Effect of Silver Nanoparticles Synthesized with NPsAg Ethylene Glycol (C2H6O2) on Brown Decay and White Decay Fungi of Nine Tropical Woods

Copyright © 2017 American Scientific Publishers All rights reserved

Printed in the United States of America

Article

Journal of

Nanoscience and Nanotechnology

Effect of Silver Nanoparticles Synthesized with

NPsAg-Ethylene Glycol (C

₂

H

₆

O

₂

) on Brown Decay and

White Decay Fungi of Nine Tropical Woods

Róger Moya

_{, Ana Rodriguez-Zuñiga}

_{, Alexander Berrocal}

_{, and José Vega-Baudrit}

Nanotechnology applications have potential for improving decay resistance of wood under tropical conditions. In this work, nine commercial timbers from Costa Rica were treated with silver nanopar-ticles synthetized with NPsAg-ethylene glycol through pressure. White-rot (Trametes versicolor) and brown-rot (Lenzites acuta) fungi were tested. According to the results, the sizes of the synthetized silver nanoparticles were 40 to 100 nm. The retention achieved was of 16 to 112 mg of silver per kilogram of wood or 7.7 to 25.1 g of silver per cubic meter of wood. Specific gravity affected the retention inCordia alliodora,Gmelina arborea,Goethalsia meiantha,Tectona grandisandVochysia ferruginea. Loss of weight was less in wood treated with silver nanoparticles, its values ranging from 8% to 35% inL. acuta and 7% to 11% in T. versicolor. As for durability, the wood of the species treated with silver nanoparticles is classified as highly resistant toT. versicolorand moder-ately to modermoder-ately resistant toL. acuta. Moreover, the effect of retention of the nanoparticles was not significant for weight in all of the species. This parameter was positively affected inC. odorata,

E. cyclocarpum,G. arborea,T. grandisandV. ferruginea, although unaffected for other species.

Keywords:

1. INTRODUCTION

In recent years, approach to wood preservation has focused on developing environmentally friendly preser-vatives through improved formulations.1 _{The aim is to} develop preservatives that contain the least amount of active component, in order to reduce or avoid the health problems and chemical contamination associated with them while the wood product is being used and one it has fulfilled its useful life.2

Worldwide there is an intensive development of nan-otechnology applications in many areas, from improving the behavior of materials to medical purposes.3 _{As for} materials, the aim of the application of nanotechnology is to increase their mechanical and physical properties, dura-bility and improve the material’s response to chemicals.4 The aim of nanotechnology applications is using small quantities to increase the properties of the materials.5

Silver nanoparticles have gained much attention world-wide for various types of applications, including medical

∗_{Author to whom correspondence should be addressed.}

applications, because of its antibacterial activity.6 _Silver nanoparticles have also gained popularity in other areas, such as in plant biology and reinforcement of materials.7!8 Wood has also been researched to improve its properties. Silver nanoparticles have been applied to improve the pro-tection of the wood,9–12 _{its behavior against fire, improved} physical properties, densification of wood, properties of particleboard and dried-wood.5!13!14

Various methods for silver nanoparticle synthetizing are available (Tan and Cheong 2013) of those,15 _reduction methods are among the most important.16!17_For synthetiz-ing, an excess of ethylene glycol (C2H6O2" is used as reducing agent, polyvinylpyrrolidone (PVP) as a stabilizer to prevent agglomeration of the nanoparticles, and silver nitrate (AgNO3"as silver source.16

who synthetized silver nanoparticles by means of reduction with sodium borohydride and also tested its effectiveness in three commercial species (Acacia mangium, Cedrela

odorataandVochysia guatemalensis) from Costa Rica.

This study reports on the synthetizing of silver nanopar-ticles (by reduction with an excess of ethylene glycol

(C₂H₆O₂", as reducing agent, polyvinylpyrrolidone (PVP)

as stabilizing and silver nitrate (AgNO3"as silver source and their effects on the wood’s absorption capacity, retention, and resistance to the attack of two types of fungi (brown-rotL. acuta and white-rot T. versicolor) in nine tropical species (Acacia mangium,Cedrela odorata,

Cordia alliodora, Enterolobium cyclocarpum, Gmelina

arborea,Goethalsia meiantha,Ochroma pyramidale,

Tec-tona grandisandVochysia ferruginea).

2. MATERIALS AND METHODS

Three components were utilized for synthesizing the silver nanoparticles: silver nitrate (AgNO3" as a source of the reduced metal, supplied by MERCK (pureness 99,9+%), ethylene glycol (C2H6O2" as reducing agent, supplied by Baker (pureness 99.9%) and polyvinylpyrrolidone (PVP) as stabilizing agent supplied by Magnacol Ldt. Wood sam-ples from 9 tropical species in Costa Rica were used:

A. mangium, C. odorata, C. alliodora, E. cyclocarpum,

G. arborea, G. meiantha, O. pyramidale, T. grandis and

V. ferruginea. These are woods traditionally employed in

Costa Rica to manufacture doors, wood-based products and products used in engineering, which requires intensive use of adhesives. A. mangium, G. arborea and T. gran-dis are grown in forest plantations, while G. meiantha,

O. pyramidale and V. ferruginea come from secondary

forests and C. odorata, C. alliodora andE. cyclocarpum

are found in agroforestry systems. The wood was obtained in various sites of commercialization of lumber. The fungal test was performed with two types of fungi:T. versicolor

(white-rot fungus) andL. acuta(brown-rot fungus).

