Encapsulation and release of essential oils in functional silica

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Encapsulation and release of essential oils in functional silica nanocontainers Arjaree Jobdeedamrong, Ratchapol Jenjob, and Daniel Crespy Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01652 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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Encapsulation and release of essential oils in functional silica nanocontainers Arjaree Jobdeedamrong, Ratchapol Jenjob, Daniel Crespy* Department of Materials Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand KEYWORDS: Core-shell nanoparticles; encapsulation; essential oils; hyaluronic acid; silica nanocapsules.

ABSTRACT: We describe the fabrication of mesoporous silica nanocontainers (SiO2NCs) encapsulating simultaneously different antiseptic agents. Peppermint oil (PO), thyme oil (TO), cinnamon oil (CnO), and clove oil (CO), which are known to display antibacterial properties, are loaded in the core of the silica nanocontainers that are stabilized by antiseptic surfactants. The encapsulation efficiency, surface area and pore size are controlled by the type of oil and surfactant. The release of essential oils is further controlled by grafting oxidized hyaluronic acid on silica nanocontainers functionalized with amino groups.

INTRODUCTION Healthcare-associated infections (HCAIs) remain one of the most critical issue in patient care.1 Consequences include prolonged hospital stay, long-term disability, increased resistance of

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microorganisms to antimicrobial agents, significant economic burden for the healthcare system, additional pain for patients, and massive deaths.1-2 Only in the European Union, HCAIs were estimated to cause 16 million extra-days of hospital stay and was attributable to 37,000 deaths resulting in a 7 billion € financial burden annually.3 In the United States of America, there were about 99,000 deaths attributed to HCAIs and a cost of approximately 6.5 billion $ in 2004.3 The microorganisms typically associated with HCAIs include Gram-positive bacteria such as Staphylococcus aureus and Gram-negative bacteria such as Escherichia coli and Pseudomonas aeruginosa. These micro-organisms expand the progress of infection and influence biofilm formation that has posed a challenge in treating HCAIs.4 Essential oils (EOs) such as peppermint oil (PO),5-6 thyme oil (TO),5-7 clove oil (CO),5-9 and cinnamon oil (CnO)6 have been used for their antibacterial, antiviral, anti-inflammatory, antifungal, and antioxidant properties. The composition of EOs generally consists of terpenes, terpenoids, and phenylpropenes that can attach to the cell surface of organisms and penetrate the phospholipid bilayer of cell membrane, eventually leading to cells death.10-12 Terpenoids such as menthol and thymol and phenylpropenes such as eugenol and cinnamaldehyde are components of EOs that mainly influence antibacterial activities. For example, thymol is able to disturb micromembranes by integration of its polar head-groups in lipid bilayers and increase of the intracellular ATP concentration.13 Eugenol was also found to affect the transport of ions through cellular membranes.14 Cinnamaldehyde inhibits enzymes associated in cytokine interactions and acts as an ATPase inhibitor.13 However, the main limitations of EOs are their inherent volatility and propensity to oxidize.15 These drawbacks limit the long-term antibacterial efficacy of EOs. In order to overcome this issue, nanoencapsulation has been introduced.


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Due to small size of submicron droplets of emulsions, antibacterial agents can be efficiently delivered to microbial cells and disturb the cell membrane by interfering with the phospholipid bilayer.16 Therefore, the encapsulation of antibacterial agents such as essential oils in nanoparticles or nanocapsules has drawn attention for antimicrobial applications. For example, essential oils were encapsulated in polysaccharide,17-18 protein,19 and in inorganic20 nanocarriers. Furthermore, nanocarriers are attractive materials for protecting payloads from environmental influence and allowing a controlled release of encapsulated payloads. Indeed, nanocarriers can play the role of barrier between encapsulated molecules and the environment to reduce evaporation, photodegradation, and physicochemical stress for the payloads.21 Different groups reported the use of silica nanocapsules as nanocarriers for the delivery of payloads22-23 because of their high surface area, tunable size dimension, and adjustable porous structure surface. Moreover, silica nanocarriers are mechanically robust, chemically stable, and biocompatible.24-25 However, the porous structure of silica nanocapsules induces an initial burst release of encapsulated payloads and therefore a potentially low therapeutic efficiency.26 This burst release can be reduced by either using silica nanocapsules (SiO2NCs) with integrated redox-responsive groups,27 or by capping mesoporous silica nanocarriers with DNA,28 dextran,29 poly(beta-amino ester),30 a drug,31 or photoactive ruthenium complexes.32 Another strategy consisted in encapsulating a responsive material containing a payload inside mesoporous silica nanocarriers. The payload was released only after activation by change of pH value33 or chemical reduction.34 Besides, coating silica nanocarriers with a polymer allows to tune the release profile of encapsulated payloads.35 In a previous work, we encapsulated peppermint oil (PO) in SiO2NCs that were subsequently embedded in crosslinked poly(vinyl alcohol) nanofibers. The nanofibrous membranes were used


