Self-template impregnated silver nanoparticles in coordination polymer gel: photocatalytic CO₂ reduction, CO₂ fixation, and antibacterial activity

Abstract

CO₂ fixation and light-assisted conversion of CO₂ in the presence of water into fuels and feedstocks are clean and sustainable techniques to alleviate the energy crisis and global climate change. In this regard, herein, a waterborne multifunctional metal–organic coordination polymer gel (Ag@GMP) was prepared from silver nitrate and guanosine 5′-monophosphate. Electron microscopy exhibits that Ag@GMP has a flower-like structure, which is composed of vertically grown sheets, and corresponding high magnification images display the presence of silver nanoparticles on the vertically grown sheets. Ag@GMP demonstrates remarkable photocatalytic performance, achieving a CO₂ conversion rate of 18.6 μmol g⁻¹ with approximately 85% selectivity towards CO at ambient temperature without using sacrificial agents. In situ diffuse reflectance infrared Fourier transform spectroscopy was employed to elucidate the proposed mechanism for photocatalytic CO₂ reduction. Additionally, Ag@GMP exhibits significant catalytic activity in the fixation of CO₂ with epoxides, leading to the formation of valuable chemicals under atmospheric pressure. Ag@GMP demonstrated efficient antibacterial activity against both Gram-negative and Gram-positive bacteria. The highest zone of inhibition was observed against S. aureus MTCC 3160 (15.83 ± 1.1 mm), and for E. coli, P. aeruginosa PAO1, and B. subtilis, it was found to be 12.66 ± 0.9, 14.33 ± 0.8 and 12.8 ± 0.8 mm, respectively.

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Introduction

The ever-increasing concentration of carbon dioxide in the earth's atmosphere is a threat for the existence of life on the earth.^1,2 CO₂ concentration has steeply risen from 280 ppm in the pre-industrial era to 410 ppm at present.^3,4 The high level of CO₂ in the atmosphere will directly affect humans.^5,6 The utilization of renewable energy (nuclear power, wind, hydrogen, and solar energy) is still lower than that of fossil fuels. Therefore, since the last few decades, there is a global demand for carbon dioxide capture and utilization (CCU).⁷ CCU technique is an exciting strategy for achieving sustainable energy.^8–10 Particularly, the photocatalytic conversion of CO₂ in the presence of water into fuels and value-added feedstocks is a viable technique to alleviate the greenhouse effect and energy demand.^11–13 Similarly, the coupling of CO₂ with epoxides to produce cyclic carbonates is an eco-friendly approach for the development of value-added products from CO₂ as cyclic carbonates find a wide range of applications as electrolytes in batteries, in polymer synthesis, and as green solvents.¹⁴ However, the activation of carbon dioxide is extremely challenging owing to its high endergonic nature; therefore, catalysts play a key role in the activation of CO₂.^15,16

To this end, the design and development of advanced, “soft” processable metal–organic coordination polymer gel (CPG) hybrid materials with high catalytic activity for CO₂ fixation and photocatalytic CO₂ reduction is a stirring and emerging area of research in heterogeneous catalysis.^17–20 CPGs are prepared through the coordination-driven self-assembly of metal ions and organic linkers and have attracted plenty of attention because of their wide range of applications ranging from catalysis to medicine.^21–24 The choice of metal ions plays an important role in designing CPGs for desired applications.^25–27 Plasmonic nanoparticle (PNP)-incorporated CPGs are attractive materials for catalysis because of their remarkable absorption and nanoscale control of light and heat.²⁸ An increasing number of these so-called hybrid plasmonic nanomaterials have been studied for chemical reactions in recent years, resulting in the development of plasmon-assisted catalysis.²⁹

An increasing number of these so-called hybrid plasmonic nanomaterials have been studied for chemical reactions in recent years, resulting in the development of plasmon-assisted catalysis.²⁹ As silver-based plasmonic materials have been established for photocatalytic CO₂ reduction, therefore, the use of silver metal ion and π-conjugated organic linkers could be an ideal combination for the preparation of soft metal–organic CPG-based active catalytic materials.³⁰ However, only a handful of CPG-based heterogeneous catalysts have been reported for photocatalytic carbon dioxide reduction as well as CO₂ fixation.^{11,17–20,31} Therefore, there is wide open space for the design and the development of metal–organic CPG hybrid materials for catalysis.

Antibiotics entered in their golden era with the discovery of Penicillin in 1928; since then, several antibiotics have been either extracted from natural sources or chemically prepared, and countless lives have been saved.³² However, antibiotic resistance in bacteria has been rapidly growing.^33,34 Therefore, it is essential to develop the next generation of antibiotics against a variety of life-threatening bacterial infections. Engineered nanoparticles (NPs) have the capability to mitigate the issue by providing a viable alternative for treating a variety of illnesses, most notably those generated by multidrug-resistant (MDR) bacteria.^35–37 Amongst various engineered nanoparticles that have been utilized as antibacterial agents, silver nanoparticles (AgNPs) or AgNPs-containing materials have been found to be excellent antibacterial agents because of their robust antimicrobial effectiveness against different fungi, viruses, and bacteria.^38–40 The bare AgNPs are susceptible to aggregation when disseminated in aqueous solution, which decreases their stability and drastically reduces their antibacterial activity.³⁶ This problem with AgNPs can be overcome by affixing them to a support material.³⁹ CPGs exhibit remarkable adaptability for incorporating various polymers, nanomaterials, and metal ions by in situ co-gelation without affecting the basic gelation process.^41,42 Therefore, CPGs could be utilized to prepare highly dispersed AgNPs-loaded CPG for excellent antibacterial activity, wherein the gel matrix can act as a template (or fixed platform) and offers anchoring sites for in situ reduced or presynthesized AgNPs to avoid aggregation.

