Development of New Thio-Functionalized Resins for the Study of Confinement of High Solubility Nanoparticles: An Alternative to Reduce the Future Ecological Impact

Abstract

The rapid advance of nanotechnology has led to the proliferation of a wide range of water-soluble nanomaterials and therefore, a profound impact on the environment, underscoring the need to devise effective methods for isolating and confining these nanomaterials. This study presents the development of a highly effective system capable of removing and storing gold nanoparticles in solution using modified resins with different linking groups, such as tris(2-aminoethyl) amine, 3-mercaptopropionic acid, 11-mercaptoundecanoic acid, and L-cysteine. The catalytic properties of the resins functionalized with the thiol group containing gold nanoparticles were evaluated, using the reduction of the nitro compounds (3,5-dinitrosalicylic acid, 4-nitroguaiacol and 4-nitroaniline) as model reaction. Solid-phase organic synthesis was employed to prepare the modified resins and their interaction with gold nanoparticles was characterized using infrared spectroscopy, thermogravimetric analysis, field emission scanning electron microscopy-energy dispersive spectroscopy, and X-ray photoelectron spectroscopy. The removal capacity of the resins was evaluated using ultraviolet-visible optical emission spectroscopy and inductively coupled plasma techniques. Ultraviolet-visible technique was suitable to assess the catalytic properties of the functionalized resins containing gold nanoparticles. It was determined that the functionalized resins showed remarkable capacity for complex gold nanoparticles, removing them from the solution. The most significant removal was with 11-mercaptoundecanoic acid.

Keywords:
solid-phase; resins; remotion of AuNPs; catalysis

Introduction

In the last 30 years, nanotechnology and nanomaterials have transformed the field of chemistry. This transformation began with the ability to characterize these nanomaterials at the atomic scale, and has rapidly evolved with new techniques for describing and manipulating materials at the nanometer scale. The study of the intrinsic properties of nanoparticles has been crucial in understanding the unique features they exhibit at the nanoscale. Consequently, nanomaterials are increasingly being incorporated into a wide range of consumer products and commercial processes. An exceptional example of nanomaterials commonly found in daily life are metal nanoparticles (M-NPs); due to their unique properties, they are used in electronic, optical, catalytic, medical coatings, and sensor applications, among others.¹ Nonetheless, the lack of information on environmental toxicity associated with the disposal and exposure to nanoparticles poses risk to both the environment and human beings. Therefore, gaining more knowledge about its properties and interaction with the environment is crucial.² Additionally, these materials have been utilized in catalysis and treatment of industrial waste.³

Given the potential risk of nanoparticles disposal to the environment, searching for new tools for their detection and entrapment is pivotal. One of the methods that can address the problem of nanoparticle contamination is using resins as adsorbents. The thio functionalized polymeric materials have enormous potential for environmental remediation.⁴,⁵,⁶,⁷,⁸ Polystyrene has the advantage of supporting gold nanoparticles (AuNPs) of various sizes, diversity of functional groups, chemical stability, and it is easy to handle and recover using simple filtration.⁹ Most current absorbents are functionally modified poly-(styrene-divinylbenzene)-resins, which have been extensively studied in recovering and removing heavy metal ions in aqueous solutions due to their mechanical strength and easy chemical modification.¹⁰ Additionally, chelating groups can be produced or constructed on these polymeric resins to correspond to an analyte, which in this work could be the nanoparticles in the solution.¹¹

A significant organic pollutant is 4-nitrophenol due to its numerous residual sources which increase the discharge of these residues in the water. Therefore, it is evident that an efficient decomposition of this compound is crucially required.¹² A solution to this problem is developing a system that allows the catalytic reduction of this compound and is environmentally friendly.

In our group, a variety of research related to solid-phase organic synthesis (SPOS) has been conducted using polystyrene-based polymeric resins, with Merrifield and Wang being the most widely explored.¹³, ¹⁴, ¹⁵, ¹⁶, ¹⁷ These resins have been modified for use as metal sensors of species of environmental concerning species,¹⁶, ¹⁷ as a metal ion sensor of biological interest,¹³ in the preparation of supported chemical reagents,⁸ to monitor chemical reactions in the solid phase,¹⁴ and the syntheses of peptides with high affinity to enzymatic complexes and their signaling, among others applications.¹⁵, ¹⁸ Mesoporous materials that contain thiadiazole have been prepared, which include a high degree of nitrogen and sulfur and are very efficient in removing mercury [Hg^II] from contaminated water.¹⁹ Nanospheres have also been prepared with triazine covalently bonded to a thioether, proving to be an excellent robust adsorbent for removing Hg^II.²⁰ Gold has a high affinity when interacting with sulfur, forming a covalent bond, and releasing molecular hydrogen. This interaction is powerful due to the soft character of both gold and sulfur. Therefore, thiol groups bind strongly to gold,²¹ and NPs have shown excellent stability and can be stored for years.²²

