Visualizing Reversible Uranyl (UO₂ ²⁺) Capture and Release With Self‐Indicating Porous Organic Polymers

Abstract

Seawater uranium extraction is a promising sustainable energy pathway; however, practical application requires advanced adsorbents that combine reversible, high uranium‐uptake capacity with the ability to track sorption cycles. Herein, we report self‐indicating porous organic polymers (POPs), Sal‐POP and Im‐POP, containing uranyl (UO₂ ²⁺) ion‐coordinating salphen and o‐hydroxy imine cores with UO₂ ²⁺ binding energies up to −71.94 kcal mol⁻¹ as uranium adsorbents, and demonstrate their reversible, high uranium extraction capacities up to 0.114 g g⁻¹ h⁻¹. Enabled by the extended conjugation of salphen unit and strong UO₂ ²⁺ binding, Sal‐POP readily changes color at uranium saturation from yellow to black and allows visual tracking of uranium adsorption/desorption cycles. With a high density of UO₂ ²⁺ binding sites, both Sal‐POP and Im‐POP demonstrate rapid, reversible, and high uranium uptake capacity, with distribution coefficients (K _d) of 1.88 × 10⁵ and 2.94 × 10⁵ mL g⁻¹, and removal efficiencies up to 70% and 78%. The uranyl capture is highly reversible, and the POP color changes from black to yellow upon simple elution with Na₂CO₃, thus acting as a self‐indicating POP adsorbent.

Uranium capture from seawater is a promising sustainable strategy, and the adsorbents showing high uptake capacity and sorption trackability are essential. The work describes self‐indicating porous organic polymers that exhibit visible color changes during uranium adsorption and desorption cycles allow tracking of uranium‐free, saturated states without the need for sophisticated tools, and are suitable for real‐world practical deployment.

1. Introduction

Nuclear energy is widely regarded as a promising low‐carbon power source, and the vast uranium reserves (ca. 4.5 billion tons) in seawater make it a truly sustainable alternative for future energy demands [1, 2, 3, 4, 5]. In contrast to current industrial uranium ore mining and fuel processing methods, which inevitably risk releasing uranium into soil and aquatic environments, seawater extraction is a promising alternative [6, 7]. In seawater, uranium exists as a uranyl (UO₂ ²⁺) carbonate at ultradilute concentrations (ca. 3–4 ppb) [8]; thus, the seawater extraction requires adsorbents that possess high binding energy and are selective to UO₂ ²⁺ [9, 10, 11], particularly in the presence of common competing ions such as vanadium (V⁵⁺) [12, 13, 14, 15, 16]. Moreover, adsorbent materials that exhibit reversible visual color changes during uranium capture and release cycles are advantageous as they allow real‐time tracking of the uranium sorption process, circumvent the need for sophisticated tools, and are suitable for practical applications.

Porous organic polymers (POPs) [17, 18, 19] have recently emerged as a highly promising platform for both uranium detection and capture [20, 21]. Unlike metal–organic frameworks [22, 23, 24], POPs offer undoubtedly high chemical stability through robust C─C/C─N linkages and a rigid aromatic skeleton, enabling reliable performance under harsh conditions (acidic, basic, saline) [25, 26, 27]. POPs possess high accessible surface areas and a rigid, three‐dimensional porous structure that facilitates rapid uranyl transport and superior capture efficiencies [28, 29, 30]. Furthermore, their structural tunability allows integration of chromophoric units for intense color changes, while the extended π‐conjugation renders an amplified transduction signal response, even for ultratrace chemical perturbations [31, 32, 33, 34, 35]. Relying on high selectivity of uranyl ions to amidoxime (AO) in the presence of a complex matrix of interfering ions of seawater, several AO‐POPs have been reported for uranium capture from seawater; however, high‐temperature synthesis and laborious postsynthetic modification raise the cost of the process, and also the pore blockage by large AO groups restricts the access of uranyl cations to the bulk of the POPs and limit uptake [36, 37, 38, 39]. In this context, POPs containing intrinsic uranyl‐binding motifs avoid pore occlusion or extra steps, and postsynthetic modifications are significant for efficient UO₂ ²⁺ capture but require a rational design strategy [40, 41, 42].