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for antibacterial wound dressing.36 A synergistic effect of the antibacterial effect of PO, the antiseptic agents octenidine (OCT) and chlorhexidine was observed. Herein, we explore the encapsulation of various essential oils in SiO2NCs using different surfactants that are templating agents, stabilize the emulsion droplets, and display antiseptic agents. Furthermore, we investigate the relationship between porosity and release profile of encapsulated payload. Finally, the release profile was tune by functionalizing the surface of the SiO2NCs with hyaluronic acid (HA). This study gives a rational basis for designing and synthesizing nanocapsules with superior loading and tunable release.

EXPERIMENTAL SECTION Materials. Tetraethyl orthosilicate (TEOS, Acros Organics, 98%), peppermint oil (PO, Sigma Aldrich), clove oil (CO, Sigma Aldrich), cinnamon oil (CnO, Sigma Aldrich), thyme oil (TO, Sigma

Aldrich),

cetyltrimethylammonium

chloride

(CTMA-Cl,

TCI,

95%),

didecyldimethylammonium chloride (DDAC, Sigma Aldrich), octenidine dihydrochloride (OCT, TCI, 98%), hexadecane (HD, Acros Organics, 99%), (3-aminopropyl)triethoxysilane (APTES, Sigma Aldrich, 99%), phosphate buffered saline (PBS, Sigma Aldrich), hyaluronic acid (HA, average molecular weight of 1420 kDa, SK Bioland), sodium periodate (NaIO4, Univar, 99.8 %), ethylene glycol (Sigma Aldrich), sodium hydroxide (NaOH, Carlo Erba, 98%), methyl orange (Labchem), and ethanol (EtOH, Carlo Erba, 99.9%) were used as received. Deionized water was used through all the experiments.


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Encapsulation of essential oils in silica nanocapsules (SiO2NCs). Silica nanocapsules (SiO2NCs) encapsulating essential oils were synthesized by interfacial miniemulsion.37 Briefly, the oil phase was prepared from a mixing of 2 g TEOS precursor, 125 mg HD, and 1 g essential oils (PO, TO, CO, and CnO). Then, 30 ml of 0.77% (w/w) aqueous solution of DDAC, CTMACl, or OCT was mixed with oil phase. The mixture was continuously stirred at 540 rpm for 10 min to form an emulsion. Subsequently, miniemulsions were prepared by ultrasonication (3 min, 70% amplitude in a pulsed regime with 30 second on and 10 second off, power output 89 Watts) under ice cooling. Then, the miniemulsion was transferred to a flask for carrying out sol-gel process while stirring for 20 h.

Scheme 1. Schematic representation of A) Hydrolysis and condensation of TEOS providing silica nanocapsules. B) The functionalization on the silica nanocapsules with 3-aminopropyl triethoxysilane (APTES) and subsequent reaction with oxidized-hyaluronic acid.