In this work, different acids such as 3-mercaptopropionic acid (MPA), 11-mercaptoundecanoic acid (MUA), and L-cysteine (Cys) were used to chemically modify the polymeric resin trizamine (R-TrisA) chemically. AuNPs in aqueous solution were removed by resins modified with thiols. The resin’s ability to remove the AuNPs in the solution was evaluated. The catalytic properties in the reduction of nitro compounds²³, ²⁴ are assessed, similar to the work done by Shah and Kaur.²⁵ Contamination by traces of nitroaromatic dyes is also a significant environmental concern due to the high toxicity of these dyes and their widespread use in daily life.²⁶ The confinement of the AuNPs allows them to be stored and catalytically reused more than six times without losing their capacity.

Experimental

General methods

Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectra were recorded using PerkinElmer FT-IR/FT-NIR model Spectrum 400 spectrophotometer (Shelton, USA). The analyzes were carried out directly with the pure compounds on the ATR device. The transmittance was expressed in cm⁻¹ between 4000 and 500 cm⁻¹.

The ultraviolet-visible (UV-Vis) spectra were collected with a Varian CARY 50 Scan UV-Vis spectrophotometer (Santa Clara, CA, USA). The analyzes were carried out in a cell with an optical path of 1 cm with solutions of the nitro-aromatic compounds at a concentration of 5.55 mM. The absorbance was expressed in nm between 250 and 550. The reaction was analyzed by UV-Vis and the spectra were collected every 10 min for 60 min.

The reactions performed in a microwave (MW) reactor were performed using a Discover System from CEM Corporation at a 2.45 GHz frequency with maximal power output of about 200 W with internal magnetic stirring. For the reactions, the power of the microwave reactor used was 100 W (CEM Corporation, Montvale, NJ, USA).

Field emission scanning electron microscopy (FESEM) with a JEOL 7800F Prime determined the morphology (Peabody, MA, USA). Energy dispersive spectroscopy (EDS) determined the surface elemental composition in a Bruker QUANTAX 200 detector (Boston, MA, USA). The analyzes were carried out by depositing the resin beads on an aluminum tape with special rubber to fix them. Excess solid was removed with pressurized air. The samples were exposed to 350 nm radiation with an air flow for 10 min to clean the surfaces with the formation of ozone. The samples were placed in a special support and introduced into the main chamber of the microscope for observation.

Thermogravimetric analysis (DSC-TGA) were collected with a TA Instruments, SDT 2960 Simultaneous (Waltham, MA, USA). To carry out the analysis, 8 mg of sample was used in a nitrogen source to avoid oxidation of the organic material.

X-ray photoelectron spectroscopy (XPS) spectra were collected from an ESCALAB 250 Xi XPS (Thermo Fischer Scientific Inc. Waltham, MA, USA). The analyzes were carried out with ca. 8 mg of sample.

Inductively coupled plasma optical emission spectroscopy (ICP-OES) determined the elemental analysis in a PerkinElmer 6300 (Shelton, USA). The analyzes were done with the AuNP solutions before and after the interaction with all resins used in this project.

Chemical reagents

The following reactants: mercapto propionic acid (99%), cysteine (> 97%), and mercapto undecanoic acid (> 95%), hydroxy benzotriazole (> 97%), diisopropyl carbodiimide (> 98%), diisopropylethylamine, (> 95%), dimethyl amino pyridine (> 98%), chloroauric acid (> 49% Au), sodium citrate, 3,5-dinitrosalicylic acid (> 98%), 4-nitroaniline (> 99%), 4-nitroguaiacol (97%), sodium borohydride (> 98%), were purchased of Sigma-Aldrich, Merck (Toluca, Mexico), and used without further purification.

Tris (2-aminoethyl) amine Resin (R-TrisA) with a loading of 0.8 mmol g⁻¹ was purchased from Creosalus, Inc.

The solvents (dichloromethane, dimethylformamide, acetone, and methanol) were purchased from Sigma Aldrich, Merck (Toluca, Mexico) and were used without further purification.