Here, we report self‐indicating POP uranium adsorbents, Sal‐POP and Im‐POP, containing salphen and o‐hydroxy imine uranyl binding units for rapid, reversible uranyl (UO₂ ²⁺) extraction and exhibit visible color changes during the uranium capture‐release cycles essential for practical deployment (Scheme 1). Both POPs exhibit strong coordination interactions with uranyl cations and showed exceptionally high uranium uptake capacities of 0.114 g g⁻¹ within 1 h under a 10 ppm UO₂ ²⁺ concentration of simulated seawater. The UO₂ ²⁺ uptake capacities have significantly increased with increasing UO₂ ²⁺ concentration, with the highest uptake of ca. 1.05 g g⁻¹ d⁻¹ UO₂ ²⁺ at 100 ppm in 24 h of POPs. These UO₂ ²⁺ uptake values are higher than those reported to date (Scheme 1). Such high uptakes were also corroborated with the high K _d and removal efficiencies of 1.88 × 10⁵, 2.94 × 10⁵ mL g⁻¹ and 70% and 78% for Sal‐POP and Im‐POP, respectively. Of the POPs, Sal‐POP also displays a striking reversible colorimetric change (yellow ⇌ black) during uranium adsorption–desorption cycles due to uranyl binding and subsequent changes in the optical absorption of the POP. Such complexation was confirmed by the Stern–Volmer quenching analysis at a lower concentration of UO₂ ²⁺. Both POPs are easily regenerated after uranium saturation by simple elution with Na₂CO₃ and retain high uptake capacity over multiple cycles.

SCHEME 1.

Top: illustration of self‐indicating uranium adsorbing POP that allows tracking of the initiation of uranium adsorption (by fluorescence turn off), saturation (by colorimetric changes), and the desorption process (both colorimetric and fluorescence turn on). Bottom: comparison of uranyl uptake properties of Sal‐POP and Im‐POP with previously reported POPs.

2. Results and Discussion

2.1. Structural Characterization

Both POPs, Sal‐POP and Im‐POP, were synthesized via a Schiff‐base condensation, yielding high‐density salphen and o‐hydroxy imine coordinating sites within a three‐dimensional porous network for UO₂ ²⁺ capture. Sal‐POP and Im‐POP were synthesized by the condensation reaction of M2 with naphthalene‐2,3‐diamine (M1) or p‐phenylenediamine (M3), respectively, in a solvent mixture of n‐butanol/o‐dichlorobenzene under solvothermal conditions (Figure 1 and Scheme S1) (see Supporting Information for details). The successful formation of imine‐linked porous networks was initially supported by Fourier transform infrared (FT‐IR) spectroscopy. Sal‐POP showed C═N stretching vibration (ν _C═N) at 1610 cm⁻¹ and a broad band at 3046 and 3592 cm⁻¹ corresponding to stretching vibration of OH (ν _O─H) [43]. Similarly, Im‐POP showed a C═N stretching vibration (ν_C═N) at 1616 cm⁻¹, indicating the formation of the POPs via the imine linkages (Figures 2a, S1, and S2) [44]. Moreover, both POPs showed no carbonyl or amine stretching vibrations related to monomers, indicating the absence of any monomer impurities in the polymers. Formation of the POPs was further strengthened by solid‐state ¹³C CP/MAS NMR, as shown in Figure 2b. Sal‐POP and Im‐POP displayed signals at 160 and 159 ppm, corresponding to imine (C═N) carbon, confirming the formation of the imine linkage. Similarly, both POPs showed signals at 152–156 and 144–148 ppm that correspond to phenolic carbons. The other peaks in the 105–136 ppm range are ascribed to aromatic carbons of POPs [13, 31, 45]. X‐ray photoelectron spectroscopy (XPS) offered additional insight into the bonding environments of the POPs. For Sal‐POP, the C 1s spectrum was fitted into multiple peaks with binding energies of 284.30, 285.24, and 286.49 eV, corresponding to the C═C, C═N, and phenolic C─O bonding environment (Figure 2c). The relative percentage of C, N, and O atom are 76.36%, 6.07%, and 17.57%, respectively, indicating a high density of accessible O/N‐containing binding sites in the POPs (Table S1), while the N 1s and O 1s deconvoluted spectra showed peaks at 398.54, 400.34 eV and 532.26, 533.31 eV corresponding to C═N, C─N and ─OH, C─O bonding environment related to the salphen unit (Figure 2d,e). These binding energies are consistent with literature‐reported salen‐based polymers [46]. Similarly, Im‐POP exhibited similar C 1s, N 1s, and O 1s XPS spectra, confirming the presence of an imine bonding environment in the POP and consistent with the literature values (Figure 2f–h) [44]. Powder X‐ray diffraction patterns of both POPs displayed broad diffraction signals in the range of 5°–50°, suggesting their amorphous nature (Figure S3) [47]. Thermogravimetric analysis indicated high thermal stability; Sal‐POP exhibited negligible weight loss up to 350 °C, while Im‐POP exhibited relatively low thermal stability (Figure S4). The POPs were degassed at 120 °C prior to performing the nitrogen (N₂) adsorption measurements. N₂ adsorption isotherms at 77 K up to a relative pressure (P/P ₀) of 1,revealed type‐II uptake profiles for both Sal‐POP and Im‐POP with a total N₂ uptake of 16.22 and 29.90 mL g⁻¹, respectively (Figures S5 and S6). The type‐II profile indicates the nonporous/macroporous nature of POPs to N₂. In this case, N₂ adsorption is not due to micropore filling but rather interparticle pore filling. Application of nonlinear DFT yielded a Brunauer–Emmett–Teller (BET) surface area of 24.15 and 45.61 m² g⁻¹ for Sal‐POP and Im‐POP, respectively. Field emission scanning electron microscope image analysis indicated irregularly fuzed micron‐sized spherical particles for both POPs, which were also observed for literature‐reported POPs (Figures 2i,j and S7) [13, 31]. Energy‐dispersive spectroscopy (EDS) mapping showed the distribution of C, N, and O throughout the polymer network structure (Figures S8–S11). Elemental analysis further confirmed the successful incorporation of nitrogen functionalities within the polymeric frameworks, with Sal‐POP and Im‐POP exhibiting nitrogen contents of 6.95% and 9.26%, respectively (Table S2). High‐resolution transmission electron microscope images showed aggregated particles at low magnification; however, at higher magnification, irregularly stacked POP layer‐like structures were observed (inset Figures 2i,j and S12).