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Surface modification of the silica nanocapsules. A certain amount of APTES precursor in 1.5 mL of deionized water was added into 30 g of SiO2NCs with CO and DDAC surfactant (SiO2NCs/CO-D) dispersion. The mixture was further stirred at room temperature at 200 rpm 12 h to obtain amino-functionalized SiO2NCs (NH2-SiO2NCs), as shown in Scheme 1. Oxidation of hyaluronic acid. 1 g of hyaluronic acid (HA, 1,420,000 gmol-1) was dissolved in 99 g of deionized water at room temperature. Then, 30 ml of a 1.7w/v% sodium periodate (NaIO4) in deionized water was gently added to the HA solution under stirring at 540 rpm. The molar ratio NaIO4:HA was 0.5:1. The oxidation reaction proceeded in a dark environment for 3 h at room temperature. The reaction was stopped by adding 1 ml of ethylene glycol. The product was then dialyzed (MWCO = 6000-8000 gmol-1) for 3 days against deionized water. The deionized water was changed two times per day. The yield of oxi-HA was 77% ± 1. The oxi-HA was freeze-dried before measuring the number of aldehydes groups in HA by titration with 0.1 M NaOH and 0.25 M NH2OHHCl-methyl orange as an indicator.38 The pH of solution was adjusted to 4 and the color of solution was changed from light orange to red orange. The calculation is shown in Equation S1, supporting information. The molar ratio of aldehyde in hyaluronic acid M was about 30% ± 2 (Equation S1). The formation of oxi-HA was also conﬁrmed by FTIR with the appearance of a new signal at 1733 cm-1 corresponding to the C=O stretching of aldehyde groups in oxi-HA (Figure. 3B2).39 Reaction of oxidized-HA (oxi-HA) with functionalized silica nanocapsules. A certain amount of oxi-HA (M = 30%) in 0.5 mL deionized water was added to 5 g of SiO2NCs dispersion modified with a certain amount of APTES (Table 2). The mixture was stirred at 540 rpm for 12 h to obtain SiO2NCs with HA (HA-SiO2NCs), as depicted in Scheme 1.


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Release of essential oils from SiO2NCs and HA-SiO2NCs. 1 g of SiO2NCs/PO-D, SiO2NCs/CO-D, SiO2NCs/CnO-D, or HA-SiO2NCs/CO-D, or 1 g of 33 % clove oil in 20wt% ETOH/PBS solution in a dialysis bag (MWCO = 3500 gmol-1) were placed 20 g of a 20wt% ETOH/PBS solution at ambient temperature. 1 g of the release medium was withdrawn at 0.062, 0.125, 0.25, 0.5, 1, 2, 4, 6, 12, 24, and 48 h and replaced with 1 g of the 20wt% ethanol/buffer medium. The absorption of the released essential oils was then measured by UV-Vis spectrophotometry. The calibrations curves for the different oils are shown in Figure S2. Characterization techniques. The hydrodynamic diameter Dh of the SiO2NCs was measured by dynamic light scattering (DLS, NanoPlus, Particulate systems) at a fixed scattering angle of 90°. The solid content of SiO2NCs was measured by gravimetry after freeze-drying the dispersion for 5 days and drying in an oven at 70 °C for 7 days. The morphologies of SiO2NCs were observed with a scanning electron microscope (SEM, JEOL, JSM-7610F, Tokyo, Japan) operating at 1.5 kV a transmission electron microscope (TEM, JEOL, JEM-ARM200F, Tokyo, Japan). The samples for SEM and TEM were prepared by drop-casting of diluted SiO2NCs dispersions on silicon wafer and copper grid, respectively. The encapsulation efficiency (EE) and loading capacity (LC) of the SiO2NCs were determined by UV-Vis spectroscopy (PerkinElmer, Lambda 25). The samples were centrifuged at 20,000 rpm for 30 min. Then, the supernatants of the samples were collected and adjusted with EtOH to obtain a water:ethanol weight ratio of 1:1. The content of antibacterial agents was examined by measuring the supernatant and comparing it to calibration curves obtained from UV-Vis spectroscopy (Figure S2). The EE corresponds to the amount of encapsulated oil compared to the initial amount of oil before emulsification while the loading capacity LC is calculated as the weight of encapsulated oil compared to the total weight of nanocapsules. The surface area and pore size distribution of


