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Silica nanosystems for active antifouling protection: nanocapsules and mesoporous nanoparticles in controlled release applications
Journal of Alloys and Compounds 798 (2019) 144e148
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Silica nanosystems for active antifouling protection: nanocapsules and mesoporous nanoparticles in controlled release applications L. Ruggiero a, *, E. Di Bartolomeo b, T. Gasperi a, I. Luisetto a, A. Talone a, c, F. Zurlo b, D. Peddis c, M.A. Ricci a, A. Sodo a degli Studi “Roma Tre”, Roma, Italy Dipartimento di Scienze, Universita Dipartimento di Scienze Chimiche e Tecnologie, Universita' di Roma Tor Vergata, Roma, Italy c Istituto di Struttura della Materia, CNR, Montelibretti (RM), Italy a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2018 Received in revised form 15 May 2019 Accepted 17 May 2019 Available online 18 May 2019
In the attempt of satisfying the quest for new antifouling products tailored for applications on Cultural Heritage, we have synthesized and characterized two different silica nanosystems. The first consists of core-shell nanocapsules and the second one of mesostructured nanoparticles. Both systems have been successfully loaded in situ with the commercial biocide 2-mercaptobenzothizole (MBT). The synthesis procedure and particles characterization measurements, as far as their dimensions and superficial properties, loading capability, and release rate in aqueous solution, are reported. We stress that both particles have the dimensions required to avoid undesired light scattering once dispersed on the artefact surface. Interestingly, the two nanoparticles have complementary properties, suggesting that combining them in a coating may have a higher potential for production of new generation active products. © 2019 Elsevier B.V. All rights reserved.
Keywords: Silica nanoparticles TEOS Encapsulation of biocides 2-Mercaptobenzothiazole Controlled release
1. Introduction The conservation of outdoor cultural heritage has always been a challenge. In particular the surface of stone buildings and monuments may deteriorate with the passage of time, due to its interaction with the environment. Biotic factors, especially microorganisms (bacteria, archea and fungi) are among the most common damage agents, affecting stone surface characteristics and aspect, due to physical and chemical interactions [1]. Bioremediation treatments, based on various biocides, are generally applied to a range of materials in order to eliminate microorganisms colonies and to prevent their recurrence. Since the effectiveness and efficiency of several biocides last only for a short period of time (between 6 months and one year), frequent treatments or many application cycles are usually needed [2]. Consequently, it is important to select new products with long lasting antifouling efficiency and, at the same time, it is mandatory to look for non-toxic and harmless formulations [3]. Commercial products consist of biocides dispersed into a
* Corresponding author. E-mail address: [email protected] (L. Ruggiero). https://doi.org/10.1016/j.jallcom.2019.05.215 0925-8388/© 2019 Elsevier B.V. All rights reserved.
coating formulation [4], giving quite short-lived anti-vegetative efficiency, due to the immediate availability of the entire amount of biocide. Indeed, most biocides are small molecules with fast selfdiffusion through the coating matrix; as a consequence, these products require very high biocide concentrations in order to increase their antifouling function [5]. Conversely, loading of biocides in nanosystems is a good strategy to reduce the amount of the bioactive compound and to obtain a satisfactory long-lasting antifouling action. This strategy allows storing the biocide, avoiding its leaching, while ensuring ecofriendly and smart release. In the literature, different loading techniques have been reported for effective storage and on demand release of biocides [6]. Generally, nanomaterials with high pore volume and large surface area are used to load active compounds through either a physical adsorption procedure or a physicochemical interaction after the funtionalization of the surface [7,8]. The physical adsorption procedure involves the use of silica nanoparticles as scaffolds on which the active compound is adsorbed. This method is relatively simple and allows loading of nanoparticles with different types of biocides depending on the solvent; nevertheless it has an intrinsic drawback, due to the fast and easy leakage of biocides from the scaffold surface [9,10]. In the chemicophysical procedure, instead, the silica surface is functionalized in
L. Ruggiero et al. / Journal of Alloys and Compounds 798 (2019) 144e148
order to promote chemical bonds with the biocides. Functional groups, targeting ligands and stimuli-sensitive molecules can be conjugated to the internal pores and the outer surface of nanoparticles [11,12]: this procedure allows longer release times, but the synthesis is much more complex, time consuming and strictly specific of each selected class of biocides. In this work, we report the synthesis protocol and characterization of two different nanosystems, which allow slow biocide release, yet being based on a purely physical confining procedure. The two synthetic strategies are based on the physical confinement of the biocide into nanocontainers: namely within core-shell nanocapsules (Si-NC) or into mesoporous nanoparticles (Si-MNP), both precluding the direct contact of the biocide with the environment. Both synthesis procedures are one-step self-assembly methods, involving the silica precoursor (TEOS) polymerization assisted by a cationic surfactant (CTAB).
2. Experimental procedure 2.1. Materials We have used acetyltrimethylammonium bromide (CTAB, Aldrich), ammonia solution (NH3aq 30%, Aldrich), tetraethoxysilane (TEOS, Aldrich) and diethylether (Et2O, Aldrich) for the preparation of nanocapsules and nanoparticles. These have been loaded with 2mercaptobenzothiazole (MBT), an active antifouling compound. All the chemicals have analytic grade purity and have been used without further purification. For HPLC analysis, acetonitrile and ethanol (RS-Plus grade), purchased from Sigma-Aldrich Fluka, have been used. Distilled water was further purified through a Milli-Q Plus apparatus (Millipore, Bedford, MA USA).