FIGURE 1.

Synthetic route to Sal‐POP and Im‐POP from corresponding monomers. Reaction conditions: [a] n‐BuOH/o‐DCB/AcOH (1:1:0.2), 120 °C, 96 h.

FIGURE 2.

POP characterization: (a) FT‐IR spectra of Sal‐POP (blue), Im‐POP (red), and M2 (green), (b) solid‐state ¹³C CP/MAS NMR spectra of Sal‐POP (top) and Im‐POP (bottom). Deconvoluted XPS spectra of C 1s, N 1s, and O 1s for Sal‐POP (c–e) and Im‐POP (f–h). SEM image of Sal‐POP (i) and Im‐POP (j) with corresponding HRTEM images shown in the inset.

2.2. Uranium Capture of Sal‐POP and Im‐POP

Interestingly, both POPs showed remarkable UO₂ ²⁺ capture properties under simulated seawater conditions across a wide concentration range (10–100 ppm) owing to the presence of high‐density coordinating salphen or o‐hydroxy imine sites. The uptake was monitored using a calibration curve obtained by titrating arsenazo dye III with known UO₂ ²⁺ concentration, a well‐established method in literature (Figure S13) [28, 48, 49]. Uranium capture experiments were performed by dispersing 5 mg of POP in 400 mL of uranium‐spiked simulated seawater (pH maintained at 2.5) and stirring the dispersion at 298 K for 24 h (Equations S1–S3, see Experimental Section for details). As shown in Figure 3a, at a uranyl concentration of 10 ppm, Sal‐POP exhibited uranium uptake capacity of 0.114 g g⁻¹ within 1 h and reached a maximum of 0.18 g g⁻¹ in 24 h. On the other hand, Im‐POP showed a uranium uptake capacity of 0.069 g g⁻¹ within 1 h, reaching a maximum of 0.23 g g⁻¹ after 24 h. Notably, uranium uptake capacities substantially increased upon systematically increasing the UO₂ ²⁺ concentration from 10 to 100 ppm (Figure 3b). For instance, at 50 and 100 ppm, Sal‐POP exhibited uptake capacities up to 0.76 and 1.05 g g⁻¹, respectively, while the Im‐POP exhibited uptake capacities up to 0.89 and 1.79 g g⁻¹, respectively (Figures S14 and S15). Such high loading capacity of both POPs underscores their strong affinity for uranyl ions through coordination interactions, as well as the accessible porosity of the POPs. Here, it should be noted that the BET surface area determined from the type II N₂ adsorption isotherm does not reflect the actual uranyl‐accessible surface area of the POPs. Also, it is reported in the literature that the BET surface area is not directly correlated with the small‐molecule uptake properties of POPs. For instance, the lowest BET surface area POPs are known to exhibit considerably high molecular iodine (I₂) uptake values compared to high surface area POPs [50, 51, 52]. Interestingly, the color of the Sal‐POP changed from yellow to black upon uranium adsorption (Figures 3b inset and S16). The distribution coefficient (K _d) reflecting the binding affinity between POP and UO₂ ²⁺ was determined to be higher; for instance, Sal‐POP showed K _d values of 1.88 × 10⁵, 1.26 × 10⁵, and 0.53 × 10⁵ mL g⁻¹ at 10, 50, and 100 ppm uranyl ion concentrations, respectively (Equation S4). On the other hand, Im‐POP exhibited