Functionalization of R-TrizA with MPA, MUA, and Cys (2a-2c)

The R-TrizA (3.0 g, 2.4 mmol) was washed with dichloromethane (DCM) (30 mL × 2) and dimethylformamide (DMF) (30 mL × 2), then dried for two hours under reduced pressure. The R-TrizA was swollen with 30 mL of DCM for 20 min thorough mechanical agitation. The resin was split into three different flasks. It was added a solution of 14.4 mmol of the corresponding compound (MPA, Cys, and MUA) and hydroxy benzotriazole (HOBt) (14.4 mmol) in 25 mL of DMF and left to react for 15 min, was added a solution of diisopropyl carbodiimide (DIC) (1.94 g, 14.4 mmol), diisopropylethylamine (DIEA) (1.86 g, 14.4 mmol,) and a catalytic amount of dimethyl amino pyridine (DMAP) in 25 mL of DMF to the mixture reaction and allowed under orbital stirring for three hours. Each resin was dried in a porous plate funnel and washed with DMF (30 mL × 3), H2O (30 mL × 3), CH3OH (30 mL × 3), and acetone (30 mL × 3). The functionalized resins were dried under reduced pressure for 18 h.

R-TrizA (1)

FTIR-ATR (neat) v / cm⁻¹ 3332 (N-H), 3062, and 3021 (=C–H), 2919, and 2840 (C–H), 1602 (C=C).

R-TrizA-MPA (3a)

A white solid was obtained. Yield 990 mg (99.0%, recovery, > 97% conversion); FTIR-ATR (neat) v / cm⁻¹ 3062, and 3027 (=CH), 2925, and 2848 (C-H), 1648 (C=O), and 1601 (C=C), 1257 (C–O).

R-TrizA-MUA (3b)

A white solid was obtained. Yield 984 mg (98.4% recovery, > 97% conversion); FTIR-ATR (neat) v / cm⁻¹ 3057, and 3024 (=CH), 2919, and 2848 (C-H), 1639 (C=O), and 1598 (C=C), 1265 (C–O).

R-TrizA-Cys (3c)

A white solid was obtained. Yield 988 mg (98.8%, recovery, > 97% conversion); FTIR-ATR (neat) v / cm⁻¹ 3060, and 3021 (=CH), 2925, and 2845 (C-H), 1680 (C=O), 1601 (C=C), 1317 (C–O).

Synthesis of AuNPs

The AuNPs were synthesized by modifying the method used by Augustine et al.,²⁷ obtaining nanoparticles with an average size of 7.7 nm. For the synthesis, the assistance of microwave radiation was required, with output conditions of 800 W and 2.45 GHz. Tetrachloroauric acid (HAuCl₄) (0.5 mM) in an aqueous solution (1.5 mL) was placed in a 10 mL container, and a solution (1.5 mL) of 3.8 mM sodium citrate was added. The reaction mixture was heated to 100 °C for 5 min. After cooling to room temperature, the size of the formed AuNPs in a colloidal solution was analyzed.²⁷

Evaluation of modified R-TrizA with AuNPs solution (4)

The resins (R-TrizA-MPA, R-TrizA-MUA, and R-TrizA-Cys) (300 mg, 0.24 mmol) were added to a solution (20 mL) of freshly prepared of AuNPs. The reaction mixture was shaken on an orbital shaker, and changes in AuNPs concentration were monitored by UV-Vis spectroscopy.

Catalytic reduction of nitro compounds

The catalytic reduction kinetics were studied in a systematic manner, using a standard quartz cell with 1 cm in length and 4.5 mL volume. The experiment was carried out by mixing water (2.27 mL) with 3.5-dinitrosalicylic acid (7 mg, 027 mmol), 4-nitroaniline (4 mg, 0.027 mmol), or 4-nitroguaiacol (5 mg, 0.027 mmol). A fresh sodium borohydride solution was added (2 mg, 0.054 mmol) as per the procedure. After mixing the solution, the R-TrizA coupled with AuNPs (5 mg) was added. The UV-Vis technique was used to analyze the reaction, and the spectra were collected every 10 min for 60 min in a 200-550 nm range.

Results and Discussion

Water-soluble metal nanoparticles (NPs) have been a significant development due to their applications in various fields and ultrasmall size which confer unique properties.²⁸ Nano or ultrafine particles of metals or oxides increase their solubility in water, contaminating it and presenting risks to humans. Therefore, it is crucial to develop new technologies for the removal and confinement of these particles to ensure their stabilization.²⁹ Thus, it is highly relevant to contemplate a specific method to confine these materials and thus avoid possible contamination. In this work, the use of polymerized, specifically functionalized resins is explored. The choice of functionalized resins is significant because they have specific chemical groups attached to them, enhancing their ability to confine nanoparticles. To carry out the work of confinement, the first part was to condense the R-TrizA resin with linking groups such as MPA, MUA, and Cys (Scheme 1). Thiol resins were added to a solution of AuNPs to determine the removal capacity of this metal. Subsequently, they can be recovered or used as catalysts in reduction processes.