In this regard, a waterborne CPG (Ag@GMP) was synthesized by the direct mixing of silver nitrate and the low molecular weight gelator guanosine 5′-monophosphate disodium salt hydrate (Fig. 1). The silver nanoparticle-loaded Ag@GMP has the ability to convert CO₂ with over ∼85% selectivity into CO in the presence of moisture and light without any sacrificial agent. Moreover, the as-synthesized Ag@GMP also exhibits excellent catalytic activity toward CO₂ conversion into cyclic carbonate at atmospheric pressure. Additionally, Ag@GMP has an inhibitory ability for Gram-negative as well as Gram-positive bacteria. The maximum zone of inhibition against S. aureus MTCC 3160 was found to be 15.83 ± 1.1 mm, and for E. coli, P. aeruginosa PAO1, and B. subtilis, it was 12.66 ± 0.9, 14.33 ± 0.8 and 12.8 ± 0.8 mm, respectively.

Fig. 1 Schematic for the synthesis of the metal–organic coordination polymer gel (Ag@GMP).

Results and discussion

The coordination-driven gelation approach for the fabrication of silver nanoparticles-anchored metal–organic CPG with silver nitrate and GMP is demonstrated in Fig. 1. The structure of guanine in GMP is rich in carbon and nitrogen, providing a rigid skeleton with multiple coordination sites. The gel nature of Ag@GMP was checked by the “inversion vial” method. Upon turning the vial upside down, the gel did not flow due to the gravitational forces, indicating the gel nature of Ag@GMP CPG (Fig. 1). The binding model of different metal ions with nucleic acids is a well-established field of research.^43,44 At neutral pH, the binding stability suggests that the best possible interaction may happen with the N7/O6 centre of the guanosine moiety to the metal ions.⁴⁵ Importantly, Ag⁺, Pt²⁺, and Hg²⁺ have the affinity to bind with nucleobase rather than phosphate groups.^46,47 Herein, Ag⁺ is prone to forming adducts with the electron-rich nitrogen (N7) and oxygen (O6) sites⁴³ in the purine ring of the guanosine moiety, which is elaborately illustrated in Fig. 2. The self-assembly process may encompass both metal–linker coordination and additional supramolecular non-covalent interactions that contribute to robust gelation. The nucleobase and phosphate groups regulate the π⋯π-stacking and hydrogen bonding, respectively, to strengthen the gel synthesis via 3D supramolecular structure formation. As we know, the semi-solid gel materials are viscoelastic in nature. Therefore, the mechanical strength and the viscoelastic nature of the synthesized CPG were investigated by rheological analysis. For gel materials, the storage modulus (G′) is always higher than the loss modulus (G′′) of the materials. Freshly prepared Ag@GMP CPG was used for rheological study. Fig. S1† demonstrates the G′ and G′′ analysis against angular frequency sweep at constant 0.01% strain, wherein the G′ and G′′ values remained almost constant in the entire test region and did not cross each other (i.e., G′ > 10⁵, G′′ < 10⁵). The rheological study confirms the viscoelastic nature of the synthesized Ag@GMP CPG.

Fig. 2 Schematic representation of the potential interactions between silver ions and the GMP moiety.

The interaction with Ag⁺ and GMP moiety was further explained through FTIR analysis (Fig. S2a, ESI†). The blue shift from 1689 cm⁻¹ to 1606 cm⁻¹ and 3311 cm⁻¹ to 3216 cm⁻¹ is due to the alteration in the C=O stretching vibration and N–H stretching band, respectively. This clearly indicates the interaction between Ag⁺ with purine ring's carbonyl groups. The hydrogen bonding in the supramolecular structure may be the reason behind the NH₂ band shifting. Furthermore, the peak at about 1523 cm⁻¹ remains unshifted, but the intensity of the peak is drastically diminished after complex formation between Ag⁺ and the GMP moiety, which may be due to the C=N vibrational stretching band of the imidazole moiety.^48–51 It also suggests the interaction between Ag⁺ and the purine unit. In addition, the effect of π–π stacking interactions on the gelation process was investigated using diffuse-reflectance spectroscopy (DRS) (Fig. S2b, ESI†). The spectrum of Ag@GMP shows that the pristine GMP absorbance band was slightly red shifted (272 nm to 276 nm) due to the π–π stacking in the supramolecular gel structure.⁵² Along with this, a clear broad absorbance peak at about 440 nm dictates the surface plasmon resonance peak (SPR) of AgNPs, which is absent in pristine GMP.⁴¹ The generation of AgNPs during the assembly process is likely associated with the heterocyclic and glycosyl groups. The electron-rich nitrogen-containing moiety and the hydroxyl group of glycosyl may take part in stabilizing the AgNPs with the gel matrix. The AgNPs were then incorporated due to the interaction between the amino or hydroxyl groups and AgNPs. Therefore, the supramolecular gel-phase network encapsulates and stabilizes these nascent metal nanoparticles.^53,54

The synthesized Ag@GMP CPG was lyophilized to get the Ag@GMP xerogel, and then PXRD analysis was performed for the phase purity and crystallinity of the synthesized xerogel (Fig. S3, ESI†). The recorded PXRD pattern of the Ag@GMP xerogel suggests the crystalline nature of the material, which contrasts with the usually amorphous nature of the CPG. The obtained diffraction was matched with that of the reported Ag(0) (ICSD No. 22434), indicating the formation of AgNPs within the Ag@GMP xerogel.

The FESEM micrograph of Ag@GMP exhibits a flower-like structure made up of vertically grown sheets. The high-magnification FESEM images reveal that these sheets contain silver nanoparticles (Fig. 3a–c). The TEM micrograph also confirms the presence of AgNPs in the as-synthesized Ag@GMP (Fig. 3d). The HRTEM images revealed the dispersion of the spherical and quasi-spherical AgNPs within the Ag@GMP xerogel, and the average particle size of AgNPs was found to be approximately 4.46 nm (Fig. 3e and Fig. S4, ESI†).