K _d values of 2.94 × 10⁵, 1.56 × 10⁵, and 1.15 × 10⁵ mL g⁻¹ at 10, 50, and 100 ppm uranyl ion concentrations, respectively (Figure 3c). The higher K _d values indicate stronger binding affinity between POP and UO₂ ²⁺ and enhanced removal efficiency. Im‐POP showed relatively better K _d values compared to Sal‐POP at all UO₂ ²⁺ concentrations, indicating its better binding affinity to UO₂ ²⁺. This is in conjunction with uranium removal efficiencies, where Im‐POP and Sal‐POP showed 78%, 66%, 59% and 70%, 61%, and 40% of UO₂ ²⁺ removal efficiencies at 10, 50, and 100 ppm UO₂ ²⁺, respectively (Figure 3d, Equation S5). Kinetic analysis of the uranium adsorption process was also investigated; the plot of t/q _t versus t fitted well with the pseudo‐second‐order equation model (Equations S6 and S7) for the UO₂ ²⁺ adsorption process [28], indicating that chemisorption is the rate‐limiting step (Figures 3e, S17 and, S18). Moreover, the adsorption isotherms fitted well with the Langmuir adsorption model (Equations S8 and S9), consistent with monolayer adsorption behavior and a uniform distribution of active binding sites (Figure S19) [30, 53]. The uranyl uptake properties were further compared with structurally similar pTPE‐Salphen [30]. As shown in Figure S20, the pTPE‐Salphen exhibited a uranium uptake value of 119.3 mg g⁻¹ for 10 ppm, which is lower than Sal‐POP and Im‐POP over the same duration. These results clearly confirm that the uranyl uptake properties of the Sal‐POP porous network are not dictated solely by monomer M2 but also by the 3D connectivity and pore topology resulting from the naphthyl‐diamine. Collectively, these results demonstrate that both Sal‐POP and Im‐POP exhibit rapid, high‐capacity uranium capture directed by the suitable salphen, o‐hydroxy imine chelating sites distributed on the three‐dimensionally accessible pore surface of a POP network [14, 37]. It is worth mentioning that the UO₂ ²⁺ adsorption capacities are relatively higher than other POPs, including COFs containing pore‐blocking amidoxime groups reported in literature (Table S3, Scheme 1). The strategy used here avoids the postsynthetic modification procedure, and the coordinating sites are easily accessible to uranium for adsorption and desorption [54, 55].

FIGURE 3.

Uranyl cation capture behavior of Sal‐POP (blue) and Im‐POP (red): (a) time dependent UO₂ ²⁺ uptake capacity of POPs with 10 ppm uranium solution (pH = 2.5), (b) uptake capacity of POPs as a function of UO₂ ²⁺ concentration in the range of 10–100 ppm (pH = 2.5), inset shows the color changes of Sal‐POP upon UO₂ ²⁺ adsorption. (c) Distribution coefficient (K _d) at varying uranyl concentrations for Sal‐POP (blue) and Im‐POP (red) and (d) corresponding removal efficiencies. (e) Time‐dependent uranium uptake by POPs fitted to pseudo‐second‐order kinetic model for Sal‐POP (blue) and Im‐POP (red, inset). (f) Uranium uptake performance of Sal‐POP at 6.3 ppb uranyl‐spiked simulated seawater.