2.2. Synthesis of nanostructures with in situ encapsulation of the 2mercaptobenzothiazole (MBT) According to a typical synthesis procedure of silica nanocapsules (Si-NC) [13,14], CTAB (0.25 g) is dissolved in deionized water (35 mL), and then 0.25 mL of ammonia aqueous solution (25e28% V) are added. Afterwards, 0.01 g of MBT dissolved in 25 mL of diethyl ether are added to the aqueous solution under constant stirring. After 30 min, when the miniemulsion is stabilized, 2 mL of TEOS are slowly dripped into the emulsion. The reaction is left to proceed at room temperature for 24 h under constant magnetic stirring. The obtained precipitate is finally filtered, repeatedly washed with water, then dried at room temperature. For the synthesis of the silica nanoparticles (Si-MNP), a solution of CTAB (0.25 g), MBT (0.01 g) in 20 mL of water is sonicated for 30 min. At the same time in other beacker, 875 mL of NaOH aqueous solution (2.0 M) are diluted into 100 mL of deionized water and then added to the first solution. The final solution is heated to 80 C. After reaching the temperature, 1.25 mL of TEOS are dripped and the final mixture is kept under fast stirring for additional 2 h. Finally, the silica nanoparticles are collected by filtration and washed several times with water [15]. A small portion of both products has been calcined at 550 C for 5 h in order to remove residual CTAB and other organic components, for control measurements. An additional control sample has been prepared, by physical adsorption of MBT on a portion of calcined Si-MNP, namely MNP-impregnated. In order to physically adsorb MBT on Si-NC, dry silica nanoparticles have been mixed with a water solution of 2-mercaptobenzothiazole (0,4 mg/mL). The MBT and Si-NC suspension was transferred to a vacuum jar, which was then evacuated by a vacuum pump.
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2.3. Silica nanosystems characterization The morphology of the nanosystems has been investigated by using a Field Emission Scanning Electron Microscope (FE-SEM, SUPRA™ 35, Carl Zeiss SMT, Oberkochen, Germany). To avoid charging effects, the samples were previously sputtered with a thin layer of gold. Particle size was investigated and the particle distribution was processed using the ImageJ software. Thermal stability and biocide content have been determined by thermogravimetric analysis, using a TG-DSC 1 (Mettler Toledo, STAR System) with simultaneous thermogravimetry and scanning calorimetry abilities. The measurements have been carried out under air atmosphere, with a heating rate of 10 C/min in the temperature range of 30e800 C. Textural properties have been investigated by the N2 adsorptionedesorption isotherms at 196 C using a Micromeritics Gemini V apparatus. The specific surface area has been calculated by the BrunauereEmmetteTeller (BET) method by using adsorption data in the range 0.04 < P/P < 0.30. The pore size distribution has been obtained from the desorption branch using the BarretteJoynereHalenda (BJH) method. The total pore volume has been calculated from the maximum adsorption point at P/P ¼ 0.98. Prior to the N2 adsorption, each sample was degassed in flowing He at 150 C by using a Micromeritics Flow Prep accessory. 2.4. Release studies of the biocide The release profiles of MBT have been monitored by High Performance Liquid Chromatography (HPLC) coupled with a diodearray ultraviolet (UV-DAD) detector, for the quantitative analysis. An isocratic mixture of water and acetonitrile has been used for the mobile phase. The correlation coefficient of the calibration curves obtained with 10 MBT standards was higher than 0.999. Samples of 5 mg of each nanosystem (Si-NC, Si-MNP, MNP-impregnated) were dispersed in 1 mL of solvent (H₂O or EtOH) and monitored at time intervals between 0 and 120 days. 1 mL of each sample has been collected and filtered through a PTFE syringe filter (Iso-Disc™ Filters PTFE-13-2 13 mm 0.2 mm, Sigma-Aldrich) prior to injection of 20 mL into the HPLC/UV-DAD system. Liquid chromatography was performed under reversed phase (RP) conditions by means of a HPLC system Infinity 1260 series (Agilent, Santa Clara, California, USA) using Phenomenex® Luna column (C18, 50 4.60 mm, Phenomenex®, Torrance, California, USA) protected by a guard column of the same type (4.0 10 mm, 5 mm) and kept at 25 C. 3. Results and discussion The synthesis of silica nanosystems, namely nanocapsules (SiNC) and nanoparticles (Si-MNP), basically follows well established methodologies, involving, at basic conditions, the hydrolysis and polycondensation of the silica precursor TEOS at the interface of pore-templating surfactant micelles [16]. In particular, in the case of Si-NC we have used the same procedure of ref. [13] also for the loading of MBT, although at different biocide molar ratio. In the case of Si-MNP, we have modified the synthesis protocol of the nanoparticles, reported in Ref. [15], in order to confine MBT in situ during the synthesis. Specifically, in order to encapsulate MBT into the Si-NCs, the biocide has been loaded after dissolution in ethyl ether, where the latter acts as cosolvent of the oil/water mini-emulsions [13,14]. In the case of the Si-MNPs, we have used CTAB as pore-templating surfactant: this gives a mesoporous matrix, hosting the MBT. Noteworthy, to the best of our knowledge, the latter entrapment procedure has not been reported so far. We have performed SEM analyses in order to investigate the