Fig. 3 The FESEM images of Ag@GMP at (a) 10k magnification, (b) 25k magnification, and (c) 100k magnification. (d) TEM images of Ag@GMP. HRTEM image of (e) Ag@GMP, and (f) d-spacing images of the (111) plane of Ag. (g) AFM micrographs and (h) height profile curve of Ag@GMP. (i) SAED pattern of Ag@GMP. (j–o) TEM mapping images of Ag@GMP.

The lattice fringes of Fig. 3e are presented in Fig. 3f, which exhibits a clear view of the d-spacing for the (111) plane of Ag with a d-spacing of 2.35 Å. Moreover, the topological study of Ag@GMP was performed by AFM analysis in the noncontact mode, which further confirmed the formation of AgNPs within the as-synthesized Ag@GMP material. The AFM image height profile measurement displayed that the Ag@GMP xerogel consists of about 3 nm AgNPs on the sheet-like structure (Fig. 3g and h). The SAED pattern of the material indicates the crystalline nature (Fig. 3i). The STEM study was also performed, which demonstrated the homogenous distribution of all the elements within the synthesized material (Fig. 3j–o).

Furthermore, X-ray photoelectron spectroscopy (XPS) was also performed to investigate the electronic state of the surface and the chemical composition of the synthesized Ag@GMP xerogel. The survey spectrum peaks clearly exhibit that the xerogel is composed of silver (Ag), nitrogen (N), carbon (C), oxygen (O), and phosphorus (P) elements (Fig. S5a, ESI†). The core-level spectrum of Ag 3d was deconvoluted to investigate the valence state of silver in the synthesized xerogel. The XPS spectrum of Ag 3d exhibits two peaks at 368.3 eV and 374.3 eV assigned to 3d_5/2 and 3d_3/2, respectively (Fig. S5b, ESI†). The deconvoluted spectra display the presence of both Ag(I) and Ag(0) in the material. Next, we performed a temperature-programmed desorption (TPD) experiment to evaluate the active binding sites on the synthesized Ag@GMP material (Fig. S6, ESI†). The CO₂-TPD spectrum of the Ag@GMP xerogel shows CO₂ desorption peaks at low temperatures (i.e., about 80 °C) and also at high temperatures (i.e., about 600 °C). The CO₂ desorption peaks observed at about 80 °C indicate the presence of weak binding sites, and the peaks found at higher temperatures indicate the existence of strong basic sites (i.e., –NH/–OH groups) that are capable of binding with CO₂ even at higher temperatures. The obtained CO₂-TPD results suggest the presence of binding sites for CO₂ gas at low as well as high temperatures in the as-synthesized Ag@GMP material.

The ζ-potential value of the Ag@GMP xerogel was found to be −54.9 mV (Fig. S7, ESI†). The high negative value of ζ-potential indicates that the Ag@GMP xerogel has a negative surface charge. As we know, carbon dioxide is a Lewis acid; therefore, it is expected that the negative surface of the Ag@GMP xerogel can facilitate the binding of the CO₂ molecules.

Optoelectronic properties

The optical band gap of the Ag@GMP xerogel was calculated to be 3.77 eV using the Tauc plot from the UV-vis absorption spectra (Fig. S8, ESI†). Thereafter, the photocurrent response of Ag@GMP was analysed to explore the efficiency of photoinduced charge carriers. Fig. 4a clearly shows that the photocurrent surged spontaneously under light irradiation and subsequently reduced to zero when the light was turned off, showing that the Ag@GMP catalyst exhibited photoresponse. Additionally, the photocurrent response remained stable after four ON/OFF illumination cycles, suggesting the excellent photoelectrochemical stability of Ag@GMP. However, the slight decrease in the photocurrent intensity is due to the back-electron transfer process. Furthermore, valence band (VB) XPS analysis was performed to get information of the valence band edge, which was found to be 2.76 eV (Fig. 4b). Therefore, the corresponding conduction and valence bands can be calculated from the following eqn (1).

E_VB = E_CB + E_g (1)

where E_VB, E_CB, and E_g stand for valence band edge, conduction band edge, and band gap, respectively. Based on the obtained data, the band alignment presented in Fig. 4c suggests that Ag@GMP possesses appropriate band edge positions for carbon dioxide reduction.

Fig. 4 (a) Photocurrent response and (b) valence band (VB) XPS data of Ag@GMP. (c) Experimental band alignment of Ag@GMP. (d) Formation rates of CO, CH₄ and H₂ for 4, 8 and 12 h of illumination time. (e) The carbon monoxide selectivity plot of CO over CH₄. (f) Time-dependent in situ DRIFT analysis for the Ag@GMP catalyst using moist CO₂.

Photocatalytic activity

The transformation of carbon dioxide gas into valuable chemical compounds in the presence of moisture and light is a sustainable approach for addressing environmental concerns. The band alignment of the Ag@GMP material suggests that it can reduce CO₂ gas. Moreover, the CO₂-TPD and zeta potential results also indicate the presence of basic sites within Ag@GMP.

Therefore, we checked the applicability of Ag@GMP to reduce CO₂ gas in the presence of water and light. Here, water molecules work as a source of H⁺ ions or reducing agents without adding sacrificial agents. Several catalytic measurements were performed under identical conditions without CO₂ gas, photocatalysts, and light irradiation (Fig. S9, ESI†). No products (CO, CH₄ and H₂) were detected in the GC equipped with TCD, FID, and methaniser, suggesting that the reduced products were derived only from CO₂ photoreduction. Fig. 4d demonstrates the formation rate of CO, CH₄, and H₂ over Ag@GMP catalysts for different illumination times, i.e., four, eight, and twelve hours of experiment. The yield of the photo-reduced products increased with the irradiation time. The Ag@GMP was found to be highly selective (>85%) toward CO formation over CH₄ and H₂ (Fig. 4e). The CO, CH₄, and H₂ formation rates were 18.6 μmol g⁻¹, 0.78 μmol g⁻¹, and 2.6 μmol g⁻¹, respectively, under 12 h of irradiation. The PXRD patterns after catalysis (Fig. S10, ESI†) suggest the retention of the framework of Ag@GMP catalysts. The photocatalytic performance of the Ag@GMP catalysts is highly comparable with several reported benchmark hybrid materials (Table S1, ESI†).