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SiO2NCs were measured with BET surface analyzer (MicrotracBEL, BELSORP-mini II). The measurements were performed under nitrogen atmosphere and maintained at a temperature range around 63-77 K. Prior to measurements, the samples were calcinated at 400 °C for 5 h with a rate of 4 °C min-1 and degassed in vacuum at 200 °C for 12 h. The N2 isotherms were used to determine the specific surface areas from the standard BET equation at the relative pressure (p/p0). Pore sizes of the samples were obtained from the N2 adsorption curve by the Barret– Joyner–Halenda (BJH) method with the corrected Kelvin equation. The interaction of HA with functionalized SiO2NCs (NH2-SiO2NCs) was investigated by Fourier transform infrared spectroscopy (FTIR, PerkinElmer Frontier FT-IR, Universal-ATR) and X-ray photoelectron spectroscopy (XPS, JEOL, JPS-9010MC).

RESULTS AND DISCUSSION Encapsulation of essential oils in silica nanocapsules (SiO2NCs). Silica nanocapsules containing different essential oils were synthesized in miniemulsion by hydrolysis and condensation of tetraethyl orthosilicate (TEOS) as shown in Figure 1A. Aqueous solutions of the cationic surfactants cetyltrimethylammonium chloride (CTMA-Cl), didecyldimethylammonium chloride (DDAC), or octenidine dihydrochloride (OCT) were prepared as continuous phases. The antibacterial agents such as peppermint oil (PO), thyme oil (TO), clove oil (CO), and cinnamon oil (CnO) were mixed with tetraethyl orthosilicate and hexadecane to form the dispersed phase and were then converted to miniemulsion droplets by ultrasonication. The resulting miniemulsion droplets were stirred for 20 h to form silica nanocapsules (SiO2NCs). The essential oils were hence in situ encapsulated in the core of SiO2NCs. The encapsulated liquid cores (PO,


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TO, CO, and CnO) and surfactants (CTMA-Cl, DDAC, and OCT) were varied to study the size of SiO2NCs and encapsulation efficiency of the oils (Table 1). Miniemulsions could not be formed with the combination OCT and three oils (TO, CO, and CnO) due to the precipitation of OCT when the OCT aqueous solution was mixed with TO, CO, and CnO. Thus, only nanocapsules with peppermint oil (PO) could be formed when OCT was used to stabilize the colloids. The hydrodynamic diameter of SiO2NCs encapsulating essential oils was ranging from 65 to ~150 nm (see Table 1). The average diameters of SiO2NCs/PO stabilized by CTMA-Cl and DDAC as surfactant (65-75 nm) was smaller compared with SiO2NCs/CO (77-97 nm), SiO2NCs/CnO (107-120 nm), and SiO2NCs/TO (87-146 nm), respectively, with the same surfactant. Although the particle size of SiO2NCs/CO-C could be immediately measured after the reaction (~77 nm), gelation was observed after 2 days. The gelation was attributed to high interfacial-tension between CO and the aqueous phase (see Table S1), leading to an unstable suspension. Colloidal particles readily aggregate until they interlink to form a gel.40 Generally, the types of oil and surfactant did not influence the solid content of the dispersions, which was in the range of 2-3% (Table 1). In order to quantify the encapsulation of oils in SiO2NCs, the encapsulation efficiency (EE) and the loading capacity (LC) were calculated from UV spectroscopy measurements (Figure 1). More than 80% encapsulation efficiency (EE) was obtained for SiO2NCs with peppermint oil, clove oil, and cinnamon oil (Figure 1B). However, lower EE of thyme oil (40-62%) was obtained due to the higher water solubility of p-cymene (23.4 mg/ml),41 a main component of thyme oil, compared to other oils (see Table S2) causing repelling TO from a core of SiO2NCs (Figure 1B). The loading capacity of the nanocapsules, which is expressed as the weight fraction of essential oils in nanocapsules was found to be more


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than 75% for all cases except with thyme oil (TO). Assuming that the density of the silica shell is the same as bulk silica,42 this means that the loading capacity expressed as volume fraction is approximately 71%, which is exceptionally high.