To further demonstrate the practical applicability of the POPs for uranium extraction under practical conditions, the uranyl uptake experiments were conducted at ppb level. For this, 5 mg of Sal‐POP was continuously stirred in 400 mL of 6.3 ppb uranyl in simulated seawater for 7 days, and the uranyl uptake was monitored by inductively coupled plasma mass spectrometry analysis. As seen in Figure 3f, the Sal‐POP showed an uptake of 31.6 μg g⁻¹ in 3 days, increasing to a maximum of 54.48 μg g⁻¹ in 7 days. These values are comparable to those reported in the literature [49, 56],where experiments were performed with smaller volumes of seawater. These results confirmed that the POPs can exhibit uranyl‐extraction properties even at ppb levels of uranyl ions under seawater conditions. After this, the uranyl uptake capacities of POPs were assessed using natural seawater collected from Digha Beach, West Bengal, India. After continuous stirring of Im‐POP in 400 mL of natural seawater for 8 days, the uranyl concentration decreased from 0.666 to 0.349 ppb, corresponding to a uranium uptake capacity of 25.36 μg g⁻¹ (Figure S21).

2.3. Ground–State Complex Formation of POP With UO₂ ²⁺

Ground–state complex formation between POP and UO₂ ²⁺ is essential to achieve visual color changes during UO₂ ²⁺ capture and release cycles with POPs. The ground‐state color changes are generally studied by UV absorption spectroscopy; unfortunately, the POPs under study are generally insoluble in common organic solvents and do not exhibit appropriate UV absorption spectra in their dispersed state. Therefore, to demonstrate ground‐state complexation experimentally, Stern–Volmer quenching analysis combined with the excited‐state lifetimes of uranyl‐bound POPs was used. As shown in Figure S22, Sal‐POP exhibited a strong yellow emission (${\lambda }_{\mathrm{em}}^{\max }$ = 578 nm) in DMF and hypsochromically shifted in nonpolar solvents potentially due to ESIPT. Upon addition of aqueous UO₂ ²⁺ (250 μM) to the dispersion of Sal‐POP (0.5 mg mL⁻¹), the fluorescence was quenched significantly with a quenching efficiency >80%, potentially due to UO₂ ²⁺ binding to the coordinating salphen units of Sal‐POP (Figure 4a). The fluorescence titration experiments were carried out by gradual addition of aq. UO₂ ²⁺ (0–250 μM) also resulted in gradual fluorescence quenching (up to 85%) (Equation S10). The plot of F ₀ /F versus [UO₂ ²⁺] revealed a nonlinear Stern–Volmer response at higher UO₂ ²⁺ concentrations indicating combined static and dynamic quenching pathway [57] where the UO₂ ²⁺ binds to the salphen‐unit present in the POP, forming a dark ground state complex (Figure 4b). The optical response was also evident to the naked eye under 365 nm UV illumination, where the characteristic yellow emission of Sal‐POP visibly faded in presence of UO₂ ²⁺ ions. Interestingly, linear Stern–Volmer response was observed at low concentrations static quenching pathway at the lower UO₂ ²⁺ concentrations. The static quenching pathway can be represented by the Stern–Volmer Equation (1) [58],

FIGURE 4.

Uranyl‐induced fluorescence quenching of Sal‐POP: (a) Emission spectra of Sal‐POP recorded upon incremental addition of UO₂ ²⁺ (λ _ex = 390 nm) and (b) variation of F ₀ /F with increasing UO₂ ²⁺ concentration, revealing a nonlinear Stern–Volmer response; inset shows changes in excited state lifetime with the addition of UO₂ ²⁺ (0–200 μM).

$$F_0/F=1+K_{\mathrm{sv}}\left[Q\right]$$ (1)