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morphology of the synthesized nanosystems. Figs. 1 and 2 show the SEM micrographs of the Si-NCs, both empty and loaded, along with the particle size distributions. These show that, according to previous studies [13,14], the nanocontainers have spherical and regular shape in both cases. Nevertheless we notice that the average particle diameter, as calculated from the histograms of Figs. 1b and 2b, increases when the MBT is loaded during the synthesis. This suggests that the dimension of the biocide molecule shapes the form of the surfactant micelles and thus of the Si-NCs. We stress that the larger diameter of the loaded particles, namely (128 ± 15) nm compared to (80 ± 11) nm for the empty ones, is an evidence of a successful encapsulation. The core-shell structure of the Si-NCs is evident in Fig. 2a, where the black circles indicate two broken particles; moreover the larger particles clearly show a porous surface. The Si-MNP nanoparticles entrapping MBT appear as spherical grains with a diameter distribution function centered at (39 ± 4) nm (Fig. 3 (a) and (b)). Remarkably, the average diameter of the SiMNPs is one order of magnitude smaller than that of the Si-NC ones. Such large difference in size is due to the different synthesis procedure. Indeed, the dimensions of the Si-NC nanocontainers is limited by the dimensions of the micelle containing the biocide, while in the synthesis procedure of Si-MNP the biocide is interpenetrated with CTAB in the skeletal structure of the nanoparticle. The textural properties of the silica nanosystems loaded with the biocide have been evaluated by Nitrogen adsorption-desorption measurements and summarized in Table 1. Both nanocapsules and the nanoparticles show type IV adsorption isotherms, with a hysteresis loop characteristic for materials with mesoporous structure (see Fig. 4a) [17]. Si-NC nanocontainers have a Brunauer-Emmett-Teller (BET) specific surface area of 965 m2/g with an average pore volume of
Fig. 2. SEM micrograph of Si-NC-MBT (a) and the corresponding particle size histogram (b). The black circles in Fig. 2(a) indicate two broken nanoparticles, evidencing the core-shell structure.
Fig. 1. SEM micrograph of empty silica nanocapsules (a) and the corresponding particle size histogram (b).
Fig. 3. SEM micrograph of Si-MNP-MBT (a) and the corresponding particle size histogram(b).
L. Ruggiero et al. / Journal of Alloys and Compounds 798 (2019) 144e148
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Table 1 Textural properties of the samples determined from the adsorption-desorption isotherms of N2. Sample
Diameter/nm
SBET/m2g1
Pore volume/cm3g1
Fpores BJHa/nm
Si-NC Si-NC-MBT Si-MNP Si-MNP-MBT
80 128 e 39
965 720 960 924
1.31 1.62 0.71 1.86
3.6 3.8 3.9 3.6
a
The values represent the most frequent value (mode) of the distribution of pore diameter.
Fig. 4. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution of Si-NC-MBT and Si-MNP-MBT.
1.31 cm3/g. The Barrett-Joyner-Halenda (BJH) distribution of the cylinder pore diameters is centered at 3.6 nm. When loaded the BET specific surface area is 720 m2/g with an average pore volume of 1.62 cm3/g. The BJH distribution of the cylinder pore diameters is centered at 3.8 nm (Fig. 4b). The lower surface area of the loaded containers (Si-NC-MBT ) suggests the presence of MBT; the biocide determines also larger pore size parameters. The surface area of the empty Si-MNPs is 960 m2/g and their BJH pore size distribution gives an average pore diameter at 3.9 nm with a pore volume of 0.71 cm3/g. The Si-MNPs loaded with MBT have a BET specific surface area of 924 m2/g with average BJH cylinder pore diameter of 3.6 nm and average pore volume of 1.86 cm3/g. The change of specific surface area from empty to loaded particles is smaller in this case compared to the previous one, and reflects the different structure of the nanocomposites. We
also observe a large variation of the pore volume after loading: this may be explained as a consequence of a change of the network due to the interpenetration and entrapment of MBT. Both nanosystems exhibit a mesoporosity: in the case of Si-NCs the pores are located on the shell and are significantly smaller than the diameter of the nanostructure; in the case the Si-MNPs the structure is similar to a sponge. The high surface area and the distribution of mesopores are very important properties for a controlled biocide release, as we will show below. The thermal stability of silica nanosystems and the amount of the encapsulated biocide have been determined by thermogravimetric tests. In order to take into account, the possible contribution to the weight loss due to residual non-hydrolysed/condensed TEOS and cationic surfactant CTAB [18], a preliminary measurement on calcined neat nanoparticles has been performed for systematic error subtraction. The thermo-gravimetric curves corresponding to loaded and empty nanosystems show a similar profile in the entire temperature range, with exception of the interval 200 ÷ 300 C, where a more pronounced weight loss of loaded nanosystems is observed and attributed to the decomposition of MBT. The amount of biocide loading is estimated as 10%w/w in the case of nanocapsules and 8.2%w/w