Transition metal-modification of carrageenan-silica hybrids by a sol–gel method

Transition metal (TM)-modification of silica matrices are found in numerous materials for diverse applications. In other related hybrid materials, one tries to explore properties that result from combining the silica network with organic moieties, such as in the covalent grafting of polysaccharides onto amorphous nanosilicas. However, sol–gel routes for modification with TM have been less explored for hybrid siliceous materials. The present study demonstrates the effective modification of hybrid siliceous materials with TM (TM = Co2+, Ni2+, Cu2+, Zn2+) that result from a sol–gel method that uses as a precursor the polysaccharide κ-carrageenan that was modified with a covalently alkoxysilane linked. Structural analysis and characterization studies of the derived carrageenan-silica hybrids were undertaken, and, in particular, the effects of the TM ions on the hybrids’ properties have been assessed. This work clearly indicates that the modification with TM imposes changes on the morphological, optical, and thermal properties of the hybrids compared to the unmodified analogs. Hence, the practical applicability of the modification with TM using the sol–gel described here is not limited to the presence of the guest ion but also provides a tool for changing the properties of the host particles. Well-defined spheroidal shape ĸ-carrageenan silica particles doped with transition metals (Co2+, Cu2+, Ni2+, and Zn2+) prepared using a sol–gel method. Well-defined spheroidal shape ĸ-carrageenan silica particles doped with transition metals (Co2+, Cu2+, Ni2+, and Zn2+) prepared using a sol–gel method. Transition metal (TM) doped carrageenan-silica hybrids were prepared using a sol–gel method. The synthetic strategy reported does not require surfactants as templates. Hybrid particles of greater monodispersity and well-defined spheroidal shape have been obtained. TM doping changes the morphological, optical, and thermal properties of the hybrids compared to the non-doped analogs. Transition metal (TM) doped carrageenan-silica hybrids were prepared using a sol–gel method. The synthetic strategy reported does not require surfactants as templates. Hybrid particles of greater monodispersity and well-defined spheroidal shape have been obtained. TM doping changes the morphological, optical, and thermal properties of the hybrids compared to the non-doped analogs.


Introduction
The incorporation of transition metal ions in silica-based materials is well known and is part of technologies widely used in the manufacture of glass materials for various applications [1][2][3]. The exchange of silicon sites in the silica network with ions of Co(II) in a tetrahedral environment imparts the well-known cobalt-blue color that can be appreciated in several decorative glasses [4]. Colored silica gel, commonly used as a desiccant, also contains Co(II) as a colorimetric indicator and rare-earth ions (e.g., Er 3+ ) doping of silica glass optical fibers are explored for signal amplification in telecommunications [3], among many other examples of technological relevance. In this regard, sol-gel routes are quite effective for homogeneous doping because the metal guest species are incorporated within the host as the silica network is formed due to a series of hydrolytic and condensation reactions of silica oligomers [5]. The adaptation of such metal doping methodologies to the fabrication of hybrid biomaterials is not straightforward, namely because of the challenges associated with using different types of building blocks in the fabrication of such materials and their influence on the coordination chemistry involved. For example, in using polysaccharides as the organic component in siliceous hybrid materials, the metal coordination environments that result depend on the metal ion exchange of silicon sites and the metal chemical affinity for functional groups existent in the organic moieties [6].
Polysaccharides/silica hybrids prepared through the sol-gel method have been studied as models for a new generation of hybrid silica-based materials for several applications [7]. Nevertheless, due to the poor compatibility between the sol system and natural biopolymers, the formation of polysaccharides/silica hybrid materials is not a trivial task. Some polysaccharides can be more easily incorporated into sol-gel processes than others. The most commonly used polysaccharides have been chitosan [8,9], alginate [10,11], and cellulose [12,13]. So far, few studies have been reported on preparing hybrid polysaccharidesilica materials using carrageenan [14].
