Amanda
Alonso
*a,
Núria
Vigués
b,
Rosalía
Rodríguez-Rodríguez
c,
Xavier
Borrisé
d,
María
Muñoz
e,
Dmitri N.
Muraviev
e,
Jordi
Mas
b and
Xavier
Muñoz-Berbel
*f
aChemical Engineering Department, Escola d'Enginyeria, Universitat Autònoma de Barcelona, UAB, Spain.. E-mail: amanda.alonso@uab.cat
bDepartment of Genetics and Microbiology, UAB, Spain
cDepartment of Pharmacology, Universidad de Sevilla, Spain
dInstitut Català de Nanociencia i Nanotecnología (ICN2), Spain
eDepartment of Chemistry, UAB, Spain
fCentre Nacional de Microelectrònica (IMB-CNM, CSIC), Bellaterra, Spain
First published on 15th October 2015
This work proposes the use of cationic ion exchange resins as a platform for in situ formation and recrystallization of nanoparticles as a way to dynamically modulate their activity by changing their structure/composition. Here applied to Ag@Co-nanoparticles in cationic resins, this protocol may be expanded to other materials, opening the possibility to modulate activity with a simple and economic approach.
The role of the polymeric matrix in IMS is twofold. First, it acts as supporting material, retaining the ionic precursors for subsequent NPs formation. Additionally, it also acts as stabilizing agent, ensuring homogeneous distribution of the NPs thus avoiding their aggregation and leakage. Conversely, NPs are the functional element in the structure, conferring the nanocomposite with the desired activity. This may be catalytic capacity,6,7 antibacterial activity2,8 and/or magnetism.9 As a step forward, the activity of nanocomposites containing bimetallic NPs is the combination of the activity of both NPs.10 In some cases, synergetic effects, due to the combination of two metals in a single core–shell NP (shell@core), have been reported. For instance, in the Ag@Co-nanocomposite reported by our group, the presence of Co in the core enhanced bactericidal activity of the Ag shell.4 Hence, IMS provides with a very versatile synthetic route to obtain nanocomposites with different activities through a simple and fast synthetic protocol. Their main limitation may be in terms of versatility. Traditional IMS protocols are designed to produce nanocomposites with a selected activity, which remain stable over time. However, it is not possible to modulate or change their activity once fabricated.
Regarding this, and according to bibliography,11 NPs activity can be modulated by altering their size, shape, composition or state of aggregation. Hence, the activity of a nanocomposite may be tailored by modifying some of the previously exposed properties of the NPs.
Based on the latter, this communication proposes an IMS protocol to produce dynamic and activity-tunable nanocomposites based on the modification of Ag@Co-NPs stabilized on Ion Exchange Resins (IER).
Incubation solutions contained phosphate and chloride; two ions that interacted with Ag-NPs. Phosphate ions are capable to change the morphology of pre-synthetized Ag-NPs.13 Chloride ions, on the other hand, can etch Ag-NPs, leading to the formation of regular aggregates.14 Moreover, both chloride and phosphate react with Ag ions, being able to induce Ag re-crystallization to produce NPs of different composition (e.g. AgCl, AgCl@Co, Ag3PO4, etc.) and aggregation state. Apart from them, other components of the incubation solution may influence NPs dissolution and re-crystallization. This is the case of Co2+ ions. According to bibliography,4,15 the presence of Co2+ traces, in this case from dissolved Ag@Co-NPs, may also induce Ag ions reduction and Ag-NPs formation. Thus, the equilibrium between both dissolution and recrystallization reactions may lead to the formation of NPs with different structure/function, allowing the dynamic modulation of nanocomposite activity.
In order to study the incubation-induced modulation of activity, nanocomposites were subjected to different incubation conditions. These included phosphate concentrations between 0 and 0.1 M, chloride concentrations from 0 to 0.67 M and temperatures from 4 to 80 °C. SEM, EDX and XRD techniques were used to determine nanocomposite structure and composition at the different incubation situations (detail in Experimental section).
Equilibrium between dissolution and recrystallization reactions was reached under three experimental conditions ranges, detailed in Fig. 1.
At these conditions, regular aggregate structures, i.e. fractal-like (FLS) and cubic structures (CS), were obtained. More precisely, FLS were observed in nanocomposites after incubation with phosphate solutions (0.05 or 0.1 M phosphate) at a temperature between 4 and 65 °C. CS, on the other hand, were obtained after incubation with 0.5 M chloride at temperatures between 30 and 65 °C. Both FLS and CS were observed in nanocomposite samples incubated with chloride (0.14 and 0.67 M) and phosphate (0.01 and 0.07 M) in a wide temperature window (4–80 °C). The number and size of FLS and CS in the sample depended on the incubation conditions. Representative images of nanostructures obtained under these experimental conditions and others used during activity modulation evaluation are available in ESI (SI.1†). It should be mentioned that FLS in nanotechnology are commonly associated to processes involving formation of NPs. Concretely, it is reported that FLS are composed of multiple primary particles which act as nucleation centers in the synthesis of NPs.16
The formation of these regular aggregates of NPs was associated to a dissolution–recrystallization process. XRD data from nanocomposite samples containing NPs, CS and FLSs is illustrated in Fig. 2.