for the nanoparticles. Following previous literature [13], the controlled release of MBT from the two systems has been examined by re-dispersing the loaded nanosystems in water and analysing over the time the aqueous phase by means of HPLC. Unlike the previous literature [19], all tests have been conducted under weak stirring and without buffer. The logic behind these choices is to mimic as closely as possible the release conditions in future applications, without enhancing the biocide release through mechanical stirring, or inducing changes in the silica structure due to pH variation led by the buffer. Moreover, control release tests in ethanol have been performed, for a fast check of the effective release of the biocide, giving a MBT concentration of 0.6 mM at time zero and of 2.5 mM after 72 h of extraction for the Si-NCs; and 4.9 mM at time zero and 5.5 mM after 72 h for the Si-MNPs. The differences in the released amount of MBT confirm that the biocide are encapsulated in two different ways. The release profiles obtained in water as a function of time (from 0 to 120 days) for the two nanosystems suggest a different release rate (Fig. 5). Bearing in mind that during the release experiment the solvent has not been refreshed, we notice that the total amount of biocide released after 4 months is about the same, instead, the release kinetics of the two samples is different. In the case of loaded Si-NC-MBTs we notice an almost linear and slow release during the first z70 days, followed by a much faster release regime at longer times. In the case of the Si-MNP-MBTs we find an initial linear release, faster than in the previous system (0.00092 mM/day, to be compared with 0.00046 mM/day for the Si-NC-MBTs), followed by a saturation at about 0.12 mM. The faster initial release for the latter system may be ascribed to a more direct contact between the biocide and water, in particular at short time, possibly due to the textural properties (e.g. surface area and average pore volume). In order to definitively verify that in both nanoparticles
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References
Fig. 5. Profiles release of MBT from Si-NC-MBT (blue curve) and Si-MNP- MBT (red curve). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
formulation MBT is entrapped into the mesoporous structure, and not physically adsorbed on the surface, we have performed the same release test on the calcined nanoparticles impregnated by MBT. In this case we have observed a complete release of MBT in less than five days. 4. Conclusion The present paper reports two encapsulation protocols of 2mercaptobenzothiazole in silica nanosystems by pore-templating surfactant strategy, labeled Si-NC-MBT and Si-MNP-MBT respectively. Both products have regular and spherical morphology, with a different mesoporosity. Size and textural properties of the two nanoparticles are influenced by the synthesis procedure (e.g. presence of co-solvent, and MBT loading). In particular, the two nanoparticles have different size, with a factor ~4 between their diameters. The MBT content of the final products is similar, but the release rate is different. In particular the release in water within the first 3 months is slower for the Si-NC system. Further tests with other biocides are needed in order to check the versatility of the method. The nanosystems synthesized in this work are promising media for different applications and in particular for control release of biocides in Cultural Heritage applications. The different size and release properties of the nanosystems are particularly fit to applications as part of protective antifouling coatings for outdoor artefacts. In this respect, we stress that the dimension of both nanoparticles is such that no light diffusion will limit or change the look of the artefact. Moreover the observed differences in the release rate may be functional to the production of tailored coating products. Acknowledgments The authors gratefully acknowledge the Grant of Excellence Departments, MIUR (ARTICOLO 1, COMMI 314337 LEGGE 232/ 2016) and the SUPERARE grant “Gruppi di Ricerca”, Regione Lazio.
[1] C. Dresler, M.L. Saladino, C. Demirbag, E. Caponetti, D.F. Chillura Martino, R. Alduina, Development of controlled release systems of biocides for the conservation of cultural heritage, Int. Biodeterior. Biodegrad. 125 (2017) 150e156. https://doi.org/10.1016/j.ibiod.2017.09.007. [2] M.E. Young, H.-L. Alakomi, I. Fortune, A.A. Gorbushina, W.E. Krumbein, I. Maxwell, C. McCullagh, P. Robertson, M. Saarela, J. Valero, M. Vendrell, Development of a biocidal treatment regime to inhibit biological growths on cultural heritage: BIODAM, Environ. Geol. 56 (2008) 631e641. https://doi.org/ 10.1007/s00254-008-1455-1. [3] European parliament, Directive of the European Parliament and of the Council of 16 February 1998 Concerning the Placing of Biocidal Products on the Market, European Union: Official Journal of the European Communities, 24 th April 1998 (1998), 61. na, Use [4] M.A. Trojer, L. Nordstierna, J. Bergek, H. Blanck, K. Holmberg, M. Nyde of microcapsules as controlled release devices for coatings, Adv. Colloid Interface Sci. 222 (2015) 18e43. https://doi.org/10.1016/j.cis.2014.06.003. [5] S.K. Ghosh, Functional Coatings and Microencapsulation: A General Perspective in Functional Coatings by Polymer Microencapsulation, WILEY-VCH Verlag GmbH & Co. KGaA Weinheim, 2006, pp. 1e25. [6] L. McCusker, F. Liebau, G. Engelhardt, Nomenclature of structural and compositional characteristics of ordered microporous and mesoporous materials with inorganic hosts(IUPAC Recommendations 2001), Pure Appl. Chem. 73 (2) (2009) 381e394. https://doi.org/10.1351/pac200173020381. [7] A.C. Chan, M. Bravo Cadena, H.E. Townley, M.D. Fricker, I.P. Thompson, Effective delivery of volatile biocides employing mesoporous silicates for treating biofilms, J. R. Soc. Interface 14 (2017), 20160650. https://doi.org/10. 