Over the last few years, we have reported a series of silica-polysaccharide hybrid materials that use alkoxysilane-modified biopolymers as precursors in a new sol-gel method [15][16][17][18]. This sol-gel method has also been explored to coat magnetic iron oxide cores with hybrid silicious shells, thus providing a series of functional materials for magnetic-assisted environmental and medical nanotechnologies [18][19][20][21][22]. Noteworthy, the surfaces of the ensuing magnetic nanomaterials have specific chemical functionalities provided by the biopolymer, which are instrumental for capturing target species, such as in watercleaning nanosorbents and in biomolecule immobilization substrates. Hence, the magnetic hybrids effectively removed several emerging pollutants, namely pharmaceuticals (diclofenac, naproxen, ketoprofen, sulfamethoxazole, and ciprofloxacin) [19,[23][24][25] and pesticides (glyphosate) [18], with high adsorption capacity, reusability and applicability in natural water samples. Moreover, a drug delivery system comprising an antitumor agent (doxorubicin) loaded magnetic hybrids for anticancer therapy were developed and opens the way towards the development of theranostic agents [21]. Another study showed the application of magnetic silica hybrids in the purification of an Immunoglobulin (IgG) seemed to have high potential as a new downstream platform for biologically active biomolecules [22]. Modification with TM provides a new way to expand the multifunctionality of such hybrid materials, yet this strategy remains unexplored. For this reason, the main goal of this research was to investigate the in situ sol-gel modification with TM of carrageenan-silica hybrids, which is a straightforward method and takes advantage of the chemistry employed in the fabrication of silica-polysaccharide materials.

Synthesis of the κ-carrageenan precursor (SiκCRG)
An alkoxysilane containing κ-carrageenan (SiκCRG) covalently linked was prepared by reacting the biopolymer with the silane coupling agent ICPTES, following a procedure that was previously reported by us [15]. The reaction was performed in a deprotonated solvent (N, N-dimethylformamide) (DMF). Typically, SiκCRG resulted from the reaction between dry κ-carrageenan (1 g), dry DMF (13 mL

Instrumentation
Fourier transform infrared (FTIR) spectra of the particles were measured in the solid state. The spectra of the materials were collected using a Bruker Optics Tensor 27 spectrometer coupled to a horizontal attenuated total reflectance (ATR) cell, using 256 scans at a resolution of 4 cm −1 . The elemental analysis of carbon, nitrogen, hydrogen, and sulfur was obtained on a Leco Truspec-Micro CHNS 630-200-200. The specific surface area of the particles was assessed by nitrogen adsorption Brunauer-Emmett-Teller (BET) measurements, performed with a Gemini V2.0 Micromeritics instrument. The pore volume was evaluated from the adsorption branch using the Barret-Joyner-Halenda method. The morphology and size of the particles were analyzed by scanning electron microscopy (SEM) using a Hitachi SU-70 instrument operated at an accelerating voltage of 15 kV and by scanning transmission electron microscopy (STEM), using a 200 kV Hitachi HD-2700 STEM microscope equipped with energy-dispersive X-ray spectroscopy (EDS) and secondary electron detectors. Samples for SEM analysis were prepared by placing an aliquot of a dilute suspension of the particles in ethanol over a glass slide glued to the sample holder using double-sided carbon tape, and then coating the sample with carbon sputtering. Samples for STEM analysis were prepared by evaporating the diluted suspensions of the particles on a grid coated with an amorphous carbon film. Thermogravimetric analysis (TGA) of the materials was performed by using a TGA 50 instrument from Shimadzu. Samples were heated from 25 to 900 at 10°C min -1 under a nitrogen atmosphere. The 29 Si MAS/CP MAS NMR and 13 C CP MAS NMR spectra were recorded on a Bruker Avance III 400 MHz (9.4 T) spectrometer at 79.49 and 100.61 MHz, respectively. 29 Si MAS/CP MAS NMR spectra were recorded with 4.5 μs 1 H 90°pulses, a recycle delay of 60 s, at a spinning rate of 5 kHz and using a probe for a rotor with a diameter of 4 mm. 13 C CP/MAS NMR spectra were recorded with 3.65 μs 1 H 90°pulses, 1.5 ms contact time, a recycle delay of 5 s, and at a spinning rate of 9 kHz. Diffuse reflectance UV-VIS spectra of the powder samples were recorded on a Jasco U-560 UV/VIS spectrophotometer. The surface charge of the materials was assessed by zeta potential measurements, using a Zetasizer Nano series equipment from Malvern Instruments. The content of the transition metals was determined using inductively coupled plasma-optical emission spectroscopy (ICP-OES) using a model Horiba Jobin Yvon Activa M.