In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and photocatalytic CO₂ reduction mechanism

The in situ DRIFTS technique was employed under UV-vis illumination at 1800–1300 cm⁻¹ to investigate the light-induced generation of active intermediates throughout the catalytic process, thereby offering essential insights into the plausible CO₂ reduction mechanism (Fig. 4f). Initially, Ag@GMP was subjected to vacuum at 100 °C for 2 hours to remove any adsorbed species, followed by the passage of water-moist CO₂ from the reaction vessels to maintain the adsorption–desorption process until a stable IR signal was obtained. The IR spectrum was initially acquired under dark condition, labeled as 0 min. Subsequently, the xenon arc lamp was activated to facilitate the monitoring of intermediate products formed under UV-vis light exposure. Upon light exposure, new peaks emerged, and the intensity of some peaks changed over time, indicating a reaction intermediate transformation. Initially, upon light illumination, CO₂ accepts photoconducting electrons from the catalyst surface to undergo a single-electron reduction process, resulting in the formation of reactive intermediate COO⁻ species, which is indicated by the emergence of a peak at 1522 cm⁻¹.⁵⁵

In addition, two peaks that arise with time at 1379 and 1571 cm⁻¹ could be attributed to formic acid (HCOOH) and formate (HCOO⁻) species, respectively.⁵⁶ The peak appearing at 1321 cm⁻¹ is attributed to the bidentate carbonate species, while the emergence of the peak at 1231 cm⁻¹ corresponds to bicarbonates.^57,58 These peaks arise due to the interaction between reactive intermediate molecules and the available free OH group on the catalyst's surface. The peak appearing at 1488 cm⁻¹ corresponds to monodentate carbonates. Additionally, the emergence of the peak at 1231 cm⁻¹ corresponds to the bicarbonates. Moreover, a notable peak emerged at 1692 cm⁻¹ over time, corresponding to the CO peak, thus confirming the formation of CO due to light exposure over time.⁵⁹

We have proposed the plausible reaction pathway for photocatalytic CO₂ reduction based on the obtained experimental results and in situ DRIFT analysis (R1–R10, ESI†). The zeta potential and CO₂-TPD analyses indicate the basic nature of the catalyst surface, which could facilitate CO₂ interaction with the material. Fig. 4f implies that Ag@GMP would have a multielectron reduction process. The photo-generated electron–hole pair reacted with CO₂ on the catalyst surface, yielding the COO⁻ species (Fig. 4f, 1522 cm⁻¹, R1 & R2, ESI†). This implies the formation of a reactive intermediate, which subsequently undergoes a series of intricate reactions to yield CO and CH₄ (R4–R10, ESI†). The signals correspond to HCO₃⁻ (1231 cm⁻¹), CO₃⁻ (1321 & 1488 cm⁻¹), and HCOOH (1379 cm⁻¹), which appeared with time in the in situ DRIFT analysis, affirming the formation of intermediates for product formation. Notably, these entities can receive both protons and electrons, leading to carbon monoxide production, as depicted in the reactions (R5–R9, ESI†).

Catalytic CO₂ cycloaddition reactions

Silver nanoparticles functionalized metal–organic hybrid materials have shown efficient catalytic activity in facilitating the carboxylation of epoxides to yield the corresponding esters. The as-synthesized Ag@GMP material contains catalytically active sites comprising silver nanoparticles, alongside polar functional groups such as –OH and –NH, which endow them with great potential for catalyzing CO₂ cycloaddition reactions. Therefore, we have investigated the applicability of Ag@GMP as a catalyst for CO₂ cycloaddition reactions under solvent-free conditions. Bifunctional catalysts are crucial for driving both photocatalytic CO₂ reduction and CO₂ cycloaddition reactions efficiently. However, to the best of our knowledge, there are no reports on specifically tailored CPGs for these dual functions.

Ag@GMP was activated prior to catalysis by keeping it under vacuum for 24 h. The catalytic reactions were carried out in the presence of carbon dioxide at atmospheric pressure (balloon) and under solvent-free conditions at room temperature. 20 mmol epoxide and 5 mol% tetrabutylammonium bromide (TBAB) were used as the substrate and cocatalyst, respectively. The ¹H NMR spectrum of the filtrate was recorded to calculate the conversion. Controlled reactions were carried out without the catalyst, without the cocatalyst, using ligands (GMP) and AgNO₃ as the catalyst (Table 1). In all the cases, the conversion was found to be very low. A time-dependent kinetic study was also carried out to check the efficiency of the catalyst (considering the conversion of epichlorohydrin to the corresponding cyclic carbonate (CC) as a model reaction).

Table 1 Controlled reaction data for CO₂ cycloaddition reactions

Sr. No.	Catalyst	Temperature (°C)	Conversion (%)^a
a Percentage of conversions were determined using ^1H-NMR analysis.
1	GMP	RT	35
2	AgNO3	RT	31
3	Without TBAB	RT	25
4	Without Ag@GMP	RT	9
5	Ag@GMP	RT	99

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Fig. 5 shows the conversion of the CO₂ cycloaddition reaction of epichlorohydrin in the presence of Ag@GMP catalyst at different time intervals. The conversion was found to be 9%, 22%, and 36% after 2 h, 5 h, and 10 h respectively. Finally, the conversion was found to be 99% after 48 h.

