Optical and Chiroptical Stimuli-Responsive Chiral AgNPs@H-Leu-Poly(phenylacetylene) Nanocomposites in Water

Optical and Chiroptical Stimuli-Responsive Chiral AgNPs@H-Leu-Poly(phenylacetylene) Nanocomposites in Water Manuel Fernández-Míguez, Manuel Nunez-Martínez, Esteban Suárez-Picado, Emilio Quinoá, and Félix Freire* Cite This: ACS Nano 2024, 18, 28822−28833 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Dynamic macroscopically chiral nanocomposites are prepared by combining silver nanoparticles (AgNPs) and dynamic helical poly(phenylacetylene)s (PPAs) bearing pendants functionalized with amino groups. These amino groups provide the nanocomposite with the ability to disperse in water along with high stability due to the interaction between the ammonium group and the AgNP. Moreover, the equilibrium between NH3+/NH2 produces a “blinking” contact between the PPA and the AgNPs, which allows total control of the dynamic helical behavior of the polymer. The use of acidic or neutral pH allows controlling the morphology of the nanocomposite, which consists of a nanosphere that has trapped inside it a single AgNP (pH = 2) or several AgNPs (pH = 7) with ca. 30 nm of diameter. These nanocomposites combine the optical and chiroptical stimuli-responsive properties of both components, AgNPs and PPAs. Thus, the controlled aggregation of the nanocomposite produced variations in the LSPR band of the AgNPs in a reversible manner. In turn, given that the chiral coating is selective to Ba2+, the presence of this metal ion caused a helical inversion of the chiral coating of the nanocomposite detected by electronic circular dichroism. Moreover, it is possible to distinguish between three metal ions in different oxidation states, such as Ce4+, Fe3+, and Hg2+, which produce different responses of the nanocomposite when oxidizing the AgNP to Ag+. KEYWORDS: chirality, silver nanoparticles, stimuli responsiveness, poly(phenylacetylene)s, nanocomposites INTRODUCTION Silver nanoparticles (AgNPs) have attracted the attention of the scientific community due to their applications in various fields such as medical therapy, 1 sensors, 2 and catalysis, 3 among others. However, to keep these AgNPs stable over time in specific environmental conditions, it is necessary to coat them with a certain organic molecule or polymeric coating. 4−7 Thus, their natural tendency to aggregate and oxidize is disrupted by steric or electrostatic effects. 8 Moreover, by playing with the structure and functional groups of the coating agents, extra functionalities can be introduced in the hybrid material that could combine the properties of both systems (MNPs and coating) 9 generating chiral plasmonic nanostructures. 10−13 Helical biopolymers including DNA, 14 peptides, 15 and polysaccharides 16 have been used to decorate AgNPs. However, Received: June 27, 2024 Revised: September 26, 2024 Accepted: October 2, 2024 Published: October 9, 2024 Article www.acsnano.org © 2024 The Authors. Published by American Chemical Society 28822 https://doi.org/10.1021/acsnano.4c08622 ACS Nano 2024, 18, 28822−28833 This article is licensed under CC-BY 4.0 Downloaded via UNIV DE SANTIAGO DE COMPOSTELA on December 10, 2024 at 14:13:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. their pool of building blocks (chirality, functional groups) in biopolymers is limited because Nature only used a limited number of molecules to create organisms, such as the 20 natural amino acids or the 4 nucleotides. Furthermore, these biomacromolecules are generally static, a fact that makes it not possible to tune their helical sense or elongation in the presence of external stimuli. Scheme 1. Graphical Illustration for the Preparation AgNPs@PPAs Nanocomposites Dispersed in Water and the StimuliResponsive Properties Addressed in This Work Figure 1. Chemical structure of (a) m-(L)-1, m-(D)-1, and (b) their protonated forms. (c) Scheme of the polymerization reaction to obtain Poly(L)-1/H+and Poly(L)-1/H+. ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.4c08622 ACS Nano 2024, 18, 28822−28833 28823 Our group has been working lately on the preparation of hybrid materials that combine the properties of MNPs (M = Au or Ag) and stimuli-responsive helical polymers such as poly(phenylacetylene)s (PPAs). 