Results and discussion
The first step of this work involved the preparation of a hybrid precursor (SiκCRG) by reaction of κ-carrageenan with a functionalized alkoxysilane containing isocyanate groups. Covalent urethane bonds (−NHCOO−) can be formed between the hydroxyl groups of κ-carrageenan with the isocyanate groups (−NCO) of the silane coupling agent ICPTES ( Fig. 1). In order to obtain TM-modified κ-carrageenan-silica hybrid particles, in a second step, the SiκCRG and TEOS were mixed, in the presence of aqueous solutions of selected TM ions (TM = Co 2+ , Cu 2+ , Ni 2+ , Zn 2+ ) ( Fig. 1). For the sake of comparison, a similar method was applied for preparing the TM-modified SiO 2 based counterparts, i.e., in the absence of the κ-carrageenan containing precursor. The synthesized particles were analyzed using ATR FTIR spectroscopy (Fig. 2). The spectra of amorphous  [33] that usually appear within the range of 400-670 cm −1 , can barely be seen in the FTIR spectra due to the lower concentration of each TM and presence and overlap of silica peaks that mask these specific vibrations. Regarding the TM-modified hybrid particles (Fig. 2), the FTIR spectral bands of the particles have confirmed the main characteristics of silicate network grafted to ĸ-carrageenan. Briefly, Fig. 1 Schematic representation of the synthesis of the hybrid precursor, SiκCRG (step 1) by reaction of the hydroxyl groups of κcarrageenan with isocyanate groups of ICPTES. The scheme also illustrates the case of Co-modified hybrid siliceous materials using TEOS in the presence of aqueous solutions of the selected TM ion (step 2), suggesting a coordination environment for the cation κ-carrageenan spectrum showed typical bands in the region 1067-1033 cm −1 due to C-O and C-OH vibrations, a band at 838 cm −1 that is attributed to the α(1-3)-Dgalactose C-O-S stretching vibration, a band at 925 cm −1 that corresponds to the 3,6-anhydro-D-galactose and a broad band at 1227 cm −1 due to the S-O antisymmetric stretching of the ester sulfate groups [19]. The typical vibration bands of SiO 2 and κ-carrageenan have also been observed in the spectra of the TM-modified hybrid particles, although these particles have not shown any noticeable changes in the FTIR spectral bands position after being modified with TM.
The organic-inorganic hybrid nature of the TM-modified siliceous materials obtained by this sol-gel method was confirmed by elemental microanalysis (Table 1). While SiO 2 and TM-modified silica particles show low carbon content (<1.6 wt%), all the TM-modified hybrid particles exhibit higher carbon content (>12 wt%), which is in agreement with the formation of hybrids with a significant content of κ-carrageenan as the organic component. The specific surface area (S BET ) and pore volume (V p ) of these materials (Table 1) were assessed by nitrogen adsorption/desorption isotherms. The specific surface area decreased from 52.0 m 2 g −1 in SiO 2 to 9.2 m 2 g −1 in SiO 2 /SiκCRG, due to the increase of the particle  Table 1 Compositional and structural properties of assynthesized materials Particle diameter assessed by TEM c BET specific surface area (S BET ) and porosity volume (V p ) assessed by N 2 adsorption size and the formation of the hybrid. Identical correlation between the particle size and the surface area was found in the TM-modified hybrids. The BET-specific surface area decreased from 9.2 m 2 g −1 (SiO 2 /SiκCRG) to values in the range from 0.2 to 3.9 m 2 g −1 , along with an increase in particle size. The presence of the TM cations during the synthesis of the particles contributed to the surface area decrease [34]. Regarding the inorganic silica particles, the BET-specific surface area decreased from 52.0 m 2 g −1 in SiO 2 to values in the range 20.1-31.3 m 2 g −1 in modified silica. Since the particle size decreased after modification (except for SiO 2 /Zn), the decrease in surface area was most likely due to the blocking of some pores by the TM cations that could limit the adsorption of the probe gas (N 2 ) inside the pores [35]. The decrease in the pore volume is in agreement with this effect.