Figure 1. (A) Schematic illustration of silica nanocapsules (SiO2NCs) encapsulating peppermint oil (PO), thyme oil (TO), cinnamon oil (CnO), and clove oil (CO). Essential oils are complex mixtures of various chemicals. Therefore, only the chemical structures and the relative content (%) of the major components are shown. The SiO2NCs were prepared with different surfactants such as cetyltrimethylammonium chloride (CTMA-Cl), didecyldimethylammonium chloride (DDAC), and octenidine dihydrochloride (OCT). (B) Encapsulation efficiency (EE) and loading capacity (LC) of SiO2NCs with different essential oils and surfactants.


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Table 1. Hydrodynamic diameters, solid contents, encapsulation efficiency, surface area, and mean pore diameter of silica nanocapsules (SiO2NCs) containing essential oils stabilized by different surfactants.

Entry SiO2NCs/PO-C SiO2NCs/PO-D SiO2NCs/PO-O SiO2NCs/TO-C SiO2NCs/TO-D SiO2NCs/CO-C SiO2NCs/CO-D SiO2NCs/CnO-C SiO2NCs/CnO-D

Oil

PO

TO CO CnO

Surfactant

Dh [nm]

PDI

Solid content [%]

CTMA-Cl DDAC OCT CTMA-Cl DDAC CTMA-Cl DDAC CTMA-Cl DDAC

65 75 116 87 146 77 97 120 107

0.163 0.192 0.163 0.165 0.203 0.204 0.142 0.202 0.207

2.4 2.1 2.8 2.3 2.8 Gelation 2.7 3.0 3.1

Encapsulation efficiency [%] 87 92 98 62 40 Gelation 93 83 89

Surface area [m2 g-1]

Mean pore diameter [nm]

315 517 448

19 10 23

520 246 Gelation 460 637 513

11 6 Gelation 4 8 10

Morphology and surface area of the nanocapsules. The morphology of SiO2NCs encapsulating essential oils was observed by SEM (Figure 2). The SEM images showed a regular distribution of size and the nanocapsules displayed a spherical shape. Holes of nanocapsules were observed due to the evaporation of the oils during the preparation of the SEM samples and in the vacuum chamber of the microscope. Similar observations were already reported for polymer and inorganic nanocapsules encapsulating various liquids.27, 43 Well-defined core-shell structures with a thin silica shell (6-8 nm) were observed by TEM images as shown in Figure 2B and 2E. The Brunauer-Emmett-Teller (BET) method was used to measure and calculate the surface area of the SiO2NCs (Table 1). For SiO2NCs with CTMA-Cl surfactant, the surface area decreased following the order SiO2NCs/CnO ˃ SiO2NCs/TO ˃ SiO2NCs/PO. The highest surface area (637 m2g-1) was measured for SiO2NCs/CnO. Moreover, the mean pore diameter was calculated by the Barett-Joyner-Halenda method. The SiO2NCs/PO, SiO2NCs/TO, and


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SiO2NCs/CnO samples stabilized by CTMA-Cl exhibited a mean pore diameter of 19, 11 and 8 nm, respectively.When the surfactant was changed to DDAC, the surface area of SiO2NCs with different oils was ordered as follows SiO2NCs/CnO > SiO2NCs/PO ˃ SiO2NCs/CO ˃ SiO2NCs/TO. The pore size of SiO2NCs/PO-D (10 nm) and SiO2NCs/CnO-D (10 nm) were larger than SiO2NCs/TO-D (11 6 nm), and SiO2NCs/CO-D (4 nm). Noticeably, the silica nanocapsules with the different type of oils and surfactants therefore influenced surface area and pore diameter. The effect of the chemical structure of the surfactant was previously reported for other silica materials. For example, Sharma et al. synthesized silica monoliths by varying ionic and chain length of surfactant and investigated the effect on the pore size and surface area.44 With cationic surfactant such as cetyl trimethylammonium bromide (CTAB), the positive charge neutralized the negatively charged silica particles and generated chain-chain interactions by surfactant adsorption, resulting in an increment of pore size. 44 In comparison, the anionic sodium dodecyl sulfate formed negatively charged micelles and induced strong electrostatic repulsion with negatively charged silica particles, resulting in a decrease of pore size and surface area. 44 Moreover, due to increasing chain length of CTAB, the surface area of silica monoliths increased because of a decrease of the textural mesopore size. In summary, the type of surfactant such as ionic surfactants and chain length had strong effect on the surface area and mean pore diameter. In order to investigate the effect of oils on the pore structure, the hysteresis loop formed by N2 adsorption/desorption isotherms of SiO2NCs with different oils was examined. N2 isotherms of SiO2NCs with all oils were type IV(a) (Figure 2C and 2F), indicating that the prepared SiO2NCs were mesoporous structure.45 The form of the hysteresis loops also suggested different shapes for the pores of SiO2NCs. The hysteresis loop of SiO2NCs/PO indicated a combination of slit-shaped pores and ink-bottle-shaped pores, while the hysteresis loop of SiO2NCs/CnO represented an