1098/rsif.2016.0650. [8] A. Popat, J. Liu, Q. Hu, M. Kennedy, B. Peters, G. Q. (Max) Lu, S.Z. Qiao, Adsorption and release of biocides with mesoporous silica nanoparticles, Nanoscale 4 (3) (2012) 970e975. https://doi.org/10.1039/c2nr11691j. € hwald, [9] Z. Zheng, X. Huang, M. Schenderlein, D. Borisova, R. Cao, H. Mo D. Shchukin, Self-healing and antifouling multifunctional coatings based on pH and sulfide ion sensitive nanocontainers, Adv. Funct. Mater. 23 (2013) 3307e3314. https://doi.org/10.1002/adfm.201203180. [10] M. Michailidis, I. Sorzabal-Bellido, E.A. Adamidou, Y.A. Diaz-Fernandez, J. Aveyard, R. Wengier, D. Grigoriev, R. Raval, Y. Benayahu, R.A. D’Sa, D. Shchukin, Modified mesoporous silica nanoparticles with a dual synergetic antibacterial effect, ACS Appl. Mater. Interfaces 9 (44) (2017) 38364e38372. https://doi.org/10.1021/acsami.7b14642. zga-Wijas, U. Mizerska, W. Fortuniak, J. Chojnowski, R. Hałasa, W. Werel, [11] K. Ro Quaternary ammonium salts (QAS) modified polysiloxane biocide supported on silica materials, J. Inorg. Organomet. Polym. Mater. 17 (4) (2007) 605e613. https://doi.org/10.1007/s10904-007-9153-0. [12] K. Kermannezhad, A.N. Chermahini, M.M. Momeni, B. Rezaei, Application of amine-functionalized MCM-41 as pH-sensitive nanocontainer for controlled release of 2-mercaptobenzoxazole corrosion inhibitor, Chem. Eng. J. 306 (2016) 849e857. https://doi.org/10.1016/j.cej.2016.08.004. [13] F. Maia, J. Tedim, A.D. Lisenkov, A.N. Salak, M.L. Zheludkevich, M.G. Ferreira, Silica nanocontainers for active corrosion protection, Nanoscale 4 (2012) 1287e1298. https://doi.org/10.1039/c2nr11536k. [14] L. Ruggiero, L. Crociani, E. Zendri, N. El Habra, P. Guerriero, Incorporation of the zosteric sodium salt in silica nanocapsules:synthesis and characterization of new fillers for antifouling coatings, Appl. Surf. Sci. 439 (2018) 705e711. https://doi.org/10.1016/j.apsusc.2017.12.228. [15] N.Z. Knezevic, C. Mauriello Jimenez, M. Albino, A. Vukadinovic, A. Mrakovic, E. Illes, D. Janackovic, J. Durand, C. Sangregorio, D. Peddis, Synthesis and characterization of core-shell magnetic mesoporous silica and organosilica nanostructures, MRS Adv. 2 (19e20) (2017) 1037e1045. https://doi.org/10. 1557/adv.2017.69. [16] H. Chen, J. He, H. Tang, C. Yan, Porous silica nanocapsules and nanospheres: dynamic self-assembly synthesis and application in controlled release, Chem. Mater. 20 (2008) 5894e5900. https://doi.org/10.1021/cm801411y. [17] A. Zeid, ALOthman, a review: fundamental aspects of silicate mesoporous materials, Materials 5 (12) (2012) 2874e2902. https://doi.org/10.3390/ ma5122874. [18] G. Sponchia, R. Marin, I. Freris, M. Marchiori, E. Moretti, L. Storaro, P. Caton, A. Lausi, A. Benedetti, P. Riello, Mesoporous silica nanoparticles with tunable pore size for tailored gold nanoparticles, J. Nanoparticle Res. 16 (2014) 2245e2256. https://doi.org/10.1007/s11051-014-2245-1. [19] A. Chenan, S. Ramya, R.P. George, U. Kamachi Mudali, Hollow mesoporous zirconia nanocontainers for storing and controlled releasing of corrosion inhibitors, Ceram. Int. 40 (2014) 10457e10463. https://doi.org/10.1016/j.cej. 2016.08.004.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Silica nanosystems for active antifouling protection: nanocapsules and mesoporous nanoparticles in controlled release applications L. Ruggiero a, *, E. Di Bartolomeo b, T. Gasperi a, I. Luisetto a, A. Talone a, c, F. Zurlo b, D. Peddis c, M.A. Ricci a, A. Sodo a degli Studi “Roma Tre”, Roma, Italy Dipartimento di Scienze, Universita Dipartimento di Scienze Chimiche e Tecnologie, Universita' di Roma Tor Vergata, Roma, Italy c Istituto di Struttura della Materia, CNR, Montelibretti (RM), Italy a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2018 Received in revised form 15 May 2019 Accepted 17 May 2019 Available online 18 May 2019
In the attempt of satisfying the quest for new antifouling products tailored for applications on Cultural Heritage, we have synthesized and characterized two different silica nanosystems. The first consists of core-shell nanocapsules and the second one of mesostructured nanoparticles. Both systems have been successfully loaded in situ with the commercial biocide 2-mercaptobenzothizole (MBT). The synthesis procedure and particles characterization measurements, as far as their dimensions and superficial properties, loading capability, and release rate in aqueous solution, are reported. We stress that both particles have the dimensions required to avoid undesired light scattering once dispersed on the artefact surface. Interestingly, the two nanoparticles have complementary properties, suggesting that combining them in a coating may have a higher potential for production of new generation active products. © 2019 Elsevier B.V. All rights reserved.