The morphological characteristics of the TM-modified and unmodified materials were investigated by SEM (Fig. 3). The SEM analysis showed that unmodified bulk SiO 2 and SiO 2 /SiκCRG samples have both uniform and spheroidal particle morphology (Fig. 3a, b). As shown in Table 1, the average size of the SiO 2 particles was 131 ± 9 nm, and decreased to 60 ± 20 nm, 13 ± 2 nm, and 6 ± 1 nm, for the SiO 2 /Cu, SiO 2 /Ni and SiO 2 /Co particles, respectively (Fig. 3c, e and g). Compared with the unmodified SiO 2 , the average size of the TM-modified SiO 2 particles markedly decrease, suggesting that the incorporation of the TM cations, such as Co 2+ , Cu 2+ , and Ni 2+ , limits the growth of the SiO 2 particles. Interestingly, the above observation follows the decreasing tendency of the TM ionic radii for the respective coordination geometry, which might suggest a charge density effect of the TM ions when already interacting with the silica oligomers. In the case of SiO 2 /Zn particles, as compared to the unmodified SiO 2 sample, the average particle size increased to 180 ± 20 nm, but the particles show a distinct nanoplatelets-like morphology (Fig. 3i) instead of a spheroidal morphology [36,37]. The TM-modified κ-carrageenan silica particles (Fig. 3d, f, h and j) presented well-defined spheroidal shape with an average size ranging between 750 nm and 1300 nm in diameter (Table 1). Furthermore, there is not a clear trend on the effect of the TM cation employed on the final average particle size of the modified hybrid materials. These observations are a strong indication of the important role of the alkoxysilanemodified polysaccharide precursor during the sol-gel process, namely by providing diverse oxygen donor groups for coordinating TM cationic species present in the reacting mixture, such as sulfate groups. It should be stressed that the above sol-gel route led to morphological uniform spherical particles of the TM-modified hybrid materials without surfactants and emulsions.
XRD patterns of the unmodified and TM-modified particles are shown in Fig. S3 (Supporting Information). For all the materials, the only broad peak detected is the one ascribed to amorphous silica at 2θ ≈ 21°.
The surface charge measured as zeta-potential revealed a negative surface charge for all the silica and hybrid particles ( Table 2). The zeta potential of unmodified SiO 2 and SiO 2 / SiκCRG particles was negative (−46 and −68 mV, respectively). The TM-modified κ-carrageenan silica particles presented more negative zeta-potential values, compared with amorphous inorganic SiO 2 , indicating that the anionic polysaccharide κ-carrageenan was bonded to the silica network.
A first indication of the presence of the TM cations in the silicious network was their characteristic color, which remained after thoroughly washing the solid samples (Fig. 4). This aspect was further investigated by diffuse reflectance visible (DR-UV/VIS) spectroscopy, as shown in Fig. 4. Overall, the DR-UV/VIS spectra show the absorption features in the visible region as expected for d-d electronic transition bands for the respective coordinated TM cations (Fig. 4a-c), except for the Zn 2+ samples in which the metal has the d orbitals totally filled (Fig. 4d). Furthermore, the DR-UV/VIS spectra suggest different coordination environments for the TM cations in the hybrid materials. The analysis of the spectra for the Co-modified siliceous materials (SiO 2 /Co and SiO 2 /SiκCRG/Co) is particularly instructive because it showed bands peaked at 527, 584, and 643 nm, which is a triplet characteristic of Co 2+ in a tetrahedral environment [38][39][40], thus in agreement with the observed blue color of the respective samples (Fig. 4a). It is known that Co 2+ changes in color from pink to blue, from octahedral coordination in the corresponding hydrated samples to tetrahedral coordination in the dehydrated samples [39,40]. Additionally, the DR-UV/VIS spectra of Cumodified silica (SiO 2 /Cu) and hybrid (SiO 2 /SiκCRG/Cu) materials (Fig. 4b) show two bands at around 330-350 nm, that can be ascribed to charge transfer between mononuclear Cu 2+ ion and oxygen and between Cu 2+ and oxygen in oligonuclear [Cu-O-Cu] n surface species [41]. The band in the 600-800 nm range is usually attributed to d-d transitions of the Cu 2+ ions in an octahedral or tetragonal distorted octahedral surrounding [42]. These results are in agreement with the structures proposed in refs. [43][44][45], showing the typical fingerprint of hexacoordinated Cu 2+ ions. Figure 4c shows the DR-UV/VIS spectra of Nimodified silica and hybrid materials. For these materials, two bands located between 401 and 680 nm are observed, corresponding to the d-d transitions of the Ni 2+ cations [46][47][48], indicating an octahedral coordination of Ni 2+ [49]. Figure 4d shows the DR-UV/VIS spectra of Zn-modified silica (SiO 2 /Zn) and hybrid (SiO 2 /SiκCRG/Zn) particles, which are in agreement with other studies reported in the literature for Zn-modified silica particles [50]. Although the above interpretation is consistent with the data available, a To determine the TM content, the modified materials were analyzed by ICP-OES. The ICP-OES results showed that TM-modified particles contain between 0.2 and 2.8% of metal (Table S1, Supporting Information), which corroborates the results previously discussed and indicates the presence of the TM cations in the silicious network of the materials.
