosensor for CO2. J. Am. and biocompatible choline hydroxide in an aqueous medium. Synth. Commun. 48, Chem. Soc. 134, 17846–17849. doi:10.1021/ja306891c 208–215. doi:10.1080/00397911.2017.1395053 Guo, S., Zhang, H., Lu, X., Xiao, H., and Zhao, Y. (2019). Sensing carbon dioxide Pietsch, C., Schubert, U. S., and Hoogenboom, R. (2011). Aqueous polymeric sensors through a solution transparency change in gas-responsive polymers. J. Mater. Chem. C based on temperature-induced polymer phase transitions and solvatochromic dyes. 7, 4049–4056. doi:10.1039/c8tc05346d Chem. Commun. 47, 8750–8765. doi:10.1039/c1cc11940k Guo, J., Wei, X., Fang, X., Shan, R., and Zhang, X. (2021). A rapid acid vapor detector Prakash, K., Sahoo, P. R., and Kumar, S. (2016). A substituted spiropyran for highly based on spiropyran-polymer composite. Sens. Act. B Chem. 347, 130623. doi:10.1016/j. sensitive and selective colorimetric detection of cyanide ions. Sens. Actuators B Chem. snb.2021.130623 237, 856–864. doi:10.1016/j.snb.2016.06.170 Haghi, R. K., Chapoy, A., Peirera, L. M. C., Yang, J., and Tohidi, B. (2017). pH of CO2 Puligundla, P., Jung, J., and Ko, S. (2012). Carbon dioxide sensors for intelligent food saturated water and CO2 saturated brines: Experimental measurements and modelling. packaging applications. Food control. 25, 328–333. doi:10.1016/j.foodcont.2011.10.043 Int. J. Greenh. Gas. Control 66, 190–203. doi:10.1016/j.ijggc.2017.10.001 Qi, Q., Li, C., Liu, X., Jiang, S., Xu, Z., Lee, R., et al. (2017). Solid-state photoinduced Hammarson, M., Nilsson, J. R., Li, S., Beke-Somfai, T., and Andréasson, J. (2013). luminescence switch for advanced anticounterfeiting and super-resolution imaging Characterization of the thermal and photoinduced reactions of photochromic spiropyrans applications. J. Am. Chem. Soc. 139, 16036–16039. doi:10.1021/jacs.7b07738 in aqueous solution. J. Phys. Chem. B 117, 13561–13571. doi:10.1021/jp408781p Quin, M., Huang, Y., Li, F., and Song, Y. (2015). Photochromic sensors: A versatile He, D., Arisaka, Y., Masuda, K., Yamamoto, M., and Takeda, N. (2017). A approach for recognition and discrimination. J. Mater. Chem. C 3, 9265–9275. doi:10. photoresponsive soft interface reversibly controls wettability and cell adhesion by 1039/c5tc01939g conformational changes in a spiropyran-conjugated amphiphilic block copolymer. Acta Biomater. 51, 101–111. doi:10.1016/j.actbio.2017.01.049 Rad, J. K., Ghomi, A. R., Karimipour, K., and Mahdavian, A. R. (2020). Progressive readout platform based on photoswitchable polyacrylic nanofibers containing Huang, X.-R., Pan, X.-H., Lee, G.-H., and Chena, C. (2011). C2-symmetrical spiropyran in photopatterning with instant responsivity to acid−base vapors. bipyridyldiols as promising enantioselective catalysts in Nozaki–Hiyama allylation. Macromolecules 53, 1613–1622. doi:10.1021/acs.macromol.9b02603 Adv. Synth. Catal. 353, 1949–1954. doi:10.1002/adsc.201100023 Rad, J. K., Balzade, Z., and Mahdavian, A. R. (2022). Spiropyran-based advanced Jia, S., Fong, W.-K., Graham, B., and Boyd, B. J. (2018). Photoswitchable molecules in photoswitchable materials: A fascinating pathway to the future stimuli-responsive long-wavelength light-responsive drug delivery: From molecular design to applications. devices. J. Photochem. Photobiol. C Photochem. Rev. 51, 100487. doi:10.1016/j. Chem. Mater. 30, 2873–2887. doi:10.1021/acs.chemmater.8b00357 jphotochemrev.2022.100487 Julià-López, A., Hernando, J., Ruiz-Molina, D., González-Monje, P., Sedó, J., and Sahoo, P. R., Prakash, K., and Kumar, S. (2018). Light controlled receptors for heavy Roscini, C. (2016). Temperature-controlled switchable photochromism in solid metal ions. Coord. Chem. Rev. 357, 18–49. doi:10.1016/j.ccr.2017.11.010 materials. Angew. Chem. Int. Ed. 55, 15044–15048. doi:10.1002/anie.201608408 Sakata, T., Yan, Y., and Marriott, G. (2005). Optical switching of dipolar interactions Klajn, R. (2014). Spiropyran-based dynamic materials. Chem. Soc. Rev. 43, 148–184. on proteins. Proc. Natl. Acad. Sci. U. S. A. 102, 4759–4764. doi:10.1073/pnas. doi:10.1039/c3cs60181a 