17−20 These polymers are dynamic from a structural point of view, a fact that allows their secondary structure (elongation and/or helical sense, scaffold) to be modulated by the presence of external stimuli such as temperature, solvents, metal ions and pH, among others. 21−31 These characteristics make them excellent candidates to protect MNPs and obtain hybrid materials (MNPs@PPAs) that combine the dynamic chiroptical properties of PPAs and the optical properties of MNPs. However, to date, these studies were carried out only in organic media (e.g., CHCl3, DCM), due to the limitation of creating water-soluble PPAs with the ability to interact with MNPs. The main reason is that the Rh(I) catalyst used during the polymerization reaction is poisoned by monomers containing amino groups. This problem was recently overcome Figure 2. (a) Schematic illustration of the Poly(L)-1 helix inversion after complexation with 1.5 equiv of Ag+at pH = 7 and (b) pH = 2 and the formation of HPMC particles. ECD spectra of Poly(L)-1 and Poly(L)-1/Ag+at (c) pH = 7 and (d) pH = 2. (e) DLS studies of Poly(L)-1/Ag+1/ 1.5 mol/mol prepared at pH = 7 and pH = 2. (f) SEM image of Poly(L)-1/Ag+nanospheres at pH = 7. [Poly(L)-1] = 0.3 mg/mL H2O, [AgClO4] = 10 mg/mL H2O. ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.4c08622 ACS Nano 2024, 18, 28822−28833 28824 by performing the polymerization reaction with protonated ammonium phenylacetylene monomers (NH3+-PA). 32 In this acidic conditions, the Rh(I) catalyst is not poisoned, allowing the creation of water-soluble amino-PPAs required to prepare water-soluble MNPs@PPA nanocomposites. Furthermore, by using amino-PPAs and playing with the pH, it is possible to adjust the NH3+/NH2ratio along the helical polymer. These amino and ammonium groups are in equilibrium and the positive charges, from a macroscopic point of view, exchange from one amino group to another along the polymer scaffold. 33−35 Therefore, the NH3+�AgNP contacts, which stabilize the metal nanoparticle, are dynamic and move along the polymer−metal nanoparticle interface, without blocking the flexibility of the pendant groups attached to the AgNP. As a result, the polymer is expected to vary its helical structure� elongation and helix sense�when it interacts with external stimuli in the nanocomposite, similarly to its behavior in the molecularly dissolved state. In this work, we will explore the formation, chiroptical properties, and morphology of dynamic aqueous AgNPs@PPAs nanocomposites by reducing an amino-PPA/Ag+complex at acidic and neutral pH (2 and 7 respectively, Scheme 1) and their stimuli-responsive properties in water against different stimuli such as pH, divalent metal ions, and oxidizing agents. Figure 3. (a) Schematic illustration of the formation of AgNPs@Poly(L)-1 nanocomposite at pH = 7. Comparison of (b) ECD and (c) UV−vis spectra of Poly(L)-1/Ag+and AgNPs@Poly(L)-1 at pH = 7. (d) DLS studies of Poly(L)-1/Ag+and AgNPs@Poly(L)-1. (e) SEM and (f) TEM studies of AgNPs@Poly(L)-1 at pH = 7 (Gaussian size distribution of 100 AgNPs). ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.4c08622 ACS Nano 2024, 18, 28822−28833 28825 RESULTS AND DISCUSSION To prepare a chiral AgNPs@PPA nanocomposite that can be dispersed in water, we designed an aminophenylacetylene monomer bearing the 4-ethynyl anilide of (L)- or (D)-leucine with a deprotected amino group (Figure 1a). These monomers were prepared following peptide chemistry (Supporting Information) and were further protonated with HCl (1 equiv) to generate the corresponding ammonium salt (Figure 1b). These protonated monomers were polymerized with [Rh- (cod)2]BF4in water to obtain the desired polymers (Poly(L)-1/ HCl and Poly(D)-1/HCl) with low polydispersity and high content of cis-configuration of double bonds (Figure 1c). These polymers show good water solubility at different pH, e.g., pH = 2 and 7, where the NH2/NH3+ratio will be different along the polymeric scaffolds. Both protonated and deprotonated forms of the amino group are needed to prepare the desired chiral AgNPs@PPA nanocomposite, with the NH2groups being necessary to form complexes with Ag+ions, while the ammonium groups are essential to providing water solubility. Under these conditions (pH = 2 and pH = 7), Poly(L)-1shows a Pscrew sense excess inferred from ECD (ECD380 nm> 0), while Poly(D)-1shows an Mscrew sense excess (ECD380 nm< 0) due Figure 4. (a) Schematic illustration of the formation of AgNP@Poly(L)-1 nanocomposite at pH = 2. Comparison of (b) ECD and (c) UV−vis spectra of Poly(L)-1/Ag+and AgNP@Poly(L)-1 at pH = 2. (d) DLS studies of Poly(L)-1/Ag+and AgNP@Poly(L)-1. (e) SEM and (f) TEM studies of AgNP@Poly(L)-1 at pH = 2 (Gaussian size distribution of 50 AgNPs). ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.4c08622 ACS Nano 2024, 18, 28822−28833 28826 to their enantiomeric relationship (Figures 2c,d and S10). The adoption of a Phelix by Poly(L)-1and an Mhelix by Poly(D)-1 is due to the presence of a preferred antiperiplanar conformation between the carbonyl and NH2/NH3+groups in the pendant (Figure 2a,b), which places the side chain of the amino acid, i.e., the isobutyl group, in such a way that it induces a Phelix in the main chain of the polyene (Figure 2a). Next, the ability of Poly(L)-1to form complexes with Ag+ions at different pHs was verified by adding AgClO4(1.5 equiv, c= 10 mg/mL MQ H2O) to an aqueous solution of Poly(L)-1at pH = 7 and 2 (Figure 2a,b). Interestingly, when complexation is carried out at pH = 7 (Figure 2a), a helix inversion from P (ECD380 nm> 0) to M(ECD380 nm< 0) is observed (Figure 2c). This is a consequence of a change in conformation in the pendant group from an antiperiplanar orientation between the carbonyl and amino (NH2) groups to a synperiplanar orientation after chelation of both functional groups with a silver(I) ion (Figure 2a). This antiperiplanar to synperiplanar conformational switch places the amino acid side chain, i.e., the isobutyl group, in two very different spatial orientations, producing a helix inversion effect. However, at pH = 2, the addition of AgClO4to a Poly(L)-1 solution produces a slight decrease in the Pscrew sense excess (Figure 2b,d). This result can be explained by considering the ratio of NH2/NH3+groups within Poly(L)-1at these pHs. Thus, while at pH = 7, the amount of neutral amino groups, which can coordinate silver ions, is high (major component); at acidic pH (pH = 2), this number is very low (minor component), the helix being mostly populated by ammonium groups that have lost the ability to coordinate with Ag+ions. Interestingly, these helical polymer metal complexes (HPMC, Poly(L)-1/Ag+), prepared at pH = 7 and pH = 2, form nanospheres in water as inferred from dynamic light scattering (DLS) (Figure 2e) and scanning electron microscopy (SEM) studies (Figures 2f and S23−S26). HPMC nanospheres are generated due to the ability of silver ions to act as cross-linking agents. Their size is larger at pH = 7 due to the greater number of Ag+that form complexes with the amino groups (NH2) of different polymer chains, which are more abundant at pH = 7 than at pH = 2. Analogous studies were carried out with Poly(D)-1, showing identical stimuli-responsive properties but opposite helical senses due to their enantiomeric relationship (Figure S10). Next, AgNPs@Poly(L)-1nanocomposites were prepared at pH = 7 (Figure 3a) and pH = 2 (Figure 4a) from the Poly(L)-1/ Ag+complexes (1/1 mol/mol ratio) by adding 0.45 equiv of NaBH4as the reducing agent. The mixtures were kept under vigorous stirring for 60 min, and the color of the solutions changed from yellow to brown, indicative that reduction of silver ions was taking place. Immediately thereafter, a metal scavenger resin (Quadrapure TU) was added to the solution mixtures to remove unreacted Ag+ions and Na+from the reducing agent. ECD studies at pH = 7 show that the formation of the AgNPs@Poly(L)-1nanocomposite, after metal ion reduction, does not affect the preferred Pscrew sense adopted by Poly(L)- 1/Ag+�ECD380 nm< 0 for Poly(L)-1/Ag+and AgNPs@Poly- (L)-1(Figure 3b). UV−vis studies of AgNPs@Poly(L)-1show the localized surface plasmon resonance (LSPR) band centered at 427 nm, indicative of AgNPs formation (Figure 3c). Moreover, a variation in the size of the aggregate during the formation of the composite, from 94 to ca. 250 nm, is observed by DLS and SEM. These studies also reveal the presence of spherical particles for the AgNPs@Poly(L)-1nanocomposite (Figures 3d,e and S30). These nanospheres are filled with several AgNPs whose size is ca. 30 nm (Figure 3f), as could be observed in the images obtained through transmission electron microscopy (TEM) studies. The large number of AgNPs within the nanosphere that forms AgNPs@Poly(L)-1is attributed to the presence of several nucleation points (amino groups complexed with silver ions: NH2�Ag+) along the helix at pH = 7. Considering the morphology of the helical polymer metal complex and the nanocomposite, it is necessary for the reducing agent to diffuse inside the Poly(L)-1/Ag+nanosphere to create nucleation sites. Moreover, FT-IR studies show that in addition to the ammonium groups, the AgNPs are also stabilized by the carbonyl group of the anilide (Figure S11). As a result, these nanocomposites show good thermal and temporal stability once the solvent is removed and are redispersed in water, which confirms that Poly(L)-1is a good protecting agent for obtaining AgNPs (Figure S12a). In addition, photostability studies show that poly(L)-1is more stable to light irradiation when the nanocomposite is prepared at pH = 7 than when it is molecularly dissolved (see Figure S12). Similarly, VT-ECD studies show how, in a molecularly dissolved state, the screw sense excess is lost at 340 K, while in the nanocomposite, it remains unaltered at the same temperature (see Figure S12). Interestingly, when the AgNPs@Poly(L)-1nanocomposite is prepared at pH = 2 (Figure 4a), ECD studies show a helical inversion of the Poly(L)-1/Ag+complex after reduction of the silver ion to form the AgNP@Poly(L)-1nanocomposite (Figure 4b). At this pH, there are many ammonium groups in the polymer that do not interact with Ag+ions (Figure 4a). However, once the metal ion is reduced to Ag(0), the ammonium groups have a high affinity for the metal in the ground state (Figure 4a). Thus, the chelation of carbonyl and ammonium to Ag(0) nanoparticles changes the preferred conformation of the pendant group, which evolves from antiperiplanar to synperiplanar during the reduction of the metal ion, placing the helix directing group (isobutyl) in different spatial orientations. As a result, this conformational switch produced a helix inversion of PPA that allowed us to monitor the reduction of Ag+to Ag(0) by ECD (Figure 4b). Therefore, Poly(L)-1in addition to stabilizing the metal nanoparticles can be used to monitor the reduction of Ag(I) to Ag(0) in water. UV−vis studies of AgNP@Poly(L)-1show the localized surface plasmon resonance (LSPR) band centered at 412 nm, indicative of the formation of AgNPs (Figure 4c). This plasmon band is hypsochromically shifted by 15 nm compared to the plasmon band of AgNP@Poly(L)-1obtained at pH = 7. DLS studies show a small variation in the aggregate size during the formation of the nanocomposite from 70 to 83 nm (Figure 4d), in agreement with electron microscopy studies, SEM and TEM, which show the presence of spherical particles for AgNP@ Poly(L)-1whose average size is ca. 80 nm (Figures 4e,f and S25). Interestingly, TEM studies reveal that within each nanosphere of the AgNP@Poly(L)-1nanocomposite prepared at pH = 2, there is only one AgNP whose size is ca. 30 nm (Figure 4f). Considering the different morphologies of the AgNP@ Poly(L)-1nanocomposites prepared at pH = 7 and pH = 2, it is possible to explain the UV−vis data considering a different environment for the AgNPs in both nanocomposites. More precisely, a bathochromic shift is observed for the plasmon band of the nanocomposite prepared at pH = 7 with respect to the ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.4c08622 ACS Nano 2024, 18, 28822−28833 28827 band observed at pH = 2, although the size of the AgNPs is ca. 30 nm in both cases. This variation in the morphology of the AgNP@Poly(L)-1 nanocomposites prepared at pH = 7 and pH = 2 is related to the different numbers of amino groups coordinated to the metal ion before adding the reducing agent. Thus, while at pH = 7, the number of amino groups within the polymer is greater than at pH = 2, the number of Ag+coordinated to these amino groups is also greater. These amino/Ag+interactions are the nucleation sites within the nanosphere, which produce more AgNPs growth sites within the nanocomposite at pH = 7 than at pH = 2. As a result, the morphology of the dispersed nanocomposites depends on the pH used to prepare them. These