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ink-bottom shape pore. As shown in Figure S3A, the SiO2NCs/CO displayed narrow slit-like pore in the structure. Moreover, cylindrical or tubular type pores were found when TO was encapsulated in SiO2NCs (Figure S3B) which indicated that the pore shape was influenced by the nature of encapsulated oil.

Figure 2. (A, D) SEM micrographs, (B, E) TEM micrographs, and (C, F) N2 adsorption/desorption isotherms of SiO2NCs/PO-D and SiO2NCs/CnO-D. Functionalization of silica nanocapsules with hyaluronic acid. Due to the fact that the SiO2NCs/CO-D displayed the highest EE and LC, we therefore selected these nanocapsules to be further functionalized with amine groups by grafting APTES on their surface in dispersion. TEOS:APTES molar ratios were varied from 100:0.16 to 100:2.5 (Table 2). The average hydrodynamic diameter of SiO2NCs/CO-D increased from 97 to 121 when 0.16 mol% of APTES was added in the system. The hydrodynamic diameter of modified-SiO2NCs after addition of APTES further increased from 121 to 154 nm when the TEOS:APTES ratio increased from


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100:0.16 to 100:0.63. Upon addition of larger amount of APTES, sedimentation of the SiO2NCs occurred. This phenomenon was attributed to self-condensation of hydrolyzed silane groups that can occur after a certain threshold.46 Therefore, NH2-SiO2NCs_3 was selected for further modification with oxidized HA (oxi-HA) via the formation of a Schiff base reaction between the aldehyde groups of oxi-HA and the amine groups of APTES. The SiO2NCs functionalized with amines were characterized by infrared spectroscopy (Figure 3C). The samples based on SiO2NCs/CO-D displayed the typical vibration bands of silica materials such as the Si-O-Si asymmetric stretching band at 1095 cm-1 and Si-O symmetric stretching band at 805 cm-1.47-48 The band at 1630 cm-1 corresponded to the bending vibration of O-H bond.47 The free silanol (SiOH) groups located on the surface of the capsules can be normally detected at around 955 cm-1.47 Moreover, the NH2-SiO2NCs/CO-D displayed a sharp peak at 1655 cm-1, which is the

δ

vibration of -NH2,49 confirming a successful functionalization of the surface of the SiO2NCs/COD by APTES (Figure 3C4). Figure 3D1 shows a detailed C1s X-ray photoelectron spectroscopy (XPS) analysis of SiO2NCs/CO-D. C-C signal at 284.6 eV were observed, as well as a signal at 285.8 eV that was assigned to C-N in DDAC surfactant as previously reported for C-C and C-N binding energies from CTAB surfactant on Au nanospheres.50 After the reaction with APTES, a signal for NH2-SiO2NCs/CO-D_3 appeared at 282.3 eV corresponded to C-Si bond of APTES, similar to a value reported for the binding energy of C-Si bond from APTES coated on silicon and silicon carbide (Figure 3D2),51

indicating a successful grafting of APTES on the

nanocapsules. After functionalization of the silica nanocapsules with -NH2 groups, hyaluronic acid bearing aldehyde groups (oxi-HA) was added to form a HA shell via the formation of Schiff bases. Hyaluronic acid (HA) is a ubiquitous extracellular matrix component synthesized by fibroblasts