Fig. 5 ¹H NMR (CDCl₃, 500 MHz) spectra for the optimization of the reaction with time for the cycloaddition reaction of epichlorohydrin with CO₂ using Ag@GMP as a catalyst.

The catalytic activity was also explored using other epoxides (Table 2), viz., a small epoxide, propyleneoxide; relatively larger epoxide, 1,2-epoxybutane; and an aromatic epoxide, styrene oxide. The conversion was found to be 82% for propyleneoxide towards the corresponding CC (Fig. S11 and Table 2, 2.2, ESI†). The relatively lower conversion may be due to the absence of an electron-withdrawing group. However, the catalytic activity was slightly reduced for considerably larger epoxides. 60% conversion was found for 1,2-epoxybutane under the same reaction conditions (Fig. S12 and Table 2, 2.3, ESI†). A low conversion of about 46% was observed in case of styrene oxide (Fig. S13 and Table 2, 2.4, ESI†).

Table 2 Substrate scope for the Ag@GMP-catalyzed CO₂ cycloaddition reaction

Sr. No.	Epoxides	Product	Conversion^a (%)
a Percentage conversions were determined using ^1H NMR analysis.
1			99
2			82
3			60
4			46

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The cycloaddition reaction is directly linked to the ring-opening process. The ring-opening process of the epoxide acts as the rate-determining step of the catalytic conversion reaction.^60,61 The electron-withdrawing effect of the chloromethyl group eases the pathway and helps in the ring-opening process. Consequently, the conversion to the respective cyclic carbonates occurs far more rapidly compared to other substrate scopes. Meanwhile, larger substituents on the aromatic ring provide steric hindrance, which impedes the diffusion rate of larger epoxides to the Lewis acidic site. It is reflected in the conversion rate of the corresponding epoxide to cyclic carbonates.⁶² The CO₂ cycloaddition reaction of epichlorohydrin towards the respective carbonate compound (CC) shows 99% conversions in the presence of the catalyst Ag@GMP (Fig. 5). After the first cycle, the catalyst was isolated via filtration and then washed with acetone for the next cycles. The recyclability of Ag@GMP was studied for up to three cycles and 99, 97, and 95% conversions were obtained, respectively (Fig. 6).

Fig. 6 ¹H-NMR (CDCl₃, 500 MHz) spectra of recyclability for the cycloaddition reaction of epichlorohydrin with CO₂ using Ag@GMP as a catalyst.

A comparison table (Table S2, ESI†) has been made to compare the activity of the catalysts Ag@GMP within the family of CPGs as well as other well-known reported compounds for the conversion of epichlorohydrin to CC via CO₂ cycloaddition reactions. Ag@GMP is well compared within the family of the reported gel-based materials. Cu(II)-MOG shows 80% conversion in 48 h at RT.¹⁹ MOG at RT exhibits about 78% conversion in 48 h.³¹ NiXero shows 48% conversion at 80 °C under 5 bar.²⁰ UMCM-1-NH₂ at RT shows about 78% conversion in 24 h,⁶³ whereas MOF-5 shows a slightly higher conversion of 93% in 12 h.⁶⁴ Almost complete conversion was found using Zn-DAT as a catalyst at 8 bar pressure in 24 h.⁶⁵ Eu-MOF and Cu–K-MOF show full conversions at higher temperatures of about 80 °C and 60 °C, respectively.^66,67 In this context, the utilization of CPG as a catalyst for CO₂ cycloaddition remains largely unexplored. Ag@GMP could convert epichlorohydrin completely to its respective CC only within 48 h. The obtained results affirmed that the performance of the catalyst Ag@GMP stands out as highly competitive within the coordination polymer gel family and the best-reported catalysts.^19,20,31 Moreover, Ag@GMP proves to be a particularly advantageous catalyst due to its straightforward scalability for bulk production.

Based on our results and through the literature survey,^68–72 the mechanism of the catalytic cycle is shown in Fig. 7. The Lewis acidic silver centre acts as a catalytic active centre. The unsaturated acidic metal centres bind the nucleophilic oxygen atoms of the epoxide. The nucleophilic bromide ion obtained from TBAB attacks the polarized epoxide for ring opening, which subsequently interacts with carbon dioxide to create an intermediate. This anion has a high tendency to close the ring to form a cyclic carbonate product. The XPS spectra of Ag@GMP indicate that the material comprises both Ag(0) and Ag⁺ states. The zero-oxidation state of silver is present as nanoparticles (NPs), and the +1 oxidation silver ions interact with the GMP moiety to form the structural arrangement.⁷³ The electron-rich NPs takes part as an electron donor source and Ag⁺ stabilizes the intermediates during the catalytic process. Hence, the surface motif featuring Ag⁺ oxidation state and the electron-rich NPs containing Ag(0) both play a synergistic role in enhancing the catalytic activity.

Fig. 7 Proposed mechanism for the CO₂ cycloaddition reaction of epoxides.

Antibacterial studies

Silver nanoparticle-containing materials are well-known to demonstrate significant antibacterial activity. Therefore, we further explored the applicability of these materials as antibacterial agents.⁴⁰ The antibacterial activity of the as-synthesized silver nanoparticles containing the Ag@GMP material was tested against Gram-negative (E. coli ATCC 25922 and P. aeruginosa PAO1) and Gram-positive (S. aureus MTCC 3160 and B. subtilis) bacteria. Ag@GMP showed varying degrees of inhibitory impacts against the tested bacteria (Fig. 8a–d). The maximum zone of inhibition (15.83 ± 1.1 mm) was observed against S. aureus MTCC 3160. Similarly, the zone of inhibition against E. coli, P. aeruginosa PAO1, and B. subtilis were found to be 12.66 ± 0.9, 14.33 ± 0.8, and 12.8 ± 0.8 mm, respectively. Furthermore, the inhibitory impact of Ag@GMP materials on biofilm development was also investigated through microscopy. The bacterial cultures were grown in the absence and presence of Ag@GMP on glass coverslips in 24-well polystyrene tissue culture plates.