![]() | ||
Fig. 2 XRD analysis of nanocomposite samples containing NPs, FLS or CS. Inset, a magnification of the crystalline area of each samples indicating the crystalline peaks obtained with the analysis. |
As shown, all nanocomposites, independently on their composition/aggregation state, presented two phases. The first phase, the principal component of the spectrum was amorphous, and may be associated to the polymeric IER. The second one corresponded to the crystalline components, in this case, the NPs. Different crystalline peaks were observed depending on the nanostructures present in the nanocomposite. Unfortunately, only those corresponding to nanocomposites containing CS could be assigned to a crystalline structure. Concretely, these peaks presented strong similarity to AgCl crystals, which suggested that this may be the main component of the cubes. Spectra corresponding to nanocomposites containing NPs or FLSs could not be assigned to any single Ag and/or Co crystalline pattern, probably due to their low intensity or the presence of NPs with different composition. Even though, the differences in the crystalline phase corroborated NPs recrystallization during the incubation and their change of composition, depending on the incubation conditions.
In terms of composition, nanocomposite samples containing NPs, CSs and FLSs were analysed by EDX and compared with IER without nanostructures. EDX peaks comparison is illustrated in Fig. 3 (EDX spectra in SI.2 ESI†).
According to these results, CSs and FLSs presented Ag and Co concentrations higher than those recorded in other regions of the nanocomposite. Since no additional metal precursors were added during incubation, the formation of these structures with high Ag and Co concentration confirmed NPs dissolution and concentration before recrystallization. It was demonstrated by statistical analysis of aggregate size in the nanocomposite sample. Thus, nanocomposite samples containing CSs presented aggregates of NPs significantly bigger than those observed in nanocomposites without NPs. An example is illustrated in Fig. SI.3.† In this figure, the size of the aggregates observed after incubation with solutions containing 0, 0.5 and 1.0 M chloride at 30 °C was determined and compared. According to results, 0.5 M chloride solutions, leading to CSs, presented bigger aggregates (163 ± 26 µm average) than the others (140 ± 34 µm for 0 M and 110 ± 30 µm for 1.0 M chloride, respectively). Additionally, the concentration of chloride in cubes was higher than in other structures or other regions of the same nanocomposite. This result may confirm the presence of AgCl crystals in the nanocomposite structure.
The structural and composition changes observed (by SEM, EDX and XRD) may involve a modification of the nanocomposite activity. Bactericide and cytotoxic activity of Ag@Co-NPs synthesized in situ on IER has been already reported.12 Following the same protocols, bactericide and cytotoxic activities of the modified nanocomposites reported here were determined and compared with previous results.
Nanocomposites containing either FLSs or CSs presented significantly lower bactericide activity than those containing NPs (MIC 50FLS = 3.2 ± 0.4 particle per 200 µL; MIC 50CS = 2.8 ± 0.2 particle per 200 µL; MIC 50NP = 1.01 ± 0.15 particle per 200 µL; Fig. 4).
This decrease may be the combination of factors. First, NPs aggregation to build big aggregates such as CSs or FLSs may drastically reduce the specific exposed bactericide surface. Apart from that, activity reduction may be also attributed to composition changes. In this sense, AgCl is reported to be less bactericidal than Ag.17
In terms of cytotoxicity, nanocomposites containing CSs were found much more cytotoxic (cell viabilityNP = 87 ± 5%; Fig. 5) than those containing FLSs or NPs, which presented similar cytotoxicities (cell viabilityFLS = 113 ± 1%; cell viabilityNP = 114 ± 2%; Fig. 5).
According to previous experiences, most of cytotoxicity associated to most of cytotoxicity should be associated to Ag@Co-NPs may be attributed to Co.12 In fact, Co is reported to be much more cytotoxic than Ag or any other component of the nanocomposite.12 Then, this result suggested that the amount of Co in contact with cells was higher in samples with CS than in those only containing NPs or FLSs. This was partially corroborated by EDX analysis of nanocomposites containing CSs, where it was possible to find areas containing high concentrations of Co (Fig. 6).
![]() | ||
Fig. 6 EDX spectrum of nanocomposites containing CSs where Co peak intensity is similar to that obtained by Ag. |
In this region, Ag may not be completely covering Co-NPs and hence, it may be in direct contact with cells, increasing samples cytotoxicity. Therefore, it may be possible to postulate that, apart from AgCl crystals (as we concluded above), nanocomposites containing CSs may also present Co-NPs with high cytotoxic activity.
Thus, we observed here a modification of the bactericide activity, in comparison with the previous results reported for Ag@Co-NPs,12 when this material is treated under different incubation conditions.
Footnote |
† Electronic supplementary information (ESI) available: SI.1. SEM images for the nanostructures obtained by modifying the incubation conditions, SI.2. EDX spectra corresponding to nanocomposites containing NPs, CSs or FLSs. The spectrum corresponding to the IER without modification is also added for comparison and SI.3. histograms showing the size frequency of the nanostructures for different incubation conditions. See DOI: 10.1039/c5ra16081b |
This journal is © The Royal Society of Chemistry 2015 |