Scheme 1
The general methodology for the synthesis of the micro-nano system using Resin-Trizamine (R-TrizA) (1).

R-TrizA resins, with their three arms featuring amino groups, offer a unique chemical structure. One arm is covalently attached directly to the resin, while the other two are free to couple with thiol-carboxylic acids (MPA, MUA, Cys) in the amide form. These acids contain a thiol group (–SH) that reacts with AuNPs, favoring retention due to the well-known high affinity of thiol groups for gold.³⁰ This property is of significant practical importance, as it enhances the stability of the S–Au–S bond, a critical factor in resin synthesis.

Infrared analysis

The infrared spectroscopy analyses of the different resins used in this study demonstrate consistency and precision, as they align with the characteristic functional groups of each of the resins. For R-TrizA (1), a vibration at 3335 cm⁻¹ is observed, corresponding to the stretching mode of the terminal amino group of the resin-trizamine. Similarly, in the cases of resins modified with mercapto-carboxylic acid, a vibration that occurs in the region of 1680-1639 cm⁻¹ can be seen, corresponding to the stretching mode of carbonyl group of the amide formed in the resins. This precision confirms the chemical modification of the resins with the new amide group formed.

Thermogravimetric analysis

The different modifications of R-TrisA were tracked by thermogravimetric analysis. The R-Tris A resin served as a support and modified with three compounds (MPA, MUA, and Cys). R-Tris A was used as a control for the experiments. Its thermogram shows two weight losses: the first at 320 °C, associated with the trisamine functional group, and the second at 440 °C, corresponding to the weight loss due to decomposition of the resinous matrix. Figure 1 shows the thermogravimetric analysis of R-TrisA-MUA. The first weight loss occurs at 255 °C is attributed to the decomposition of the resin’s MUA; the second weight loss occurs at 446 °C corresponds to the resin matrix decomposition. The other two resins; (R-TrisA-MPA in Figure S24 and R-TrisA-Cys in Figure S25 (Supplementary Information (SI) section)), show similar weight losses. Figure S24 shows the band between 265 °C; this new loss corresponds to the decomposition of mercapto-propionic amide. The last resin modified with Cys presents a band at 270 °C, corresponding to eliminating the Cys-amide group (Figure S25). All resins present the weight loss corresponding to the polystyrene resin matrix (446 °C). This observation is logical, as it matches to the temperature range in which an amino acid bond is broken; the height of the band is related to the amount of anchored material. The anchoring of these compounds to the polystyrene resin was through a peptide bond. The thermal-gravimetric analysis has been relevant in corroboration of the resin’s structural changes and allows us to evaluate the degree of conversion.³¹

Figure 1
Thermograms of R-TrizA and after its coupling with MUA.

Field emission scanning electron microscopy analysis

Figures 2a,2b,2c presents the resins used in the successful synthesis route of this work. Figure 2a demonstrate the successfully maintenance of particle sphericity. Figure 2b showcases the R-TrisA resin, which was successfully functionalized with the MUA through an amidation reaction. Figure 2c reveals the successfully interaction of the resin with the AuNPs solution. All images confirm successfully outcome of the synthesis, with the morphology remaining intact and no chemical degradation or mechanical damage, reassuring the reliability of our findings.

Figure 2
FESEM images are presented: (a) R-TrisA resin, (b) R-TrisA-MUA resin, and (c) functionalized resin with the retention of the AuNPS.

In Figure 3, the FESEM images are presented. Figure 3a shows the R-TrisA-MUA resin and the other images corresponding to the elemental mapping of the resin, showing the different components. Figure 3b shows nitrogen; Figure 3c carbon; Figure 3d oxygen; and Figure 3e sulfur. This result shows that this resin containing the sulfur atom can retain the AuNPs. The image in Figure 3e shows that the resin R-TrisA-MUA contains the sulfur atoms responsible for forming covalent bonds with the surface of the Au-NPs, which will be removed from the solution.²¹

Figure 3
FESEM images are presented: (a) R-TrisA-MUA resin, elemental analysis; (b) nitrogen, (c) carbon, (d) oxygen; and (e) sulfur.

A thorough elemental analysis accompanies the images of the R-TrisA-MUA-AuNPs resin in Figure 4. Figure 4a depicts the resin and its morphology, which remains undamaged, reaffirming the integrity of the resin microparticle and the unaffected kinetics of the removal of the gold nanoparticles. The other images, Figure 4b nitrogen, Figure 4c carbon, Figure 4d oxygen, and Figure 4e sulfur, correspond to the mapping of the microparticle, providing a comprehensive and reassuring elemental analysis of the atomic components. Figure 4f corresponds to the sulfur mapping. After gold removal, a lower presence of sulfur is denoted compared to Figure 3e. This directly results from the interaction between AuNPs and sulfur; the former effectively blocks the latter.