0405265102 Kortekaas, L., and Browne, W. R. (2019). The evolution of spiropyran: Fundamentals Santiago, S., Giménez-Gómez, P., Muñoz-Berbel, X., Hernando, J., and Guirado, G. and progress of an extraordinarily versatile photochrome. Chem. Soc. Rev. 48, (2021). Solid multiresponsive materials based on nitrospiropyran-doped ionogels. ACS 3406–3424. doi:10.1039/C9CS00203K Appl. Mater. Interfaces 13, 26461–26471. doi:10.1021/acsami.1c04159 Kortekaas, L., Chen, J., Jacquemin, D., and Browne, W. R. (2018). Proton-stabilized Shamsuzzaman, M., Shamsuzzoha, A., Maged, A., Haridy, S., Bashir, H., and Karim, A. photochemically reversible E/Z isomerization of spiropyrans. J. Phys. Chem. B 122, (2021). Effective monitoring of carbon emissions from industrial sector using statistical process 6423–6430. doi:10.1021/acs.jpcb.8b03528 control. Appl. Energy 300, 117352. doi:10.1016/j.apenergy.2021.117352 Kumar, A., Sahoo, P. R., Prakash, K., Arya, Y., and Kumar, S. (2021). Light controlled Shang, J., Lin, S., and Theato, P. (2017). Fabrication of color changeable CO2 sensitive dimerization of spiropyran as a tool to achieve dual responsive capture and release nanofibers. Polym. Chem. 8, 7446–7451. doi:10.1039/c7py01628j Frontiers in Chemistry 12 frontiersin.org Marco et al. 10.3389/fchem.2023.1176661 Shen, H.-Y., De Schrijver, S., Moonjai, N., Verstrepen, K. J., Delvaux, F., and Delvaux, Xia, G., Liu, Y., Ye, B., Sun, J., and Wang, H. (2015). A squaraine-based colorimetric F. R. (2004). Effects of CO2 on the formation of flavour volatiles during fermentation and F− dependent chemosensor for recyclable CO2 gas detection: Highly sensitive with immobilised brewer’s yeast. Appl. Microbiol. Biotechnol. 64, 636–643. doi:10.1007/ off–on–off response. Chem. Commun. 51, 13802–13805. doi:10.1039/c5cc04755b s00253-003-1523-0 Xia, H., Ding, Y., Gong, J., Lilienkampf, A., Xie, K., and Bradley, M. (2022). Shi, Z., Peng, P., Strohecker, D., and Liao, Y. (2011). Long-lived photoacid based upon Programmable and flexible fluorochromic polymer microarrays for information a photochromic reaction. J. Am. Chem. Soc. 133, 14699–14703. doi:10.1021/ja203851c storage. ACS Appl. Mater. Interfaces 14, 27107–27117. doi:10.1021/acsami.2c02242 Shin, C. H., Hong, K. I., Lee, J. H., and Jang, W.-D. (2022). Multimodal stimuli- Xie, J., Zhang, J., Ma, Y., Han, Y., Li, J., and Zhu, M. (2022). Spiropyran-incorporated responsive behaviors of photochromic spiropyran-bearing telechelic poly(2- honeycomb porous films with reversible multistimuli-responsive properties. J. Mater. isopropyl-2-oxazoline). Mater. Today Chem. 24, 100884. doi:10.1016/j.mtchem. Chem. C 10, 7154–7166. doi:10.1039/d2tc00749e 2022.100884 Xu, Q., Lee, S., Cho, Y., Kim, M. H., and Yoon, J. (2013). Polydiacetylene-based Steinegger, A., Wolfbeis, O. S., and Borisov, S. M. (2020). Optical sensing and imaging colorimetric and fluorescent chemosensor for the detection of carbon dioxide. J. Am. of pH values: Spectroscopies, materials, and applications. Chem. Rev. 120, 12357–12489. Chem. Soc. 135, 17751–17754. doi:10.1021/ja410557x doi:10.1021/acs.chemrev.0c00451 Zhu, M.-Q., Zhu, L., Han, J. J., Wu, W., Hurst, J. K., and Li, A. D. Q. (2006). Wimberger, L., Andréasson, J., and Beves, J. E. (2021a). Basic-to-acidic reversible Spiropyran-based photochromic polymer nanoparticles with optically switchable pH switching with a merocyanine photoacid. Chem. Commun. 58, 5610–5613. doi:10. luminescence. J. Am. Chem. Soc. 128, 4303–4309. doi:10.1021/ja0567642 1039/d2cc00805j Zhu, M.-Q., Zhang, G.-F., Hu, Z., Aldred, M. P., Li, C., Gong, W.-L., et al. (2014). Wimberger, L., Prasad, S. K. K., Peeks, M. D., Andréasson, J., Schmidt, T. W., and Reversible fluorescence switching of spiropyran-conjugated biodegradable Beves, J. E. (2021b). Large, tunable, and reversible pH changes by merocyanine nanoparticles for super-resolution fluorescence imaging. Macromolecules 47, photoacids. J. Am. Chem. Soc. 143, 20758–20768. doi:10.1021/jacs.1c08810 1543–1552. doi:10.1021/ma5001157 Frontiers in Chemistry 13 frontiersin.org