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HA was conjugated to silica nanoparticles and was found to enhance cell viability of normal cells compared to non-modified nanoparticles even at high concentration.52-54 HA has been reported as specific targeting ligand for the specific cell surface receptor Cluster determinant 44 (CD44),55 which is overexpressed in liver,56 ovarian,54, 57 breast,58-61 and colon tumor cancers.53 Consequently, nanocarriers modified with HA displayed superior cellular uptake, cancer cell death, and viability for human normal cell. Furthermore, HA could be degraded by hyaluronidase so that payloads were released upon activation by this enzyme.53-54 The nanocapsules remained a spherical shape as shown by SEM (Figure 3). The volume ratio of oxi-HA to SiO2NCs/CO-D was varied from 0.5 to 1.5 (Table 2). For V oxi-HA/VSiO2NCs = 0.5, the hydrodynamic diameter the nanocapsules increased from 154 nm before coating to 163 nm after coating with oxi-HA. TEM observations also showed an average increase of the shell thickness of 8 nm (5 to 13 nm) when the SiO2NCs was coated with oxi-HA (Figure 3B). These values were close to the theoretical shell thickness (5 nm) calculated based on the concentration of nanoparticles and the known amount of oxi-HA used for the reaction (see Equation S2). When the volume ratio of HA was increased to 1 and 1.5, the hydrodynamic diameter of HA-SiO2NCs increased (Table 2). Furthermore, aggregations were observed in the sample of HA-SiO2NCs/CO-D_3 (1.5 volume ratio). The destabilization was also confirmed by zeta potential measurements (Table S3). The value of zeta potential with the lowest amount of oxi-HA was 39 mV, close to the zeta potential of the silica nanocapsules functionalized with -NH2 groups (41 mV). However, as expected, the zeta potential decreased to 26 mV when the amount of oxi-HA increased (HA-SiO2NCs/COD_3, (volume ratio of HA to SiO2NCs at 1.5). Coating the nanocapsules with an excess of HA hence led to reduction of the electrostatic stabilization of the silica nanocapsules by the cationic surfactant. FT-IR spectra evidenced a reduction of the band at 1655 cm-1 associated to -NH2


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groups (Figure 3C5) due to the reaction of the amine groups with oxi-HA. Moreover, XPS measurements of the HA-SiO2NCs evidenced the presence of C=N imine bonds detected at 287.0 eV. This value matches the C=N binding energy of alkyl Schiff base from APTES functionalized silicon wafer described in another report,62 confirming the successful formation of Schiff bases. Additionally, we observed a signal at 286.2 eV related to the C-O bond of HA (Figure 3D3). The binding energies of C=N and C-O did not appear on non-coated SiO2NCs. Thus, the presence of XPS signals of C=N and C-O supports the fact that oxi-HA was grafted on the SiO2NCs.

Table 2. Characteristics and hydrodynamic diameter of amine-functionalized SiO2NCs) NH2SiO2NCs (and HA coated on the SiO2NCs encapsulating essential oils.

Entry NH2-SiO2NCs/CO-D_1 NH2-SiO2NCs/CO-D _2 NH2-SiO2NCs/CO-D _3 NH2-SiO2NCs/CO-D _4 NH2-SiO2NCs/CO-D _5 HA-SiO2NCs/CO-D _1 HA-SiO2NCs/CO-D _2 HA-SiO2NCs/CO-D _3

Mol% APTES and oxi-HA compared to TEOS APTES Oxi-HA 0.16 0 0.31 0 0.63 0 1.25 0 2.50 0 0.63 2.0 0.63 4.0 0.63 8.0

Volume ratio of HA to SiO2NCs

Dh [nm]