Keywords: Silica nanoparticles TEOS Encapsulation of biocides 2-Mercaptobenzothiazole Controlled release
1. Introduction The conservation of outdoor cultural heritage has always been a challenge. In particular the surface of stone buildings and monuments may deteriorate with the passage of time, due to its interaction with the environment. Biotic factors, especially microorganisms (bacteria, archea and fungi) are among the most common damage agents, affecting stone surface characteristics and aspect, due to physical and chemical interactions [1]. Bioremediation treatments, based on various biocides, are generally applied to a range of materials in order to eliminate microorganisms colonies and to prevent their recurrence. Since the effectiveness and efficiency of several biocides last only for a short period of time (between 6 months and one year), frequent treatments or many application cycles are usually needed [2]. Consequently, it is important to select new products with long lasting antifouling efficiency and, at the same time, it is mandatory to look for non-toxic and harmless formulations [3]. Commercial products consist of biocides dispersed into a
* Corresponding author. E-mail address: [email protected] (L. Ruggiero). https://doi.org/10.1016/j.jallcom.2019.05.215 0925-8388/© 2019 Elsevier B.V. All rights reserved.
coating formulation [4], giving quite short-lived anti-vegetative efficiency, due to the immediate availability of the entire amount of biocide. Indeed, most biocides are small molecules with fast selfdiffusion through the coating matrix; as a consequence, these products require very high biocide concentrations in order to increase their antifouling function [5]. Conversely, loading of biocides in nanosystems is a good strategy to reduce the amount of the bioactive compound and to obtain a satisfactory long-lasting antifouling action. This strategy allows storing the biocide, avoiding its leaching, while ensuring ecofriendly and smart release. In the literature, different loading techniques have been reported for effective storage and on demand release of biocides [6]. Generally, nanomaterials with high pore volume and large surface area are used to load active compounds through either a physical adsorption procedure or a physicochemical interaction after the funtionalization of the surface [7,8]. The physical adsorption procedure involves the use of silica nanoparticles as scaffolds on which the active compound is adsorbed. This method is relatively simple and allows loading of nanoparticles with different types of biocides depending on the solvent; nevertheless it has an intrinsic drawback, due to the fast and easy leakage of biocides from the scaffold surface [9,10]. In the chemicophysical procedure, instead, the silica surface is functionalized in
L. Ruggiero et al. / Journal of Alloys and Compounds 798 (2019) 144e148
order to promote chemical bonds with the biocides. Functional groups, targeting ligands and stimuli-sensitive molecules can be conjugated to the internal pores and the outer surface of nanoparticles [11,12]: this procedure allows longer release times, but the synthesis is much more complex, time consuming and strictly specific of each selected class of biocides. In this work, we report the synthesis protocol and characterization of two different nanosystems, which allow slow biocide release, yet being based on a purely physical confining procedure. The two synthetic strategies are based on the physical confinement of the biocide into nanocontainers: namely within core-shell nanocapsules (Si-NC) or into mesoporous nanoparticles (Si-MNP), both precluding the direct contact of the biocide with the environment. Both synthesis procedures are one-step self-assembly methods, involving the silica precoursor (TEOS) polymerization assisted by a cationic surfactant (CTAB).
2. Experimental procedure 2.1. Materials We have used acetyltrimethylammonium bromide (CTAB, Aldrich), ammonia solution (NH3aq 30%, Aldrich), tetraethoxysilane (TEOS, Aldrich) and diethylether (Et2O, Aldrich) for the preparation of nanocapsules and nanoparticles. These have been loaded with 2mercaptobenzothiazole (MBT), an active antifouling compound. All the chemicals have analytic grade purity and have been used without further purification. For HPLC analysis, acetonitrile and ethanol (RS-Plus grade), purchased from Sigma-Aldrich Fluka, have been used. Distilled water was further purified through a Milli-Q Plus apparatus (Millipore, Bedford, MA USA).
2.2. Synthesis of nanostructures with in situ encapsulation of the 2mercaptobenzothiazole (MBT) According to a typical synthesis procedure of silica nanocapsules (Si-NC) [13,14], CTAB (0.25 g) is dissolved in deionized water (35 mL), and then 0.25 mL of ammonia aqueous solution (25e28% V) are added. Afterwards, 0.01 g of MBT dissolved in 25 mL of diethyl ether are added to the aqueous solution under constant stirring. After 30 min, when the miniemulsion is stabilized, 2 mL of TEOS are slowly dripped into the emulsion. The reaction is left to proceed at room temperature for 24 h under constant magnetic stirring. The obtained precipitate is finally filtered, repeatedly washed with water, then dried at room temperature. For the synthesis of the silica nanoparticles (Si-MNP), a solution of CTAB (0.25 g), MBT (0.01 g) in 20 mL of water is sonicated for 30 min. At the same time in other beacker, 875 mL of NaOH aqueous solution (2.0 M) are diluted into 100 mL of deionized water and then added to the first solution. The final solution is heated to 80 C. After reaching the temperature, 1.25 mL of TEOS are dripped and the final mixture is kept under fast stirring for additional 2 h. Finally, the silica nanoparticles are collected by filtration and washed several times with water [15]. A small portion of both products has been calcined at 550 C for 5 h in order to remove residual CTAB and other organic components, for control measurements. An additional control sample has been prepared, by physical adsorption of MBT on a portion of calcined Si-MNP, namely MNP-impregnated. In order to physically adsorb MBT on Si-NC, dry silica nanoparticles have been mixed with a water solution of 2-mercaptobenzothiazole (0,4 mg/mL). The MBT and Si-NC suspension was transferred to a vacuum jar, which was then evacuated by a vacuum pump.