Figure 4
FESEM images are presented: (a) R-TrisA-MUA-AuNPs resin, elemental analysis; (b) nitrogen, (c) carbon, (d) oxygen, (e) gold, and (f) sulfur.

Figure 5 displays the EDX spectrum, a method that definitively identifies the components in the image. This is achieved by measuring the secondary electrons and determining their specific energy values, identifying the corresponding element type.³² The components of the original resin, such as carbon and nitrogen, are clearly shown, and the presence of sulfur indicates their functionalization. Most importantly, the extraction of AuNPs was unequivocally confirmed, providing a solid foundation for research.

Figure 5
Spectrum performed on the R-TrisA-MUA-AuNPs resin showing the determined components.

Table 1 meticulously details the elemental composition of the resin microparticles that have effectively removed the AuNPs.³² This comprehensive analysis reveals the gold content found on the surface of the microparticle selected for this study, along with other components such as carbon, oxygen, and nitrogen. A significant amount of gold, 13.91% by mass, is found within the microparticles, further demonstrating the precision and accuracy of the analysis. The low percentage of the atomic mass (4.68%) directly results from the heavy weight of gold, making its percentage appear small.

Table 1
Elemental composition of R-TrisA-MUA-AuNps resin

Element	Mass / %	Atomic mass / %
Carbon	13.34	73.65
Oxygen	3.92	16.25
Gold	13.91	4.68
Nitrogen	1.14	5.42
Total	100.00	100.00

The information obtained from the FESEM images demonstrated the integrity of the resins in the different processes, and the EDS demonstrated the removal capacity of the AuNPs.

X-ray photoelectron spectroscopy (XPS) analysis

The XPS analysis, conducted with meticulous precision, has elucidated the structural modifications of the R-TrisA-MUA resin. In the C1s spectrum (Figure 6a), the distinctive peaks at 284.6 eV,³³ corresponding to C=C bonds are observed, which are characteristic of the polystyrene framework of the resin microparticles. Furthermore, we identified signals attributed to C–N bonds, typical of the MUA and the tris amine arms, defined at 285.2 eV³⁴ are identified, providing robust evidence of the functionalization process. Notably, binding energies corresponding to C–S (286.9 eV),³⁴ HN–C=O (287.7 eV),³⁴ and C=O (289.1 eV)³⁴ affirmed the presence of thiol and amine functionalities within the MUA structure, with corresponding evidence provided by the S2p spectrum (Figure 6b) at 163.1³⁵ and 164.3 eV.³⁵

Figure 6
The XPS analysis shows the structural modifications of the R-TrisA-MUA resin: (a) spectrum that shows the bands for carbon, (b) spectrum that represents the corresponding sulfur bonds, (c) spectrum of nitrogen and its corresponding bonds, (d) spectrum that exhibits bonds with oxygen.

Analysis of the N1s spectrum (Figure 6c) has revealed signals at 400.3³⁴, ³⁶ and 401.9 eV,²⁰, ³⁴ indicative of secondary and tertiary amines within the Tris amine moiety. Finally, the O1s spectrum (Figure 6d) has peaks at 531.7³³, ³⁵ and 533.2 eV,³⁵ attributed to amide groups and carbonyl, respectively. Remarkably, the presence of amide functionalities across C1s, N1s, and O1s spectra has provided compelling evidence of the successful modification of the tris-amine moiety with the MUA structure. This is a significant achievement, and the comprehensive XPS analysis offers invaluable insights into the chemical transformations underlying the resin modification process.

The results obtained from FTIR spectroscopy, thermograms, FESEM, EDS, and XPS show the changes generated in the resins: (i) the incorporation of new functional groups, (ii) the integrity of the resins, (iii) the elemental analysis, and finally, (iv) the types of chemical bonds that are part of the polymer matrix of the resins. All analyses are consistent with the proposed structure of the resins.

AuNPs synthesis

The synthesis was carried out by modifying the method proposed by Augustine et al.²⁷ The volume ratio of the metal precursor and the reducing-stabilizing agent was varied. For all the analyses, a volumetric ratio of 2:3 was used. The synthesized AuNPs were analyzed using DLS. The analysis revealed that the nanoparticles have a hydrodynamic radius of 24 nm (see Figure S38, SI section).