Fig. 8 Antimicrobial activity of Ag@GMP against (a) B. subtilis, (b) P. aeruginosa PAO1, (c) E. coli, and (d) S. aureus MTCC 3160. Light microscopy micrographs of biofilms, showing the effect of Ag@GMP. (e) Control P. aeruginosa PAO1; (f) P. aeruginosa PAO1 treated with Ag@GMP (64 μg mL⁻¹); (g) control S. aureus MTCC 3160; (h) S. aureus MTCC 3160 treated with Ag@GMP (64 μg mL⁻¹). Confocal laser scanning microscopy images of biofilms, showing the effect of Ag@GMP. (i) Control P. aeruginosa PAO1; (j) P. aeruginosa PAO1 treated with Ag@GMP (64 μg mL⁻¹); (k) control S. aureus MTCC 3160; and (l) S. aureus MTCC 3160 treated with Ag@GMP (64 μg mL⁻¹).

The light microscopic images of P. aeruginosa PAO1 and S. aureus MTCC 3160 exhibited that both the bacterial pathogens had colonised on the glass surface in large numbers, producing a dense cluster of cells in the untreated control (Fig. 8e and h).

However, the biofilm-forming ability of both bacteria was significantly decreased when the cultures were treated with Ag@GMP material (Fig. 8f and h).

Additionally, confocal laser scanning microscopy (CLSM) was also used to examine the antimicrobial activity of the as-synthesized Ag@GMP. For that, biofilms were stained with acridine orange and then the confocal microscopy images of P. aeruginosa PAO1 and S. aureus MTCC 3160 biofilms were recorded with and without treating with Ag@GMP (Fig. 8i–l). The CLSM micrographs also exhibit that both the bacteria densely colonized on the glass surface in the absence of Ag@GMP, although their ability to form biofilm significantly reduced after treatment with Ag@GMP. Additionally, scanning electron microscopy was used to examine the influence of Ag@GMP treatment on biofilm architecture. Moreover, the SEM images of untreated and Ag@GMP-treated biofilms of the test bacteria (P. aeruginosa PAO1 and S. aureus MTCC 3160) are presented in Fig. S14, ESI.† The untreated P. aeruginosa PAO1 developed dense biofilms on a glass surface, with normal and smooth bacterial cell shapes (Fig. S14a, ESI†). Ag@GMP reduced the biofilm formation and decreased bacterial colonization (Fig. S14b, ESI†). Similarly, the biofilm development in untreated S. aureus MTCC 3160 was higher as compared to that in treated S. aureus MTCC 3160 (Fig. S14c and S14d, ESI†).

Conclusions

In summary, a waterborne silver nanoparticle-incorporated multifunctional Ag@GMP material was obtained from guanosine 5′-monophosphate and silver nitrate. Electron microscopy reveals the presence of silver nanoparticles on the vertically grown sheets. The PXRD pattern of the materials matches with that of the reported Ag NPs, which also favoured the SEM and TEM results. The significantly negative ζ-potential values suggest a predominantly negatively charged surface, which could facilitate CO₂ binding, also corroborated by the CO₂-TPD results, indicating the presence of basic binding sites. Ag@GMP displays excellent photocatalytic activity (18.6 μmol g⁻¹) with high selectivity (over ∼85%) for CO₂ conversion into CO at room temperature without any sacrificial agent. Moreover, Ag@GMP also displays excellent catalytic activity for the fixation of CO₂ with epoxides to afford cyclic carbonates at atmospheric pressure. The presence of Ag NPs makes Ag@GMP an efficient material for antibacterial agents, and the obtained results demonstrate efficient antibacterial activity for both Gram-negative and Gram-positive bacteria. The maximum zone of inhibition against S. aureus MTCC 3160 was found to be 15.83 ± 1.1 mm, and for E. coli, P. aeruginosa PAO1 and B. subtilis, it was 12.66 ± 0.9, 14.33 ± 0.8 and 12.8 ± 0.8 mm, respectively. The multifunctional nature of silver-based CPGs underscores their importance in addressing environmental and health challenges simultaneously, showcasing the immense potential of CPGs in advancing multifaceted solutions for a range of societal needs.

Author contributions

NA, SM, NO, SS, MTZ, SK and DS designed and conducted the research and co-wrote the manuscript. The final version of the manuscript is approved by all the authors.

Data availability

All the data supporting this article have been included in the main manuscript as well as ESI.†

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

Authors acknowledge IIT Patna for providing the research facilities. D. S. acknowledges the core research grant under sanction CRG/2023/003002 from the Science and Engineering Research Board, DST, Government of India. The anonymous reviewers are acknowledged for their efforts in reviewing and enhancing the quality of the work.