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2.3. Silica nanosystems characterization The morphology of the nanosystems has been investigated by using a Field Emission Scanning Electron Microscope (FE-SEM, SUPRA™ 35, Carl Zeiss SMT, Oberkochen, Germany). To avoid charging effects, the samples were previously sputtered with a thin layer of gold. Particle size was investigated and the particle distribution was processed using the ImageJ software. Thermal stability and biocide content have been determined by thermogravimetric analysis, using a TG-DSC 1 (Mettler Toledo, STAR System) with simultaneous thermogravimetry and scanning calorimetry abilities. The measurements have been carried out under air atmosphere, with a heating rate of 10 C/min in the temperature range of 30e800 C. Textural properties have been investigated by the N2 adsorptionedesorption isotherms at 196 C using a Micromeritics Gemini V apparatus. The specific surface area has been calculated by the BrunauereEmmetteTeller (BET) method by using adsorption data in the range 0.04 < P/P < 0.30. The pore size distribution has been obtained from the desorption branch using the BarretteJoynereHalenda (BJH) method. The total pore volume has been calculated from the maximum adsorption point at P/P ¼ 0.98. Prior to the N2 adsorption, each sample was degassed in flowing He at 150 C by using a Micromeritics Flow Prep accessory. 2.4. Release studies of the biocide The release profiles of MBT have been monitored by High Performance Liquid Chromatography (HPLC) coupled with a diodearray ultraviolet (UV-DAD) detector, for the quantitative analysis. An isocratic mixture of water and acetonitrile has been used for the mobile phase. The correlation coefficient of the calibration curves obtained with 10 MBT standards was higher than 0.999. Samples of 5 mg of each nanosystem (Si-NC, Si-MNP, MNP-impregnated) were dispersed in 1 mL of solvent (H₂O or EtOH) and monitored at time intervals between 0 and 120 days. 1 mL of each sample has been collected and filtered through a PTFE syringe filter (Iso-Disc™ Filters PTFE-13-2 13 mm 0.2 mm, Sigma-Aldrich) prior to injection of 20 mL into the HPLC/UV-DAD system. Liquid chromatography was performed under reversed phase (RP) conditions by means of a HPLC system Infinity 1260 series (Agilent, Santa Clara, California, USA) using Phenomenex® Luna column (C18, 50 4.60 mm, Phenomenex®, Torrance, California, USA) protected by a guard column of the same type (4.0 10 mm, 5 mm) and kept at 25 C. 3. Results and discussion The synthesis of silica nanosystems, namely nanocapsules (SiNC) and nanoparticles (Si-MNP), basically follows well established methodologies, involving, at basic conditions, the hydrolysis and polycondensation of the silica precursor TEOS at the interface of pore-templating surfactant micelles [16]. In particular, in the case of Si-NC we have used the same procedure of ref. [13] also for the loading of MBT, although at different biocide molar ratio. In the case of Si-MNP, we have modified the synthesis protocol of the nanoparticles, reported in Ref. [15], in order to confine MBT in situ during the synthesis. Specifically, in order to encapsulate MBT into the Si-NCs, the biocide has been loaded after dissolution in ethyl ether, where the latter acts as cosolvent of the oil/water mini-emulsions [13,14]. In the case of the Si-MNPs, we have used CTAB as pore-templating surfactant: this gives a mesoporous matrix, hosting the MBT. Noteworthy, to the best of our knowledge, the latter entrapment procedure has not been reported so far. We have performed SEM analyses in order to investigate the
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morphology of the synthesized nanosystems. Figs. 1 and 2 show the SEM micrographs of the Si-NCs, both empty and loaded, along with the particle size distributions. These show that, according to previous studies [13,14], the nanocontainers have spherical and regular shape in both cases. Nevertheless we notice that the average particle diameter, as calculated from the histograms of Figs. 1b and 2b, increases when the MBT is loaded during the synthesis. This suggests that the dimension of the biocide molecule shapes the form of the surfactant micelles and thus of the Si-NCs. We stress that the larger diameter of the loaded particles, namely (128 ± 15) nm compared to (80 ± 11) nm for the empty ones, is an evidence of a successful encapsulation. The core-shell structure of the Si-NCs is evident in Fig. 2a, where the black circles indicate two broken particles; moreover the larger particles clearly show a porous surface. The Si-MNP nanoparticles entrapping MBT appear as spherical grains with a diameter distribution function centered at (39 ± 4) nm (Fig. 3 (a) and (b)). Remarkably, the average diameter of the SiMNPs is one order of magnitude smaller than that of the Si-NC ones. Such large difference in size is due to the different synthesis procedure. Indeed, the dimensions of the Si-NC nanocontainers is limited by the dimensions of the micelle containing the biocide, while in the synthesis procedure of Si-MNP the biocide is interpenetrated with CTAB in the skeletal structure of the nanoparticle. The textural properties of the silica nanosystems loaded with the biocide have been evaluated by Nitrogen adsorption-desorption measurements and summarized in Table 1. Both nanocapsules and the nanoparticles show type IV adsorption isotherms, with a hysteresis loop characteristic for materials with mesoporous structure (see Fig. 4a) [17]. Si-NC nanocontainers have a Brunauer-Emmett-Teller (BET) specific surface area of 965 m2/g with an average pore volume of
Fig. 2. SEM micrograph of Si-NC-MBT (a) and the corresponding particle size histogram (b). The black circles in Fig. 2(a) indicate two broken nanoparticles, evidencing the core-shell structure.
Fig. 1. SEM micrograph of empty silica nanocapsules (a) and the corresponding particle size histogram (b).
Fig. 3. SEM micrograph of Si-MNP-MBT (a) and the corresponding particle size histogram(b).
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Table 1 Textural properties of the samples determined from the adsorption-desorption isotherms of N2. Sample
Diameter/nm
SBET/m2g1
Pore volume/cm3g1
Fpores BJHa/nm
Si-NC Si-NC-MBT Si-MNP Si-MNP-MBT
80 128 e 39
965 720 960 924
1.31 1.62 0.71 1.86
3.6 3.8 3.9 3.6
a
The values represent the most frequent value (mode) of the distribution of pore diameter.