Study of the removal of AuNPs

The resins, pivotal in removing the AuNPs, were meticulously analyzed by UV-Vis, and the spectra of these analyses are presented in Figure 7. The starting resin, R-TrisA, with its two arms featuring primary amines (Figure 7a), served as the control. Both arms were modified to yield the subsequent resins through an amidation reaction with Cys (Figure 7b) and coupling with AMP (Figure 7c) and MUA (Figure 7d). The AuNPs confinement reactions were then performed under the same conditions for the three samples and meticulously analyzed at various intervals. The decrease in the band at 520 nm, corresponding to the surface plasmon of the AuNPs and thus proportional to the concentration in the solution, was precisely denoted.

Figure 7
UV-Vis spectra for AuNPs that interact with R-TrizA (a), R-TrizA-Cys (b), R-TrizA-MPA (c), and R-TrizA-MUA (d).

In all cases, a decrease in the band at 520 nm is observed. The band corresponds to the surface plasmon of AuNPs; thus, it can be ascribed to the disappearance of AuNPs from the bulk. Figure 8 shows five samples with different concentrations of AuNPs. The color changes are observed proportionally to the concentration. These samples were analyzed using the UV-Vis technique to determine the color change quantitatively. The change of the surface plasmon is denoted in the different samples presented at various contact times.

Figure 8
Samples show changes in color depending on the contact time with the R-TrisA-MUA: (a) 0, (b) 10, (c) 30, (d) 40, and (e) 50 min.

On the other hand, it was calculated the efficiency in the removal of AuNPs for each of the resins evaluated from the equation:³⁷

(1) $\text{x}=\left({\text{C}}_0-\text{C}\right)\times \frac{100}{{\text{C}}_0}$

where C₀ and C are the concentrations before and after removal, respectively.

The results of removing AuNPs in the solution demonstrate a clear advancement: R-TrizA-MUA outperforms R-TrisA in speed and efficiency. In just 60 min (Figure 9), R-TrizA-MUA shows the highest efficiency among the resins, surpassing R-TrizA and other resins. Figure 9 also highlights the superior speed of removal for the R-TrizA-MUA resin. This finding confirms the superiority of R-TrizA-MUA and suggests potential for further advancements in the field. Additionally, it is interesting to note that R-TrizA (see Table 2) may interact through secondary amines with gold nanoparticles; however, this interaction is weaker than thiol groups.³⁸

Figure 9
Removal of AuNPs in a solution using the different synthesized resins (λ = 520 nm).

Table 2
AuNPs removal with functionalized resins

Compound	Resin	Remotion / %
1	R-TrizA	88.5
3a	R-TrizA-MPA	84.6
3b	R-TrizA-MUA	97.7
3c	R-TrizA-Cys	71.4

Table 2 describes the excellent capacity of the resins modified with thiol groups to remove the AuNPs in solution; in particular, the R-TrizA-MUA resin quantitatively removes the AuNPs. This removal capacity is mainly because the length of the binder contains 11 carbons, which presents better mobility and more degrees of freedom to interact with the solution. This property has already been confirmed with the working group.³⁹

Catalytic evaluation in the reduction of nitro compounds

Three different nitro compounds were used to study the catalytic effect of AuNPs confined in polymeric resins (R-TrizA, R-TrizA-MPA, R-TrizA-MUA, and R-TrizA-Cys): (a) 3,5-dinitrosalicylic acid, (b) 4-nitroaniline, and (c) 4-nitroguaiacol (see Scheme 2). The catalysis of the reduction of aromatic nitro groups with sodium borohydride (NaBH₄) has been reported by the direct deposition of AuNPs on commercially available resins using the sorption reduction method.²⁶, ⁴⁰,⁴¹, ⁴², ⁴³ No reports have been found where AuNPs are covalently bound to the thiol group of polymeric resins. The results of each of the nitro compounds used in the study of catalytic reduction are shown.

Scheme 2
Catalytic reduction of nitroaromatic compounds.

Reduction of nitroaromatic compounds to their corresponding amines

Catalytic study of 3,5-dinitrosalicylic acid

The reaction kinetics were studied using UV-Vis spectroscopy, which measures light absorption in the ultraviolet-visible spectral region. The spectra obtained are shown in Figure 10, and show the characteristic signal at 369 nm for 3,5-dinitrosalicylic acid. This band decreases as the nitro group is reduced, and a new one appears at 339 nm, corresponding to the reduction of the nitro group to amine (Figure 10). Interestingly, a more significant decrease in the band at 369 nm can be observed when using the catalysts (R-TrizA-AuNPs, R-trisA-MPA-AuNPs, R-TrisA-MUA-AuNPs, and R-Tris-Cys-AuNPS).⁴⁴

Figure 10
UV-Vis spectra for the reduction of 3,5-dinitrosalicylic acid without catalyst (a), with R-TrizA (b), R-TrizA-Cys (c), R-TrizA-MPA (d) and R-TrisA-MUA (e) as a catalyst.