References

1. Z. Liu, W. Hou, H. Guo, Z. Wang, L. Wang and M. Wu, ACS Appl. Mater. Interfaces, 2023, 15, 33868–33877.
2. M. Laghaei, M. Ghasemian, W. Lei, L. Kong and Q. Chao, J. Mater. Chem. A, 2023, 11, 11925–11963.
3. E. Gong, S. Ali, C. B. Hiragond, H. S. Kim, N. S. Powar, D. Kim, H. Kim and S.-I. In, Energy Environ. Sci., 2022, 15, 880–937.
4. S. Singh, A. Modak, K. K. Pant, A. Sinhamahapatra and P. Biswas, ACS Appl. Nano Mater., 2021, 4, 8644–8667.
5. Z. Zhang, E. Jansen, S. P. Sobolowski, O. H. Otterå, G. Ramstein, C. Guo, A. Nummelin, M. Bentsen, C. Dong and X. Wang, Nat. Geosci., 2023, 16, 321–327.
6. J. Song, G. Tong, J. Chao, J. Chung, M. Zhang, W. Lin, T. Zhang, P. M. Bentler and W. Zhu, Sci. Rep., 2023, 13, 5536.
7. J.-L. K. Gbe, K. Ravi, E. K. Tillous, A. Arya, M. Grafoute and A. V. Biradar, ACS Appl. Mater. Interfaces, 2023, 15, 17879–17892.
8. J. Hegarty, B. Shindel, D. Sukhareva, M. L. Barsoum, O. K. Farha and V. Dravid, Environ. Sci. Technol., 2023, 57, 21080–21091.
9. X. Zheng, M. C. Drummer, H. He, T. M. Rayder, J. Niklas, N. P. Weingartz, I. L. Bolotin, V. Singh, B. V. Kramar and L. X. Chen, J. Phys. Chem. Lett., 2023, 14, 4334–4341.
10. E. Nishikawa, S. Islam, S. Sleep, V. Birss and J. Bergerson, Green Chem., 2023, 25, 229–244.
11. N. Alam, N. Ojha, S. Kumar and D. Sarma, ACS Sustainable Chem. Eng., 2023, 11, 2658–2669.
12. W. Dong, C. Wang, Y. Zou, W. Wang and J. Liu, ACS Appl. Mater. Interfaces, 2024 DOI:10.1021/acsami.4c01101.
13. T. Zhang, F. Meng, M. Gao, J. Wei, K. J. H. Lim, K. H. Lim, P. Chirawatkul, A. S. W. Wong, S. Kawi and G. W. Ho, Small, 2023, 19, 2301121.
14. Z. Zhang, H. Gao, H. Wu, Y. Qian, L. Chen and J. Chen, ACS Appl. Nano Mater., 2018, 1, 6463–6476.
15. B. Ge, Y. Ye, Y. Yan, H. Luo, Y. Chen, X. Meng, X. Song and Z. Liang, Inorg. Chem., 2023, 62, 19288–19297.
16. T. Zhang, T. Li, M. Gao, W. Lu, Z. Chen, W. L. Ong, A. S. W. Wong, L. Yang, S. Kawi and G. W. Ho, Adv. Energy Mater., 2024, 2400388.
17. P. Verma, F. A. Rahimi, D. Samanta, A. Kundu, J. Dasgupta and T. K. Maji, Angew. Chem., Int. Ed., 2022, 61, e202116094.
18. P. Verma, A. Singh, F. A. Rahimi, P. Sarkar, S. Nath, S. K. Pati and T. K. Maji, Nat. Commun., 2021, 12, 7313.
19. C. K. Karan, M. C. Sau and M. Bhattacharjee, Chem. Commun., 2017, 53, 1526–1529.
20. E. Saha, H. Jungi, S. Dabas, A. Mathew, R. Kuniyil, S. Subramanian and J. Mitra, Inorg. Chem., 2023, 62, 14959–14970.
21. N. Alam, S. Mondal and D. Sarma, Coord. Chem. Rev., 2024, 504, 215673.
22. N. Alam, S. Mondal, S. S. Hossain, S. Sahoo and D. Sarma, ACS Appl. Eng. Mater., 2023, 1, 1201–1212.
23. N. Rahman and I. Ahmad, Chemosphere, 2024, 351, 141272.
24. N. Rahman and I. Ahmad, J. Hazard. Mater., 2023, 457, 131783.
25. N. Alam, S. Majumder, S. J. Ray and D. Sarma, Langmuir, 2022, 38, 10601–10610.
26. S. Dhibar, B. Pal, K. Karmakar, S. Roy, S. A. Hafiz, A. Roy, S. Bhattacharjee, S. J. Ray, P. P. Ray and B. Saha, Nanoscale Adv., 2023, 5, 6714–6723.
27. Z. Zhan, T. Huang, J. Zhu, X. Cao, Y. Zhou, Y. Tang and P. Wu, CrystEngComm, 2024, 26, 639–646.
28. N. Sarfraz and I. Khan, Chem. – Asian J., 2021, 16, 720–742.
29. Y. Liu, C.-H. Liu, T. Debnath, Y. Wang, D. Pohl, L. V. Besteiro, D. M. Meira, S. Huang, F. Yang and B. Rellinghaus, Nat. Commun., 2023, 14, 541.
30. J.-J. Liu, Z.-W. Jiang and S.-W. Hsu, ACS Appl. Mater. Interfaces, 2023, 15, 6716–6725.
31. C. K. Karan and M. Bhattacharjee, Eur. J. Inorg. Chem., 2019, 2019, 3605–3611.
32. M. M. Golden, S. J. Post, R. Rivera and W. M. Wuest, ACS Infect. Dis., 2023, 9, 2386–2393.
33. W. Li, S. Hadjigol, A. R. Mazo, J. Holden, J. Lenzo, S. J. Shirbin, A. Barlow, S. Shabani, T. Huang and E. C. Reynolds, ACS Appl. Mater. Interfaces, 2022, 14, 25025–25041.
34. H. Liu, Y. Zuo, S. Lv, X. Liu, J. Zhang, C. Zhao, X. Xu, Y. Xu and X. Wang, ACS Appl. Mater. Interfaces, 2024, 16, 17182–17192.
35. S. Kumar, R. K. Majhi, A. Singh, M. Mishra, A. Tiwari, S. Chawla, P. Guha, B. Satpati, H. Mohapatra and L. Goswami, ACS Appl. Mater. Interfaces, 2019, 11, 42998–43017.