nanocomposites prepared at pH = 2 show good thermal and temporal stability once the solvent is removed and redispersed again in water, confirming that Poly(L)-1is a good protective agent to obtain AgNPs (Figure S12b). Density functional theory calculations (SI) confirm the experimentally elucidated formation mechanism of AgNP(s)@ Poly(L)-1. Thus, in the absence of any metal ion, DFT calculations [B3LYP-D3/6-311+G(d,p)] indicate that the preferred conformation in the leucine derivative, regardless of the protonation state of the amino group, is the antiperiplanar one�carbonyl and amino/ammonium groups oriented antiperiplanar�(Figures 5a,b, and Table S2). However, after addition of the Ag+ion, DFT calculations [B3LYP-D3/6311+G(d,p) for all atoms except silver, which used B3LYP-D3/ cc-pVDZ(-pp)] 36,37 show that chelation of the metal ion with the carbonyl and amino (NH2) groups, through a synperiplanar orientation between them, is the most stable conformer. Additionally, according to the Gibbs free energy shift, the interaction with the deprotonated form in the synperiplanar orientation is the most stable. EFL and NCI analyses 38 were used to analyze the noncovalent interactions. EFL studies confirmed that there are not covalent interactions, while NCI studies indicated strong noncovalent interactions between the amide carbonyl and the amino group with the Ag+ion (Figures 5c,d and S40). Similar computational DFT calculations [B3LYP-D3/6311+G(d,p) for all atoms except silver, which used B3LYPD3/cc-pVDZ(-pp)] were performed by exchanging the Ag+ion for an AgNP consisting of Ag70faces. 39−41 In this case, the synperiplanar orientation between the carbonyl and NH3+is preferred for interacting with an AgNP showing a larger Gibbs free energy shift than the deprotonated (NH2) form (Figure 5a). ELF and NCI studies indicated a strong noncovalent interaction between the ammonium group with Ag(0) and van der Waals interactions between the amide and the hydrogen in αposition with the Ag(0) (Figures 5e,f and S41). 42 Figure 5. Preferred conformations of (a) m-(L)-1 and (b) m-(L)-1/H+. (c) Preferred conformation of m-(L)-1 in the presence of Ag+. (d) Graphical illustration showing the attractive noncovalent interactions of m-(L)-1/Ag+obtained by NCI analysis. (e) Graphical illustration showing the attractive noncovalent interactions of m-(L)-1/H+/AgNP obtained by NCI analysis. (f) Preferred conformation of m-(L)-1/H+in the presence of AgNPs. All structures were optimized by DFT B3LYP-D3/6-311+G(d,p) for all atoms and cc-pVDZ(-pp) for Ag. ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.4c08622 ACS Nano 2024, 18, 28822−28833 28828 Stimuli-Responsive Studies of Chiral AgNP(s)@Poly- (L)-1. The stimuli responsiveness of the AgNP(s)@Poly(L)-1 nanocomposite prepared at two different pHs (7 and 2) was studied considering the stimuli-responsive properties of the two components of the nanocomposite, the dynamic helical polymer (Poly(L)-1) and the metal nanoparticle (AgNP). External Stimulus: pH. Poly(L)-1adopts a preferred Phelix (ECD380 nm > 0) at neutral pH�NH2/NH3+ratio > 1�while an Mhelical sense (ECD380 nm< 0) is induced in the polymer at acidic pH�NH2/NH3+ratio < 1. Thus, the chiroptical properties of the two nanocomposites were checked by varying the pH of the water dispersions of the two AgNP@Poly(L)-1 nanocomposites (0.3 mg/mL) in the ranges between 1 and 6.5. Figure 6. UV−vis after basification with NaOH (1M) of (a) AgNP@Poly(L)-1 (previously prepared at pH = 2) and (b) AgNPs@Poly(L)-1 (previously prepared at pH = 7) (c = 0.3 mg/mL of Poly(L)-1). (c) Representation of the behavior of AgNPs@Poly(L)-1 with increasing pH after the addition of NaOH (aggregated) and with decreasing pH after the addition of HCl (dispersed). Figure 7. (a) Graphical illustration of the conformational changes suffered by Poly(L)-1 in the presence of 1 equiv of M2+ and 2 equiv of Ba2+. (b) ECD studies of AgNP@Poly(L)-1 and AgNP@Poly(L)-1/M2+ in 1/2 mol/mol ratios. (c) Graph bar highlighting the response of the ECD band at 293 nm of AgNP@Poly(L)-1 and AgNP@Poly(L)-1/M2+ in 1/2 mol/mol ratios. ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.4c08622 ACS Nano 2024, 18, 28822−28833 28829 Interestingly, it was found that pH variations in the water dispersion of the AgNP(s)@Poly(L)-1nanocomposites do not affect the axial chirality adopted by Poly(L)-1. This fact is due to the stabilization of the nanocomposite through NH3+�Ag(0) interactions, which are maintained at different pHs and which keep the free amino groups, through a conformational communication mechanism, in an antiperiplanar orientation regardless of their protonation states. This result differs from studies carried out for a solution of Poly(L)-1, where changes in pH alter the NH2/NH3+ratio and the preferred synperiplanar and antiperiplanar conformations adopted by the pendant� synperiplanar for NH2and antiperiplanar for NH3+. However, although changes in the pH do not affect the chirality of the nanocomposites, their dispersion ability is reduced when the amount of NH2increases (pH > 6). As a result, the plasmon band of the AgNP(s)@Poly(L)-1nanocomposites (LSPR = 428 nm of AgNPs@Poly(L)-1prepared at pH = 7, LSPR = 413 nm of AgNP@Poly(L)-1prepared at pH = 2) is red-shifted when the pH is higher than 7, e.g., LSPR at pH = 9 is 444 nm for AgNPs@Poly(L)-1previously prepared at pH = 7 and 430 nm for the AgNP@Poly(L)-1nanocomposite previously prepared at pH = 2. This bathochromic shift is due to the agglomeration of the nanocomposite at a basic pH (Figure 6a,b). This aggregation process is fully reversible, recovering the initial LSPR value at acidic pH and works perfectly after several pH cycles, as demonstrated by UV−vis, DLS, and SEM studies (Figures 6c, S21 and S34−S37). External Stimulus: Divalent Metal Ions (M2+). Poly(L)-1can interact with Ag+as previously demonstrated. Thus, we decided to explore its ability to interact with other metal ions, such as divalent metal ions (M2+). Hence, the chiroptical properties of an aqueous solution of Poly(L)-1were explored in the presence of different divalent metal ions which were delivered to the solution as perchlorate salts [M(ClO4)2; M2+ = Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Cu2+, Ba2+, and Pb2+]. Interestingly, it was observed that in all cases, when the metal ion is added in a 1/1 mol/mol Poly(L)-1/M(ClO4)2ratio, a helix inversion occurs due to a chelation between the carbonyl, the amino, and the divalent metal ion that favors a synperiplanar conformation in the leucine pendant group (Figure 7a). Interestingly, and only in the case of Poly(L)-1/Ba2+, when the amount of metal ion added is 2 equiv, a second helix inversion is observed (Figure 7a and S18). This fact indicates that in the case of the other metal ions or when the Poly(L)-1/Ba2+ complex is in a 1/1 mol/mol ratio, the chelated form is the most favored, which orients the pendant group synperiplanar. On the other hand, when 2 equiv of Ba2+ are added to a solution of poly(L)-1�Poly(L)-1/Ba2+ complex in a 1/2 mol/mol ratio�an evolution toward a different complex occurs. In this special case, the adoption of an antiperiplanar conformation in the pendant is stabilized by the coordination of one of the Ba2+ ions to the carbonyl, while the other ion is coordinated to the amino group. As a result, helix inversion from Mto Poccurs in PPA (Figure 7a). Analogous studies were carried out for the two nanocomposites prepared at pH = 2 and pH = 7. Thus, different perchlorates [M(ClO4)2; M2+ = Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Cu2+, Ba2+, and Pb2+] were added to dispersions of these nanocomposites in water in 1/1 and 1/2 mol/mol AgNP(s)@Poly(L)-1/M(ClO4)2ratios. In these cases, Poly(L)-1adopts an Mhelix in the nanocomposites, a consequence of the interactions between carbonyl and ammonium groups with AgNPs. Thus, when 1 equiv of M(ClO4)2is used, no effects are observed in the chirality of the nanocomposite. Both AgNPs and M2+ ions induce the same helical sense in the polymer used to coat the MNPs. However, when two equiv of the metal salt is added, a helical inversion is selectively observed in the case of AgNP(s)@Poly(L)-1/ Ba(ClO4)2, similar to the effect observed in the case of Figure 8. (a) Graphical illustration of the AgNP(s)@Poly(L)-1 nanocomposites response toward oxidizing metal ions. UV−vis titration studies of the AgNPs@Poly(L)-1 nanocomposite with (b) Fe(ClO4)3, (c) Hg(ClO4)2and (d) Ce(SO4)2. ACS Nano www.acsnano.org Article https://doi.org/10.1021/acsnano.4c08622 ACS Nano 2024, 18, 28822−28833 28830