Fig. 4. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution of Si-NC-MBT and Si-MNP-MBT.
1.31 cm3/g. The Barrett-Joyner-Halenda (BJH) distribution of the cylinder pore diameters is centered at 3.6 nm. When loaded the BET specific surface area is 720 m2/g with an average pore volume of 1.62 cm3/g. The BJH distribution of the cylinder pore diameters is centered at 3.8 nm (Fig. 4b). The lower surface area of the loaded containers (Si-NC-MBT ) suggests the presence of MBT; the biocide determines also larger pore size parameters. The surface area of the empty Si-MNPs is 960 m2/g and their BJH pore size distribution gives an average pore diameter at 3.9 nm with a pore volume of 0.71 cm3/g. The Si-MNPs loaded with MBT have a BET specific surface area of 924 m2/g with average BJH cylinder pore diameter of 3.6 nm and average pore volume of 1.86 cm3/g. The change of specific surface area from empty to loaded particles is smaller in this case compared to the previous one, and reflects the different structure of the nanocomposites. We
also observe a large variation of the pore volume after loading: this may be explained as a consequence of a change of the network due to the interpenetration and entrapment of MBT. Both nanosystems exhibit a mesoporosity: in the case of Si-NCs the pores are located on the shell and are significantly smaller than the diameter of the nanostructure; in the case the Si-MNPs the structure is similar to a sponge. The high surface area and the distribution of mesopores are very important properties for a controlled biocide release, as we will show below. The thermal stability of silica nanosystems and the amount of the encapsulated biocide have been determined by thermogravimetric tests. In order to take into account, the possible contribution to the weight loss due to residual non-hydrolysed/condensed TEOS and cationic surfactant CTAB [18], a preliminary measurement on calcined neat nanoparticles has been performed for systematic error subtraction. The thermo-gravimetric curves corresponding to loaded and empty nanosystems show a similar profile in the entire temperature range, with exception of the interval 200 ÷ 300 C, where a more pronounced weight loss of loaded nanosystems is observed and attributed to the decomposition of MBT. The amount of biocide loading is estimated as 10%w/w in the case of nanocapsules and 8.2%w/w for the nanoparticles. Following previous literature [13], the controlled release of MBT from the two systems has been examined by re-dispersing the loaded nanosystems in water and analysing over the time the aqueous phase by means of HPLC. Unlike the previous literature [19], all tests have been conducted under weak stirring and without buffer. The logic behind these choices is to mimic as closely as possible the release conditions in future applications, without enhancing the biocide release through mechanical stirring, or inducing changes in the silica structure due to pH variation led by the buffer. Moreover, control release tests in ethanol have been performed, for a fast check of the effective release of the biocide, giving a MBT concentration of 0.6 mM at time zero and of 2.5 mM after 72 h of extraction for the Si-NCs; and 4.9 mM at time zero and 5.5 mM after 72 h for the Si-MNPs. The differences in the released amount of MBT confirm that the biocide are encapsulated in two different ways. The release profiles obtained in water as a function of time (from 0 to 120 days) for the two nanosystems suggest a different release rate (Fig. 5). Bearing in mind that during the release experiment the solvent has not been refreshed, we notice that the total amount of biocide released after 4 months is about the same, instead, the release kinetics of the two samples is different. In the case of loaded Si-NC-MBTs we notice an almost linear and slow release during the first z70 days, followed by a much faster release regime at longer times. In the case of the Si-MNP-MBTs we find an initial linear release, faster than in the previous system (0.00092 mM/day, to be compared with 0.00046 mM/day for the Si-NC-MBTs), followed by a saturation at about 0.12 mM. The faster initial release for the latter system may be ascribed to a more direct contact between the biocide and water, in particular at short time, possibly due to the textural properties (e.g. surface area and average pore volume). In order to definitively verify that in both nanoparticles
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References
Fig. 5. Profiles release of MBT from Si-NC-MBT (blue curve) and Si-MNP- MBT (red curve). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
formulation MBT is entrapped into the mesoporous structure, and not physically adsorbed on the surface, we have performed the same release test on the calcined nanoparticles impregnated by MBT. In this case we have observed a complete release of MBT in less than five days. 4. Conclusion The present paper reports two encapsulation protocols of 2mercaptobenzothiazole in silica nanosystems by pore-templating surfactant strategy, labeled Si-NC-MBT and Si-MNP-MBT respectively. Both products have regular and spherical morphology, with a different mesoporosity. Size and textural properties of the two nanoparticles are influenced by the synthesis procedure (e.g. presence of co-solvent, and MBT loading). In particular, the two nanoparticles have different size, with a factor ~4 between their diameters. The MBT content of the final products is similar, but the release rate is different. In particular the release in water within the first 3 months is slower for the Si-NC system. Further tests with other biocides are needed in order to check the versatility of the method. The nanosystems synthesized in this work are promising media for different applications and in particular for control release of biocides in Cultural Heritage applications. The different size and release properties of the nanosystems are particularly fit to applications as part of protective antifouling coatings for outdoor artefacts. In this respect, we stress that the dimension of both nanoparticles is such that no light diffusion will limit or change the look of the artefact. Moreover the observed differences in the release rate may be functional to the production of tailored coating products. Acknowledgments The authors gratefully acknowledge the Grant of Excellence Departments, MIUR (ARTICOLO 1, COMMI 314337 LEGGE 232/ 2016) and the SUPERARE grant “Gruppi di Ricerca”, Regione Lazio.
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