PDI

0 0 0 0 0 0.5 1 1.5

121 134 154 > 1 µm > 1 µm 163 280 685

0.163 0.157 0.129 0.556 1.137 0.178 0.181 0.363


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Figure 3. (A) SEM micrograph and (B) TEM micrograph of HA-SiO2NCs_1. (C) FT-IR spectra of SiO2NCs and HA-SiO2NCs/CO-D. (1) hyaluronic acid (HA), (2) oxidized-hyaluronic acid (oxi-HA), (3) SiO2NCs/CO-D, (4) NH2-SiO2NCs/CO-D, and (5) HA-SiO2NCs/CO-D. (D) C1s XPS spectra of SiO2NCs and modified-SiO2NCs: (1) SiO2NCs/CO-D, (2) NH2-SiO2NCs/CO-D, and (3) HA-SiO2NCs/CO-D. In vitro release of essential oils from silica nanocapsules. The release profiles of peppermint oil (PO), clove oil (CO), and cinnamon oil (CnO) from SiO2NCs were monitored by UV-Vis absorption at 201, 281, and 290 nm, respectively. 90% PO and 70% CO were released from SiO2NCs within 12 h (Figure S4). On the contrary, SiO2NCs/CnO-D released less than 10% CnO within 48 h. This fact could be attributed to strong hydrogen bonds between cinnamaldehyde and the silanol groups of the silica nanocontainers.63 SiO2NCs/PO-D displayed the largest release, which can be related with the large pore diameter of 10 nm.


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Because SiO2NCs/CO-D displayed the highest EE, they were selected as a model to observe the release behavior of clove oil (CO) from non-functionalized and functionalized SiO2NCs. As shown in Figure 4, ~70% CO was released from SiO2NCs/CO-D after 12 h. In order to compare the efficiency of HA coating on the release from nanocapsules, different amounts of oxi-HA were reacted with the amine-functionalized SiO2NCs. Interestingly, the HA coating, especially at higher amount in the sample of HA-SiO2NCs/CO-D, significantly influenced the release profile of CO from the nanocapsules. By increasing the amount of HA, the initial burst release was also reduced and a sustained release was observed until 48 h. Both HA-SiO2NCs/CO-D_2 and HASiO2NCs/CO-D_3 presented ~60% release during 24 h. The presence of HA on the shell of SiO2NCs can reduce the release rate of CO from the SiO2NCs. Indeed, non-functionalized silica nanocapsules (SiO2NCs) displayed an initial burst release of around 50% in the first 5 h. At low concentrations of HA, similar results were observed. However, the presence of HA at higher concentration (100:8 mol ratio between TEOS and Oxi-HA) reduced significantly this burst release. Indeed, 10 h, i.e. twice than without HA, were required to release 50% CO. According to TEM image, the thickness of HA shell surrounding the surface of SiO2NCs is  8 nm. Other authors have shown that 7 nm shell thickness on silica mesoporous nanoparticles was already enough to improve the biocompatibility of the material.61


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Figure 4. Release profiles of clove oil of free CO compared with SiO2NCs/CO-D and HASiO2NCs/CO-D coated with different amounts of HA at pH 7.4 and 37 °C.

CONCLUSIONS Delivery nanocarriers for antibacterial agents were synthesized by a sol-gel process at the interface of miniemulsion droplets of essential oils. The droplets, and subsequently the mesoporous silica nanocontainers, were stabilized by antiseptic surfactants. The nanocontainers exhibited a clear core-shell morphology with a pore size and controlled by the type of encapsulated oil and surfactant. The nanocontainers could be successfully functionalized and further grafting with hyaluronic acid as demonstrated by FT-IR spectroscopy and XPS


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measurements. The hyaluronic acid shell provided a prolongated release of essential oils from the silica nanocontainers. ASSOCIATED CONTENT Supporting Information Scheme for the oxidation of hyaluronic acid, calculation of the amount of aldehyde groups in oxidized hyaluronic acid, calibration curves for determining the concentrations of essential oils by UV-spectroscopy, surface and interfacial tensions, tabulated solubility of essential oils in water, pore size distribution and isotherms, calculation of the theoretical thickness of hyaluronic acid shell, zeta potential of the nanocapsules, and release profiles of non-encapsulated oils. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no conflict of interest. ACKNOWLEDGMENTS This work was financially supported by the Vidyasirimedhi Institute of Science and Technology. Instrumental support from the Frontier Research Center (FRC) at VISTEC is also acknowledged.


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Encapsulation and release of essential oils in functional silica

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