Conversely, when no catalysts are used, the decrease in the band at 369 nm is almost imperceptible.

Moreover, considering the absorbance at 369 nm, it determined that the reactions followed the first-order kinetics (Figure 11) as the conversion of the reagents into products depended only on their concentration. However, when the catalyst is not used, the reaction follows zero-order kinetics because the speed is independent of the concentration of the reagents.⁴⁵

Figure 11
Graph of ln (At/A0) against time for the reduction of 3,5-dinitrosalicylic acid.

The rate constants were obtained for each reaction (Table 3). The use of the catalyst increases the reaction speed almost 30 times, mainly because AuNPs offer a greater surface area, providing a more significant number of available sites for the catalytic reaction to take place.⁴⁶, ⁴⁷, ⁴⁸ The catalysis results of 3,5-dinitrosalicylic acid demonstrate the efficiency of the resins functionalized with the thiol group. The R-TrizA-Cys-AuNPs has two free functional groups: the amino and the thiol. The amino group interacts with the 3,5-dinitro salicylic acid, forming a salt on the surface of the resin that increases contact with the AuNPs and, consequently, enhances catalysis.

Table 3
Rate constants for catalytic reactions

Compound	Resin	Catalyst	k / (103 s−1)
1	R-TrizA	R-TrizA-Au	44.3
3a	R-TrizA-MPA	R-TrizA-MPA-AuNPs	56.1
3b	R-TrizA-MUA	R-TrizA-MUA-AuNPs	60.5
3c	R-TrizA-Cys	R-TrizA-Cys-AuNPs	61.9

• Reaction control conditions: NaBH₄ as reducing agent, without catalyst (k = 2.2 × 10³ s⁻¹). k: rate constant.

Catalytic study of 4-nitroaniline

The catalytic activity was evaluated for 4-nitroaniline (see Scheme 2) under conditions similar to those used in the previous catalysis; the results are found in Table 4. Again, the diminished effect of the reduction is denoted when the supported gold catalyst is absent. The catalyst with the best catalytic activity incorporates both the MPA and the MUA, which, with their thiol groups, covalently bind the AuNPs and play a pivotal role in promoting the catalytic effect. It is worth mentioning that R-TrizA presents good retention of AuNPs, but does not translate into enhanced catalytic activity because its amines group acts as a passivant⁴⁹ on the gold surface. Conversely, the resins containing the thiol group, such as MPA and MUA, and that have removed the AuNPs, have been the most efficient in the 4-nitroaniline catalytic process.

Table 4
Rate constants for catalytic reactions of 4-nitroaniline

Compound	Resin	Catalyst	k / (103 s−1)
1	R-TrizA	R-TrizA-Au	60.1
3a	R-TrizA-MPA	R-TrizA-MPA-AuNPs	102.4
3b	R-TrizA-MUA	R-TrizA-MUA-AuNPs	100.2
3c	R-TrizA-Cys	R-TrizA-Cys-AuNPs	1.1

• Reaction control conditions: NaBH₄ was used as a reducing agent, without catalyst (k = 0.4 × 10³ s⁻¹). k: rate constant.

Cysteine has the carboxylic acid covalently linked to the resin, forming an amide group, and possesses free amino and thiol groups. The resin R-TrizA-Cys and the nitro-aromatic contain amino groups that act as passivant; consequently, the lowest constant is presented, indicating the suppression of the catalytic reaction.

Catalytic study of 4-nitroguaiacol

The compound 4-nitroguiacol was used to continue evaluating the catalytic activity in the reduction process of the nitro group to its corresponding amine (see Scheme 2). This compound presents two activating groups: the phenolic OH and the methoxy. Once again, the R-TrizA-MUA-AuNPs demonstrated the highest efficiency in the reduction process, followed by the R-TrisA-Cys-AuNPs. The reduction was less efficient for this aromatic nitro with an activating group (Table 5).

Table 5
Rate constants for catalytic reactions of 4-nitroguaiacol

Compound	Resin	Catalyst	k / (103 s−1)
1	R-TrizA	R-TrizA-AuNPs	0.7
3a	R-TrizA-MPA	R-TrizA-MPA-AuNPs	0.02
3b	R-TrizA-MUA	R-TrizA-MUA-AuNPs	48.9
3c	R-TrizA-Cys	R-TrizA-Cys-AuNPs	43.2

• Reaction control conditions: NaBH₄ as reducing agent, without catalyst (k = 0.07 × 10³ s⁻¹). k: rate constant.