where F ₀, F represent the fluorescence intensities before and after the addition of UO₂ ²⁺, and K _SV represents the Stern–Volmer constant. The linear fitting of F ₀ /F versus [UO₂ ²⁺] at low UO₂ ²⁺ concentration revealed a relatively high K _SV value of 9.88 × 10⁶ M⁻¹, indicating a strong interaction between the uranyl ions and the salphen coordination sites within the POP network (Figure S23). Furthermore, the Benesi–Hildebrand plot yielded an association constant (K _a) of 1.05 × 10⁵ M⁻¹ for the UO₂ ²⁺–POP complex (Figure S24), underscoring the high binding affinity and effective host–guest interaction between uranyl and POP. Excited‐state lifetime measurements further confirmed the static quenching pathway via UO₂ ²⁺ complexation to the Salphen unit of POP. As shown in the τ ₀ /τ versus [UO₂ ²⁺] plot, the excited‐state lifetime of Sal‐POP upon addition of UO₂ ²⁺ ions (0–200 μM) did not change significantly at lower concentrations, indicating the formation of a dark‐ground state complex (inset Figures 4b and S25–S27); however, a positive deviation from linearity was observed at higher concentrations. These results suggest that at lower concentrations, fluorescence quenching is primarily governed by static complexation between UO₂ ²⁺and the salphen binding sites, whereas at higher concentrations, a combination of static and dynamic quenching [36, 59] occurs potentially due to interchain excited‐state energy transfer from fluorescent TPE‐salphen units to the dark UO₂ ²⁺‐bound salphen units within the polymeric framework. Interestingly, the Sal‐POP showed minimal PL quenching with other metal ions, except Cu²⁺ (Figures S28 and S29). The PL quenching of salphen‐based POPs in the presence of Cu²⁺ was also observed in our previously reported POP [31]. These results confirm that UO₂ ²⁺ indeed binds with the salphen unit of the POP and is responsible for the visual color changes. The fluorescence quenching response of Sal‐POP in simulated seawater was also appreciable for UO₂ ²⁺ among other metal ions. Furthermore, Sal‐POP showed appreciable quenching response to UO₂ ²⁺ in natural seawater (Figure S30). Unlike Sal‐POP, Im‐POP is nonfluorescent and not suitable for fluorescence sensing of UO₂ ²⁺. The fluorescence spectra and the images of Im‐POP in different solvents under UV light are shown in Figures S31 and S32.

2.4. Validation of U^VI State in UO₂ ²⁺@POP

To validate the U^VI state of UO₂ ²⁺@POP, XPS, EDS, and FT‐IR measurements were performed on the uranium‐loaded POPs, namely, UO₂ ²⁺@Sal‐POP and UO₂ ²⁺@Im‐POP. The binding of UO₂ ²⁺ was initially confirmed by XPS analysis (Figures 5a, S31, and S32); both UO₂ ²⁺@Sal‐POP and UO₂ ²⁺@Im‐POP showed peaks corresponding to uranium. The deconvoluted U 4f spectra showed peaks corresponding to U 4f_7/2 and U 4f_5/2 at ~381.39 and ~392.05 eV, respectively, confirming the successful adsorption of uranium in its +6 oxidation state [28]. Similar U 4f spectra were observed for UO₂ ²⁺@Im‐POP [13]. Further, the deconvoluted O 1s spectra revealed UO₂ ²⁺ peaks at 530.63 and 531.6 eV for UO₂ ²⁺@Sal‐POP and UO₂ ²⁺@Im‐POP, respectively (Figures S33 and S34). Further, the shifted O 1s and N 1s peaks of POPs indicated coordination of UO₂ ²⁺ with salphen‐unit and o‐hydroxy imine unit of the corresponding POPs [13, 30]. These results confirm chemisorption of uranium on the pore surface of the POPs via chelating coordinating interactions [28]. Also, the EDS analysis revealed a uniform distribution of uranium throughout the POPs, supporting the uniform distribution of the chelating sites present in the POP material (Figures 5b,c, S35, and S36). FT‐IR spectra of UO₂ ²⁺@Sal‐POP and UO₂ ²⁺@Im‐POP exhibited characteristic asymmetric stretching vibration of the O═U═O at 905 and 894 cm⁻¹, confirming the presence of coordinated UO₂ ²⁺ in both POPs (Figures 5d, S37, and S38). These values are consistent with those of other uranium‐adsorbed POPs reported in the literature [30].

FIGURE 5.

Characterization of UO₂ ²⁺ adsorbed POPs: (a) U 4f XPS spectra for UO₂ ²⁺@Sal‐POP (top) and UO₂ ²⁺@Im‐POP (bottom). EDS mapping of (b) UO₂ ²⁺@Sal‐POP and (c) UO₂ ²⁺@Im‐POP. (d) FT‐IR spectra of Sal‐POP (blue), UO₂ ²⁺@Sal‐POP (green), and Im‐POP (red), UO₂ ²⁺@Im‐POP (violet).

