Development of Intracanal Formulation Containing Silver Nanoparticles

silver nanoparticles to be used in the development of a root intracanal formulation. Silver nanoparticles (AgNPs) were obtained by reduction of silver nitrate with sodium borohydride and characterized by UV-Visible spectrophotometry, scanning electron microscopy (SEM) and dynamic light scattering (DLS). The antimicrobial activity of nanoparticle formulation was evaluated by determinations of the minimum inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) against different bacterial species by the microdilution method, according to recommendations of the Clinical and Laboratory Standards Institute (CLSI). Three potential vehicles, hydroxyethylcellulose, Carbomer and polyethylene glycol were tested as carriers for formulations containing AgNPs. The efficiency of the synthesis method chosen to produce AgNPs was demonstrated by four characterization techniques. The nanoparticles showed antibacterial activity against all species tested. Incorporation of AgNPs into all experimental vehicles produced stable formulations but the one in hydroxyethylcellulose presented better physical proprieties. The results indicate that silver nanoparticles are potential antiseptic agents to be used in root canals and incorporation in adequate vehicles may favor a broader application. Development of Intracanal Formulation Containing Silver Nanoparticles


Introduction
Physicochemical and biologic properties of nanomaterials, especially the ones containing silver, attracted investigators in recent decades (1).Furthermore, silver nanoparticles show advantageous properties in biocompatibility and antimicrobial activity when compared to the salt precursors (2).
Small quantities of silver ions adsorbed on the surface of silver nanoparticles (AgNPs) and released in the presence of water and oxygen are able to combine with sulfur, nitrogen or oxygen of bacterial organic compounds causing damage to the cell wall and osmotic unbalance (3).Complex formation between silver nanoparticles and proteins may cause bacterial death by compromising cell metabolism (4).Interaction of the particles with bacterial DNA may result in bacteriostatic action, preventing cell reproduction (3).Antibacterial properties of AgNPs can be shown against a large spectrum of microorganisms including gram-positive and gram-negative bacteria (5), fungi (6) and viruses (7).
Endodontic infections are mainly caused by bacterial etiology.Therefore, this study investigated the preparation, characterization and evaluation of the antimicrobial properties of silver nanoparticles aiming to develop root intracanal formulations for endodontic therapy.

Synthesis of Silver Nanoparticles
To prepare the suspension of AgNPs, a volume of 10 mL silver nitrate solution with 4.0x10 -4 mol/L was added dropwise from a glass burette to a flat-bottom flask (immersed in an ice bath) containing 10 mL of 1.0x10 -2 mol/L sodium citrate, used as a stabilizer, and 10 mL of 2.5x10 -2 mol/L sodium borohydride.The reaction was carried out under constant stirring and protected from light.After the addition of silver nitrate solution was finished, the mixture was then vigorously stirred during 15 min.Furthermore, a higher concentration of reducing agent, 1×10 -3 mol/L sodium borohydride and a slower rate of silver nitrate addition were also evaluated.

Characterization
The silver nanoparticles were characterized by the absorption spectrum in visible light (300 to 700 nm) Silver nanoparticles in endodontics obtained in a Cary IE spectrophotometer (Varian, Melbourne, Australia).Zeta potential and particle size were determined by using a dynamic light scattering instrument (Zetasizer Nano ZS, Malvern Instruments Ltd.Worcestershire, England).Size distribution and shape of particles were confirmed by scanning electron microscopy (SEM) in magnifications of 20,000×, 50,000×, 80,000× and 120,000×, after sample drying at room temperature and moisture.

Preparation of Intracanal Formulations
T h r e e d i f f e r e n t e x c i p i e n t s ( p o l y m e r i c hydroxyethylcellulose gel, carbomer polymer gel and polyethylene glycol) were tested to formulate the preparation containing 75% (w/w) silver nanoparticle suspension for root intracanal administration.Polymeric hydroxyethylcellulose gel (Cellosize QP100®) were tested in several concentrations (1.0 to 5.0%), associated or not to polyoxyethylene (20) sorbitan monolaurate (Tween 20).Several concentrations of carbomer polymer gel (Carbopol Ultrez 10®) (0.25 to 1.0%) neutralized with triethanolamine in sufficient quantity to reach a pH of 6.0 to 7.0 were also tested as a vehicle for the silver nanoparticles.A semi-solid formulation based on polyethylene glycol was prepared associating Carbowax® 400, 4000 and 6000 in different proportions to 15% Tween 80.The formulations were evaluated by their macroscopic aspect, considering homogeneity and fluidity.
Since the best results were attained by using the formulation containing silver nanoparticles in 1.5% hydroxyethylcellulose gel, this vehicle was selected to verify the antimicrobial activity of the nanoparticles.