The above electronic spectra indicate that TM ions have been successfully incorporated into the silica matrix. However, this has been further confirmed by energydispersive EDS and STEM performed on the samples (Figs. 5 and S4, Supporting Information). The EDS maps show a homogeneous dispersion of the TM ions over the particles for all the samples analyzed. The EDS Si signal provides maps with a higher color density than the TM EDS signals, which is consistent with the dispersion of the metal species on the siliceous matrix.
Solid-state 29 Si NMR spectroscopy of the powders was explored to investigate the effect of TM modification on the degree of condensation of the silica network. Figure 6 shows the cross-polarization (CP)/magic-angle spinning (MAS) 29 Si NMR spectra; the corresponding chemical-shift assignments are listed in Table S2 (Supporting Information). The silicon sites are labeled according to the usual NMR spectroscopy notation: Q n represents quaternary Si atoms linked to n siloxane groups and (4 − n) OH groups [51][52][53]. Figure 6a shows the 29 Si MAS NMR spectra of bulk SiO 2 and the TMmodified silica particles and Fig. 6b shows the 29 Si MAS NMR spectra of the SiO 2 /SiκCRG hybrid and the TMmodified κ-carrageenan SiO 2 particles and yields information on the connectivity of the siloxane bonds. Bulk SiO 2 particles (Fig. 6a) show two main signals at −111 and −102 ppm, attributed to the silica sites and  particles; and e schematic representation showing the labeling of Si sites according to NMR spectroscopy notation usually applied to silica networks the unreacted surface silanol sites, respectively [51]. The chemical shifts between −90.6 ppm and −111.2 ppm in SiO 2 were ascribed to geminated silanols Q 2 (Si(OSi) 2 (OH) 2 ), isolated silanols Q 3 (Si(OSi) 3 OH) and siloxane bridges Q 4 Si(OSi) 4 , respectively [52].  (Fig. 6b) occurs, which is due to the presence of TM ions in the silica matrix. Comparing the 29 Si CP MAS NMR spectra of SiO 2 materials (Fig. 6c) with the 29 Si CP MAS NMR spectra (Fig. 6d) of the SiO 2 /SiκCRG/TM materials show a decrease in the intensity of the Q 4 signals comparing with the intensity of the Q 3 signals, which means that the modified hybrid materials have an increased number of SiOH% content, i.e., the amount of Q 3 . In addition, four new signals appear at −37.4, −47.1, −56.1, and −65.8 ppm which, compared to literature values, can be ascribed to T 0 , T 1 , T 2 , and T 3 Si sites, where n denotes the number of -Si-Obonds linked to the Si site T n [15]. Thus, T 1 , T 2 , and T 3 represent the Si sites in RSi(OSi)(OH) 2 , RSi(OSi) 2 OH, and RSi(OSi) 3 , respectively [R = -(CH 2 ) 3 -NHCOO -κ-carrageenan] and further support the polysaccharide κ-carrageenan's covalent attachment to the siliceous network. The presence of T 0 [RSi(OH) 3 ] indicates that the hydrolysis of the alkoxy groups of the κ-carrageenan precursor can occur during the sol-gel reaction. The 29 Si CP MAS NMR spectra of SiO 2 /SiκCRG/Co, SiO 2 /SiκCRG/Cu, SiO 2 /SiκCRG/Ni, and SiO 2 /SiκCRG/Zn show four new signals corresponding to Si sites in T 0 , T 1 , T 2 , and T 3 , confirming the formation of the covalent bonding between the κ-carrageenan and the SiO 2 matrix, even in the presence of the selected TM ions.
Further insight into the hybrid composition was provided by 13 C CP/magic-angle spinning (MAS) NMR. The 13 C CP/ MAS NMR spectra of κ-carrageenan, and hybrid particles are shown in Fig. 7a and the chemical shifts are listed in Table S3 (Supporting Information). The spectrum of the hybrid SiO 2 /SiκCRG, when compared to κ-carrageenan spectrum, shows new signals at δ = 9.6, 23.1, and 43.6 that correspond to C10, C9, and C8 carbon atoms, respectively, of the Si-bonded propyl chain (Fig. 5b) [51]. Additionally, a new signal that is attributed to the carbon in urethane groups (C7) occurs at δ = 157.2 ppm, demonstrating the covalent bond between the polysaccharide κ-carrageenan and the siliceous network [15]. Although less intense, these new signals are also present in the 13 C CP/MAS NMR spectra of the TM-modified hybrid siliceous materials. In addition, the broad resonances between δ = 61 and 106 ppm have been attributed to the skeleton carbon atoms of κ-carrageenan (C1-C6 and C1′-C6′), according to the literature [53].