2.5. Computational Analysis of UO₂ ²⁺ Binding

To support the UO₂ ²⁺ chemisorption on the pore surface of POPs via coordination interactions, density functional theory (DFT) calculations were performed by optimizing the elementary unit of both POPs. As seen in Figure S39, both Sal‐POP and Im‐POP exhibit intramolecular H‐bonding between the corresponding phenolic OH and ─C═N moiety of salphen and o‐hydroxy imine units present, respectively. The calculated binding energy of the UO₂ ²⁺ to the salphen unit in UO₂ ²⁺@Sal‐POP and to the o‐hydroxy imine in UO₂ ²⁺@Im‐POP were calculated to be −67.10 and −71.94 kcal mol⁻¹, respectively (Figure 6a). The relatively higher exergonic binding energy for UO₂ ²⁺@Im‐POP signifies superior thermodynamic stability and stronger coordination interactions between the uranyl cation and o‐hydroxy imine moiety. These computational findings further corroborate the experimental observations, specifically the enhanced uranium uptake capacity observed for Im‐POP relative to Sal‐POP. Charge density difference iso‐surfaces further elucidate the nature of this coordination, revealing significant charge accumulation around the bound uranyl center and a corresponding delocalized charge depletion across the POP frameworks (Figures 6b and S40). Furthermore, the band decomposed charge density surfaces for the frontier molecular orbitals (HOMO − 1, HOMO, LUMO, and LUMO + 1), as depicted in Figures 6c–f and S41, highlight a robust f–π conjugation [60]. This electronic overlap confirms the strong covalent character and orbital interactions that drive the efficient sequestration of uranium within these systems.

FIGURE 6.

DFT analysis of uranium binding in Sal‐POP: (a) Optimized geometry of UO₂ ²⁺@Sal‐POP, (b) charge density difference iso‐surface of UO₂ ²⁺@Sal‐POP (yellow: positive and cyan: negative). (c–f) Band decomposed charge density iso‐surface of HOMO − 1, HOMO, LUMO and LUMO + 1, respectively, for UO₂ ²⁺@Sal‐POP.

2.6. pH Dependency and Selectivity

To understand the effect of pH on the uranyl uptake properties, pH‐dependent uranyl capture properties have been studied. As shown in Figure 7a, the uranyl uptake (q _t ) was higher at a low pH of 2.5 and gradually decreased with increasing pH to 9. These results indicate that the uranyl‐uptake properties of POPs are strongly pH‐dependent, with the lowest pH optimal for achieving high uptake capacities. Such high uptake in a low‐pH environment of the Sal‐POP indicates the applicability of these POPs for uranium extraction from highly acidic nuclear wastewater (pH < 4) [61]. To further evaluate the impact of uranyl uptake properties of Sal‐POP and Im‐POP at 10 ppm in the presence of competing ions, uranium uptake experiments were conducted in simulated seawater containing representative coexisting metal ions of Cu²⁺, VO₃ ⁻, Fe²⁺, Ni²⁺, Ca²⁺, and Mg²⁺ at concentration ratios representative of natural seawater [8]. As shown in Figures 7b, S42, and S43, both Sal‐POP and Im‐POP exhibited appreciable UO₂ ²⁺ uptake in the presence of interfering ions. Among the ions, Cu²⁺ showed the highest interference, while Mg²⁺ showed the lowest. Similar interference results were also reported for other POPs in the literature [62].

FIGURE 7.

(a) pH dependent uranyl (UO₂ ²⁺) uptake properties of Sal‐POP. (b) Uranyl (UO₂ ²⁺) uptake of Sal‐POP in presence of interfering metal ions.