Antimicrobial Activity
The minimum inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of formulation containing silver nanoparticles in 1.5% hydroxyethylcellulose gel were determined in triplicates by the microdilution broth method in 96-well microplates as recommended by the CLSI (9).Briefly, the formulation was serially diluted with Muller Hinton culture broth to silver concentrations ranging from 7,200 to 14 ng/mL and 100 mL were added to the micro wells.The original suspension was diluted with Muller Hinton medium starting with a 1:1 dilution.
The -80 °C stored standard bacterial strains were cultured in Petri dishes containing Muller Hinton Agar (Difco, Detroit, MI, USA) for E. coli, E. faecalis, P. aeruginosa, and S. aureus, and in Muller Hinton with 5% sheep blood for S.mutans.After incubation of plates for 24 h at 37 °C, isolated colonies were suspended in saline up to a turbidity equivalent to 0.5 in the McFarland scale determined in a Densimat densitometer (Bio Mèrieux, Marcy l´Etoile, France).The 0.5 value in the McFarland scale is equivalent to about 1.5×10 8 colony forming units (CFU) per mL.Bacterial suspensions were further diluted (1:10) with Muller Hinton broth (final concentration, 1.5×10 7 CFU/mL) and 5 µL aliquots added to microplate wells containing the formulation dilutions, finally leading to about 10 5 or 10 4 CFU/well of each tested bacterial strain.
All tests included wells containing only bacterial suspensions, as a control of growth, other ones containing only each culture broth (negative control) to ensure its sterility.As positive control, a 0.12% chlorhexidine gluconate solution (CHD) was used against all studied strains.
After 24 h incubation at 37 °C, plates were analyzed to determine MICs against the different strains by considering turbidity as an indication of growth and lack of it as inhibition of growth.MICs were determined as the lowest concentration of test solutions corresponding to the well without bacterial growth (100% inhibition).To determine the MBC, 10 µL aliquots were taken from selected wells that did not show visible bacterial growth and spread on plates containing Agar Muller Hinton medium and incubated for 24 hours at 37 °C.Visual observation indicated presence or absence of growth and MBC was considered as smallest concentration of test solution showing no growth.

Synthesis of Nanoparticles and Characterization
The yellow suspension produced by the redox reaction between silver nitrate and sodium borohydride, stabilized by sodium citrate, is characteristic of silver nanoparticles.
Scanning spectrophotometry in the visible range of 300 to 700 nm showed a peak absorbance around 400 nm suggesting the presence of silver nanoparticles (Fig. 1A).
Characterization by dynamic light scattering indicated nanoparticles varying in size between 1 and 100 nm (Fig. 1B) and the analysis also allowed determining the zeta potential of particles, which was -33mV (Fig. 1C).Silver nanoparticle size depending on the concentration of the reducing agent, sodium borohydride, and on the rate of silver nitrate addition (oxidant agent).A higher concentration of sodium borohydride with 1×10 -3 mol/L and a slower addition rate of silver nitrate were also evaluated.However, the obtained results were not satisfactory.Thus, the addition rate of the oxidant agent was maintained higher and the concentration of reducing agent was standardized in 2.5×10 -2 mol/L.SEM showed the presence of round particles and confirmed sizes according with the results obtained by dynamic light spreading (Fig. 2).

Intracanal Formulations
Suspension of silver nanoparticles incorporated into vehicles of different hydroxyethylcellulose concentrations, 1 to 5%, did not show visible physical alterations after 24 h.When the vehicle employed was carbomer gel, the finished product did not show physical alterations after 24 h, but it did not have adequate fluidity even when the lowest carbomer concentration was used (0.25%).Unstable formulations were obtained when 75% (w/w) silver nanoparticles suspension was added to polyethylene glycol vehicles due to the deficient homogenization obtained.
As previously stated, the formulation containing silver nanoparticles in 1.5% hydroxyethylcellulose gel was considered the best one due to its good physical properties.

Antimicrobial Activity
MIC determinations showed that formulation prepared in 1.5% hydroxyethylcellulose gel containing AgNP have bacteriostatic activity against all species tested.MIC values were very promising ranging from 1,800 to 7,100 ng/mL, depending on the bacterial species.Chlorhexidine gluconate solution (CHD), used as positive control, presented a MIC value < 1,170 ng/mL.
Tests for minimal bactericidal concentration (MBC) indicated that bactericidal activity could only be detected in more concentrated preparations ranging from 3,600 ng/ mL (for E. coli, P. aeruginosa, S. mutans) to 7,100 ng/mL (for E. faecalis).The formulation with silver concentration of 3,600 ng/mL was able to inhibit the growth of S. aureus, however, it was not able to present bactericidal activity against this species.Both MIC and MBC results are shown in Table 1.