36. R. Zhao, M. Lv, Y. Li, M. Sun, W. Kong, L. Wang, S. Song, C. Fan, L. Jia and S. Qiu, ACS Appl. Mater. Interfaces, 2017, 9, 15328–15341.
37. X. Pang, Q. Xiao, Y. Cheng, E. Ren, L. Lian, Y. Zhang, H. Gao, X. Wang, W. Leung and X. Chen, ACS Nano, 2019, 13, 2427–2438.
38. X. Li, R. Gui, J. Li, R. Huang, Y. Shang, Q. Zhao, H. Liu, H. Jiang, X. Shang and X. Wu, ACS Appl. Mater. Interfaces, 2021, 13, 30434–30457.
39. W. Zhou, Z. Jia, P. Xiong, J. Yan, Y. Li, M. Li, Y. Cheng and Y. Zheng, ACS Appl. Mater. Interfaces, 2017, 9, 25830–25846.
40. K. Y. Gudz, L. Y. Antipina, E. S. Permyakova, A. M. Kovalskii, A. S. Konopatsky, S. Y. Filippovich, I. A. Dyatlov, P. V. Slukin, S. G. Ignatov and D. V. Shtansky, ACS Appl. Mater. Interfaces, 2021, 13, 23452–23468.
41. R. Kyarikwal, N. Malviya, A. Chakraborty and S. Mukhopadhyay, ACS Appl. Mater. Interfaces, 2021, 13, 59567–59579.
42. Y. Hu, W. Xu, G. Li, L. Xu, A. Song and J. Hao, Langmuir, 2015, 31, 8599–8605.
43. K. Loo, N. Degtyareva, J. Park, B. Sengupta, M. Reddish, C. C. Rogers, A. Bryant and J. T. Petty, J. Phys. Chem. B, 2010, 114, 4320–4326.
44. H. Sigel, Chem. Soc. Rev., 1993, 22, 255–267.
45. Y. Fu, X. Wang, J. Zhang and W. Li, Curr. Opin. Biotechnol., 2014, 28, 33–38.
46. J. V. Burda, J. Šponer, J. Leszczynski and P. Hobza, J. Phys. Chem. B, 1997, 101, 9670–9677.
47. L. Berti and G. A. Burley, Nat. Nanotechnol., 2008, 3, 81–87.
48. Y. Zhang, C. He, J. T. Petty and B. Kohler, J. Phys. Chem. Lett., 2020, 11, 8958–8963.
49. I. Goncharova, Spectrochim. Acta, Part A, 2014, 118, 221–227.
50. G. I. Dovbeshko, N. Y. Gridina, E. B. Kruglova and O. P. Pashchuk, Talanta, 2000, 53, 233–246.
51. H. Fidder, M. Yang, E. T. Nibbering, T. Elsaesser, K. Röttger and F. Temps, J. Phys. Chem. A, 2013, 117, 845–854.
52. S. Mondal and D. Sarma, Soft Matter, 2023, 19, 4926–4938.
53. S. Ray, A. K. Das and A. Banerjee, Chem. Commun., 2006, 2816–2818.
54. J. Fei, L. Gao, J. Zhao, C. Du and J. Li, Small, 2013, 9, 1021–1024.
55. A. Holmgren, B. Andersson and D. Duprez, Appl. Catal., B, 1999, 22, 215–230.
56. N. Ojha, A. Bajpai and S. Kumar, J. Colloid Interface Sci., 2021, 585, 764–777.
57. A. Porta, R. Matarrese, C. G. Visconti and L. Lietti, Energy Fuels, 2023, 37, 7280–7290.
58. Y. Yang, W. Liu, Q. Zhong, J. Zhang, B. Yao, X. Lian and H. Niu, ACS Appl. Nano Mater., 2021, 4, 4735–4745.
59. L. Zou, Z. A. Chen, D. H. Si, S. L. Yang, W. Q. Gao, K. Wang, Y. B. Huang and R. Cao, Angew. Chem., Int. Ed., 2023, 62, e202309820.
60. H. Sun and D. Zhang, J. Phys. Chem. A, 2007, 111, 8036–8043.
61. R. Patra and D. Sarma, Dalton Trans., 2023, 52, 10795–10804.
62. S. S. Dhankhar and C. M. Nagaraja, New J. Chem., 2019, 43, 2163–2170.
63. R. Babu, A. C. Kathalikkattil, R. Roshan, J. Tharun, D.-W. Kim and D.-W. Park, Green Chem., 2016, 18, 232–242.
64. J. Song, Z. Zhang, S. Hu, T. Wu, T. Jiang and B. Han, Green Chem., 2009, 11, 1031–1036.
65. R. Das, S. S. Dhankhar and C. Nagaraja, Inorg. Chem. Front., 2020, 7, 72–81.
66. M. Sinchow, N. Semakul, T. Konno and A. Rujiwatra, ACS Sustainable Chem. Eng., 2021, 9, 8581–8591.
67. Y. Yang, C. Tu, J. Shi, X. Yang, J.-J. Liu and F. Cheng, Cryst. Growth Des., 2022, 22, 4813–4820.
68. P. Das and S. K. Mandal, Chem. Mater., 2019, 31, 1584–1596.
69. M. Pardakhti, T. Jafari, Z. Tobin, B. Dutta, E. Moharreri, N. S. Shemshaki, S. Suib and R. Srivastava, ACS Appl. Mater. Interfaces, 2019, 11, 34533–34559.
70. H. Chen, L. Fan, X. Zhang and L. Ma, ACS Appl. Mater. Interfaces, 2020, 12, 27803–27811.
71. L.-G. Ding, B.-J. Yao, W.-X. Wu, Z.-G. Yu, X.-Y. Wang, J.-L. Kan and Y.-B. Dong, Inorg. Chem., 2021, 60, 12591–12601.
72. R. Patra and D. Sarma, ACS Appl. Mater. Interfaces, 2024, 16, 10196–10210.
73. R. K. Aparna, S. Mukherjee, S. S. Rose and S. Mandal, Inorg. Chem., 2022, 61, 16441–16447.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr03254c

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