2.7. Saturation Indication, and Recyclability

Notably, Sal‐POP enables naked‐eye visualization of uranyl adsorption/desorption cycles with distinct colorimetric changes. Upon exposure to UO₂ ²⁺ at 250 μM, the characteristic yellow color of Sal‐POP gradually fades and ultimately transforms to deep black upon saturation (Figure 8a). This reversible color change arises from strong ground–state complex formation between UO₂ ²⁺ and the salphen units, providing a clear visual endpoint that directly signals uranyl saturation without the need for instrumental analysis. The color changes of Sal‐POP specific to UO₂ ²⁺ were also evident from the daylight images of Sal‐POP with different metal ions (Figure S16). No such color changes were observed with Im‐POP due to a differing electronic environment from Sal‐POP (Figure S44). Moreover, the initiation of the UO₂ ²⁺ adsorption process was trackable by simple fluorescence turn‐off and its highly reversible. To evaluate the operational stability of the POPs, recyclability experiments were carried out under simulated seawater conditions [35]. After each adsorption run, the uranium‐loaded materials, that is, UO₂ ²⁺@Sal‐POP and UO₂ ²⁺@Im‐POP were regenerated by treatment with 2 M Na₂CO₃ solution (60 mL) and gently stirred for 24 h. The suspensions were filtered, repeatedly washed with distilled water until the filtrate became neutral, and then dried under vacuum. The regenerated POPs were then subjected to a subsequent adsorption cycle under identical conditions. Both Sal‐POP and Im‐POP exhibited excellent cycling stability; over five consecutive adsorption–desorption cycles, Sal‐POP retained an adsorption efficiency exceeding 88%, while Im‐POP maintained an adsorption efficiency of approximately 86%, indicating minimal degradation of the coordinating sites and preserved pore accessibility (Figures 8b and S45). Similarly, the fluorescence response of Sal‐POP toward UO₂ ²⁺ was also largely preserved across repeated cycles (Figure 8c). FT‐IR spectra collected before and after cycling showed no significant structural changes, further demonstrating the robustness of the POP frameworks under repeated chemical regeneration (Figures S46 and S47) [30, 36]. These results highlight the excellent reusability of both POPs and underscore the durability of the preorganized salphen and o‐hydroxy imine binding motifs. The ability to regenerate the POP adsorbents under mild carbonate washing conditions, without loss of adsorption capacity or photophysical response, is particularly advantageous for practical applications in continuous uranium monitoring and remediation. Such high reversibility makes a clear distinction between POPs and small‐molecule salen compounds. Although salen‐based uranyl complexes are known, they often exhibit 1:1 stoichiometry of uranyl to salen [63]. Therefore, they must be used in large quantities for uranyl extraction, which makes these small‐molecule salens inefficient compared to POPs. Also, the molecular salen‐uranyl complexes are soluble in aqueous media and require specialized methods for recycling, thereby increasing the operational and regeneration costs. In contrast, POPs are nanoporous, high‐surface‐area materials containing a high density of uranyl‐binding sites distributed uniformly throughout the 3D network, thereby significantly increasing uranyl uptake capacity in relatively small POP adsorbent materials (ca. 5 mg) compared with stoichiometric small‐molecular salen complexes. POPs are chemically robust due to strong C─C/N linkages; thus, removal of uranyl from the POP network is simple. They are highly recyclable due to their insolubility in seawater or organic solvents, highlighting their potential practical applications in large‐scale seawater uranium extraction.

FIGURE 8.

(a) Schematic illustration of Sal‐POP regeneration and uranyl extraction, (b) UO₂ ²⁺ adsorption capacity of Sal‐POP in five successive adsorption–desorption cycles, and (c) reusable performance of Sal‐POP for selective uranyl sensing and recovery.

3. Conclusion

In summary, the study demonstrated postsynthetic modification‐free, self‐indicative POPs as a unique type of uranium adsorbents that exhibit high‐capacity uranyl capture. Sal‐POP showed a remarkable uranyl uptake capacity of 0.114 g g⁻¹ h⁻¹ with a high distribution coefficient, K _d = 1.88 × 10⁵ mL g⁻¹ at 10 ppm UO₂ ²⁺. Comparison studies with Im‐POP confirmed the role of hydroxy‐imine units in UO₂ ²⁺ binding to the POP frameworks. Interestingly, the uptake capacity was 0.069 g g⁻¹ h⁻¹ with K _d of 2.94 × 10⁵ mL g⁻¹ at 10 ppm UO₂ ²⁺. Computational analysis also revealed the strong binding affinity of POPs toward UO₂ ²⁺. Both POPs exhibited remarkable recyclability of the uranium capture and release process. Their visible color changes allowed tracking of uranium adsorption initiation, saturation, and desorption, making them unique among POP‐based uranium adsorbents and providing a new platform of self‐indicative uranium adsorbents that are highly suitable for practical, real‐world uranium monitoring and extraction applications.

Funding

This study was supported by Anusandhan National Research Foundation (SRG/2023/000062) and Indian Institute of Technology Kanpur (IITK/CHM/2022233).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supplementary Material

Sahoo Nilojyoti, Akhtar Laraib, Rai Rameshwar, Kancharlapalli Srinivasu, Mothika Venkata Suresh, Visualizing Reversible Uranyl (UO₂ ²⁺) Capture and Release With Self‐Indicating Porous Organic Polymers, Small Science 2026, 6, e70416. 10.1002/smsc.70416

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.