Thermogravimetric analysis (TGA) measurements were performed to evaluate the thermal properties of the materials. Figure 8a shows the TGA of bulk silica and TMmodified silica particles. The weight loss of bare silica below 200°C is 4% which is attributed to the physisorbed water [55], and the mass loss from 200 to 600°C is related Fig. 7 a 13 C CP/MAS NMR spectra of κ-carrageenan, SiO 2 /SiCRG, SiO 2 /SiCRG/Co, SiO 2 /SiCRG/Cu, SiO 2 /SiCRG/Ni and SiO 2 /SiCRG/ Zn particles; and b chemical structure, with carbons numbered, of SiO 2 /SiκCRG hybrid to silica hydroxylation [56]. The polysaccharide κ-carrageenan (Fig. 8b) shows weight loss in three distinct stages: below 200°C (16% weight loss) corresponds to the loss of adsorbed and bound water; a second stage from 230 to 400°C (61% weight loss) is due to carbohydrate-backbone fragmentation and sulfur dioxide release [57], and further decomposition at higher temperatures leads to 20% residue at 900°C, which is due to carbon. The onset temperature of the second stage decreased to 171°C in the unmodified hybrid (SiO 2 /SiĸCRG) due to the thermal dissociation of urethane bonds in aliphatic urethane. However, in the TMmodified hybrids, this temperature was higher (210-218°C), indicating that the TM decreases the resistance to thermal decomposition (Fig. 8b). Overall, the thermal degradation of native κ-carrageenan is faster than that of TM-modified κ-carrageenan silica particles. About 77% weight loss takes place in the temperature range of 400-550°C for κ-carrageenan. In the SiO 2 /SiκCRG, SiO 2 / SiκCRG/Co, SiO 2 /SiκCRG/Cu, SiO 2 /SiκCRG/Ni, and SiO 2 /SiκCRG/Zn samples, a weight loss of 52%, 56%, 66%, 47%, and 53%, respectively, was observed at 550°C. The differences observed in the weight losses by varying the TM not only confirm the presence of the TM ion, but also suggest that the respective structures of the κ-carrageenan backbones might be changed depending on the TM species. At 900°C the residue was about 30% and 32%, 25%, 39%, and 34% for unmodified SiO 2 /SiκCRG and hybrids modified with Co, Cu, Ni, and Zn TM, respectively.

Conclusions
TM-modified (TM = Co 2+ , Cu 2+ , Ni 2+ , Zn 2+ ) carrageenansilica hybrids were synthesized by an in situ sol-gel route that uses as precursor a covalently linked alkoxysilane modified κcarrageenan. This room-temperature sol-gel method is simpler in reaction conditions and allows spherical, monodispersed sub-micrometer-sized particles to be obtained without the need of surfactants and emulsions. The measured specific surface area of the TM-modified materials is significantly altered as a result of the blocking of some pores by the TM cations, and the pore volume also decreased. Compared with the unmodified SiO 2 , the average size of the TM-modified SiO 2 particles markedly decrease, suggesting that the incorporation of the TM cations, such as Co 2+ , Cu 2+ , and Ni 2+ , limits the growth of the SiO 2 particles. Related to the SiO 2 /Zn particles, as compared to the unmodified SiO 2 sample, the average particle size increased but the particles present a distinct nanoplatelets-like morphology. The incorporation of TM in the carrageenan-silica particles has not shown a clear trend in the effect of the TM cation employed on the final average particle size of the modified hybrid materials. However, samples with well-defined spheroidal shapes have been obtained. These observations are a strong indication of the important role of the alkoxysilanemodified polysaccharide precursor during the sol-gel process, namely by providing diverse oxygen donor groups for coordinating TM cationic species present in the reacting mixture, such as sulfate groups. These metal-modified hybrid particles will hopefully aid in improving several applications of technological relevance. As a perspective for future structural studies in these hybrid materials, the collection of extended X-ray absorption fine structure signals would enable the type of coordination of the TM cation in the silica shells to be established. Funding This work is financed by Portugal 2020 through European Regional Development Fund (ERDF) in the frame of

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