Discussion
Silver nanoparticles were synthesized by reduction of   silver nitrate by borohydride in sodium citrate medium (8), which was as a stabilizer agent (9) at low temperature.According to Gulrajani et al. (10) smaller nanoparticles are obtained when synthesized at temperatures around 5 °C that favor enucleation and prevent agglomeration of the particles with lower kinetic energy (5).
The presence of nanoparticles in the suspension was confirmed by the characteristic yellow color as suggested by Shukla et al. (5) and by scanning spectrophotometry between 300 and 700 nm that produced an spectrum with a 400 nm absorbance maximum (11,12).According to Pal et al. (13), nanoparticle morphologies are directly related to absorption spectra; round particles produce spectra with a single absorbance peak.Spectra with two peaks indicate disc-shaped particles and triangular ones when three or more peaks are detected (13).
Light dynamic scattering indicated the presence of AgNPs with average sizes between 1 and 100 nm (14) and allowed determination of the zeta potential, a property directly related to particle stability (13,14).Zeta potential values characterize particle surface charges and thus, electrical potential, which is influenced by particle composition and the dispersion medium (15).The particles in this study had a zeta potential of -33 mV, a value considered ideal, according to Mohanraj and Chen (15) and Moraes et al. (16).According to these authors, zeta potentials above +30 or below -30 mV demonstrate nanoparticle stability.The SEM analysis confirmed particle spherical shapes and size distribution between 1 and 100 nm as determined by light dynamic scattering.
Lok et al. (17) suggested that antimicrobial activity of silver nanoparticles is dependent on the ability to cross the microorganism cell walls.Particle sizes ranging from 1 to10 nm in diameter have higher penetration potential and bactericidal power (18).The method described in this report produced particles in this range justifying the results that confirm other observations for S. mutans (6,19), E. coli, P. aeruginosa (20), except S. aureus (21).Against this last species, silver nanoparticles had only bacteriostatic activity.It should be noted that silver particles showed bacteriostatic and bactericidal activity against E. faecalis (22) while, calcium hydroxide a commonly used agent in endodontic therapy only acts as a bacteriostatic agent against this species at pH values higher than 12.5 (23).
To be used for antisepsis in root canal therapy longer than one session, silver nanoparticles must be able to penetrate dental tubules and the lateral channel systems and accessories.In this way, an adequate fluidity is necessary to facilitate penetration and draining of the radicular canal (24).Yamazaki et al. (25) studied dentin permeability and efficiency of Endo-PTC (which is utilized as a lubricant in endodontic instrumentation) in semi-solid pharmaceutical forms with different excipients as hydroxyethylcellulose or carbomer polymeric gels and associations of polyethylene glycol and surfactants.The results did not show differences between the evaluated preparations in terms of penetration capacity into dentin tubules or efficiency.Thus, similar excipients were tested as vehicles to the silver nanoparticles obtained in this study.The formulation containing silver nanoparticles in 1.5% hydroxyethylcellulose was selected as best due to good physical properties such as homogeneity and fluidity, which certainly it would make easier the gel application into canal.
It may be concluded that synthesis of silver nanoparticles obtained by reduction of silver nitrate, as described, was effective and confirmed by different characterization methods.Antimicrobial properties of the particles were detected against E. coli, E. faecalis, P. aeruginosa, S. aureus and S. mutans.The root intracanal formulation developed with hydroxyethylcellulose polymer gel containing the silver nanoparticles showed adequate homogeneity and fluidity for the use proposed.The study suggests an innovative use of silver nanoparticles as an endodontic antiseptic agent.However, further studies are still necessary to determine in vitro and in vivo cytotoxicity, biocompatibility and pharmacokinetics for viable utilization.

Figure 1 .
Figure 1.Characterization of silver nanoparticle suspensions.A: Absorption spectrum determined in the region of visible light (300 to 700 nm) showing peak absorbance at 400 nm.B: Particle size distribution determined by dynamic light spreading.C: Zeta potential determined by dynamic light spreading.

Figure 2 .
Figure 2. SEM micrograph of a suspension of silver nanoparticles showing particle size in nanometers.

Table 1 .
Minimum inhibitory concentration (MIC) and minimal bactericide activity (MBC) of silver nanoparticle suspensions against different standard bacterial species Note: +, -presence and absence of activity, respectively.;*not determined in dilutions with negative MICsSilver nanoparticles in endodontics