A plausible mechanism of catalysis

The proposed mechanism for reducing nitro-aromatic compounds is a complex process that unfolds in three stages. (i) NaBH₄ initiates the interaction of the hydride with the surface of the AuNPs, forming a metallic hydride, (ii) the nitro-aromatic compounds are adsorbed on the surface of the AuNPs. Consequently, NaBH₄ transfers the hydride to the nitro-aromatic compounds, generating nitroso-benzene and water as products, and (iii) the nitroso group is reduced to hydroxylamine⁴² and later amino compounds.⁵⁰, ⁵¹ The intricacy of this process, as illustrated in Scheme 3, underscored the depth of this research.

Scheme 3
Proposed mechanism for catalytic reduction of nitro-aromatic compounds.

Table 6 summarizes the results of this investigation, indicating the resin with the most significant removal activity and describing the resins that presented the most excellent activity in reducing nitro groups to their corresponding amines.

Table 6
The final results of the functionalized resin show a better capacity for removal and catalysis

Compound	Effect	Nitro compound	Resin	Remotion / %	k / (103 s−1)
1	removal	-	R-TrizA-MUA	97.7	-
3b	catalyst	3,5 dinitro salicylic acid	R-TrizA-MUA-AuNPs	-	60.5
3b	catalyst	4-nitroaniline	R-TrizA-MUA-AuNPs	-	100.2
3b	catalyst	4-nitroguaiacole	R-TrizA-MUA-AuNPs	-	48.9

• k: rate constant.

In summary, among the resins analyzed and the nitro-aromatic compounds used as test compounds for the catalytic capacity of the confinement of gold nanoparticles, it was determined that the R-TrizA-MUA resin was the one that presented the most efficient in the removal of nanoparticles as well as in the catalytic process. MUA, with a chain of 11 carbons, is the longest binder, affording degrees of freedom to enhance the interactions with AuNPs solutions and those of the reduction system. This finding aligns with previous research observations.³⁹

Conclusions

The resins modified with thiol groups proved to be highly effective in AuNPs removal and served as excellent catalysts for reducing nitroaromatics in aqueous solutions at room temperature. Thermogravimetric analysis was crucial for assessing the degree of substitution in the amidation reaction with carboxylic acids (MPA, MUA, and Cys). The integrity of the morphology and the catalytic processes of the nanoparticles were evaluated in the synthesis by FESEM microscopy. The elemental analysis by EDX confirmed surface sulfur presence on functionalized resins, with post-AuNPs removal showing decreased sulfur, indicating resin interactions with AuNPs. XPS spectroscopy was crucial in demonstrating sulfur covalently linked to the resins. The microwave-assisted method proved to be more efficient in terms of time and ease than traditional methods for synthesizing AuNPs.

While R-TrizA contributed to AuNPs removal, its catalytic activity was limited by passivating amine group interactions, hence showing a less catalytic efficacy. In contrast, R-TrizA-MUA, emerged as the most efficient agent in the removal of AuNPs as well as in the catalytic activity (97.7%). This resin features MUA with its long aliphatic chain (10 methylenes) and a high degree of freedom, and the arms proved to be more effective to stabilize the AuNPs by the increase of the chelating effect, which does not occur in the MPA or Cys cases. Catalytic reactions exhibited first-order kinetics, with markedly increased rates attributed to the surface area of the nanoparticles and their interaction with various resins. Overall, the R-TrizA-MUA system demonstrated exceptional efficiency in the development of micro-nanosystem for nanoparticle removal, confinement, and catalysis.

• Supplementary Information

  Supplementary data (UV-Vis, FTIR, FESEM, EDX, XPS, and TGA spectra) are available free of charge at http://jbcs.sbq.org.br as a PDF file.

Acknowledgments

Authors gratefully acknowledge support for this project by Consejo Nacional de Ciencia y Tecnologia (CONAHCyT, grant CF-2023-I-327) and graduate scholarship (No. 583636), also acknowledge Tecnológico Nacional de México for supporting this project (Clave 7814.201-P).

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Edited by

Editor handled this article: Célia M. Ronconi (Associate)

Data availability

Supplementary data (UV-Vis, FTIR, FESEM, EDX, XPS, and TGA spectra) are available free of charge at http://jbcs.sbq.org.br as a PDF file.

Publication Dates

• Publication in this collection
  20 Sept 2024
• Date of issue
  2025

History

• Received
  15 Apr 2024
• Accepted
  27 Aug 2024