2.2. Synthesis of the Silver Nanoparticles and Determination of the Concentration

The silver nanoparticles were synthesized using the one step procedure proposed by Kheybari et al.,16 _{which is} based on a reduction reaction, where an excess of ethylene glycol (C2H6O2 as a reducing agent, polyvinylpyrroli-done (PVP) as stabilizing agent to prevent agglomera-tion of the nanoparticles and silver nitrate (AgNO3"as a source of silver, were used. The concentration of synthe-sized silver nanoparticles was prepared at a concentration of 50 ppm.17 _{The synthesis is carried out by reflux at a} temperature between 50"_{C and 65}"_{C for 15 minutes. This} reaction consists in dissolving 0.32 g of AgNO3 in 5 mL of ethylene glycol. Then, 5 g of PVP (PVP, MagnacolLdt.) are dissolved in 30 mL of ethylene glycol. Then, these two solutions were mixed together and homogenized in

the flask of the reflux system and left stirring and heating until reaching a coloration ranging from gold to brown (Eq. (1)). More details on the process of the reaction can be consulted at Kheybari et al.16

HO

2.3. Characterization of the Nanoparticles

The size of the silver nanoparticles was determined by means of 3 different techniques: UV observation and TEM and AFM images. To perform the UV measurements, a sample was taken from the nanoparticle solution and then poured into the UV sample holder. The characteri-zation was based on UV-visible spectroscopy (T18 man-ufactured by PG Instruments, Leicestershire, UK). The measurement was performed with respect to a blank of distilled water. A small sample was again taken to obtain the TEM images, which was then placed in the microscope using 100-kV acceleration and 10,000X and 30,0000X amplification. The images of the silver nanopar-ticles were observed with a JEOL (Akishima-Shi, Tokyo, Japan) TEM, JEM-2100 model. In the case of observa-tion with AFM images, a 5-mL sample of the nanoparticle solution was taken, dissolved in 5 mL of ethanol, and then centrifuged for 5 min. Then, a small drop of this solu-tion was observed with the AFM on a ceramic surface and ethanol was needed to evaporate in order to make the mea-surement. The asylum research model MFP 3D was used to take images of the AFM.

2.4. Preservation of Wood Samples with Silver Nanoparticles

2.5. Absorption Capacity and Nano-Silver Retention in Wood Samples

The samples were weighed before and after the preserva-tion process, and two parameters were generated: absorp-tion capacity and nano-silver retenabsorp-tion. The absorpabsorp-tion capacity for each sample was calculated as the absorp-tion of soluabsorp-tion in liters by timber volume (Eq. (2)), while nano-silver retention was determinate considering the nano-silver absorption in weight per timber in weight, according to Eq. (3).

=#Weight after imprenation−Weight before imprenation"#kg"∗Density_solution#kg/m3_"

/Volume of wood sample#m3_"

∗L_1msolution₃ (2)

Retention= Absortion capacity#Lsolution/m3" Volume of wood sample#m3_"

The effectiveness of silver nanoparticles in the 30 wood samples treated (A. mangium, C. odorata, C. alliodora,

E. cyclocarpum,G. arborea,G. meiantha,O. pyramidale,

T. grandis and V. ferruginea) was measured by the

per-formance of wood against fungal attack, specifically as regards to resistance to brown decay and white decay

(L. acuta andT. versicolor, respectably). These samples

were oven-dried to 0% MC and then placed into a des-iccator with water for 2 week. Thirty samples (20×20× 20 mm) without any treatment were also conditioned and used as control. This conditioning allowed the wood sam-ples to reach 30% MC. Subsequently, the samsam-ples were sterilized and placed in a soil-block medium into glasses previously inoculated with the fungi, according to ASTM D-2017 Standard and exposed for four months.19_The sam-ple was then cleaned, oven-dried to determine the final weight, and weight loss percentage was calculated. For fungal exposition in glasses in the accelerated degradation test, an environmental control chamber was used (Darwin Chambers Company SA, St. Louis, MO).

2.7. Statistical Analysis

A descriptive analysis was developed (mean and stan-dard deviation) for absorption parameters (density of the wood, absorption capacity and retention) and weight loss percentage. In addition, it was verified if the variables met the assumptions of normal distribution, homogeneity of variances, and the presence of extreme data. Subsequently, an ANOVA was applied to verify the effect of treatment

(two levels: treated and untreated) with the silver nanopar-ticles, on the previously indicated properties of the wood of each species studied. Tukey’s test was set at 99% confi-dence level to determine the statistical difference between the means.

Additionally, a regression analysis was established between the retention, measured in g_Ag/m3

wood and density of the wood for each one of the species, in order to estab-lish the correlation between the two parameters. Also, the correlation between retention and weight loss was estab-lished for each one of the species in the two tested fungi

(L. acutaandT. versicolor). In both cases the significance

level was 99%.

3. RESULTS AND DISCUSSION

The polyol synthetizing method (Ag-polyol NPs) of silver nanoparticles with ethylene glycol produced dark yellow to brown colloidal dispersions. The size of the nanopar-ticles ranges from 40 to 100 nm (Figs. 1(a and b)). The AFM analysis shows irregularities on the surface in a solu-tion of nanoparticles which evidently can reach 11.7 nm (Fig. 1(c)). These measurements were verified with the TEM, which shows that the diameter of the nanoparti-cles may range between 40 to 100 nm (Figs. 1(a and b)). The particles used were spherical (Fig. 1(a)) and prismatic (Fig. 1(b)). Also, the UV-Vis spectrum of the nanoparticles showed a distinct band centered around 410 nm (Fig. 1(d)), which is the measurement for particle sizes of 40 nm.

Various methods are available for synthetizing silver nanoparticles, of which those based on chemical reduction, photo-reduction and thermal decomposition are among the most popular.17 _{These methods have been developed to} control growth of bacteria, for wood protection and in human medicine.20!21 _{The method applied in this work,} considered one of the most simple and effective,17_enabled production of silver nanoparticles of adequate size (Fig. 1). This is demonstrated by the TEM (Figs. 1(a–b)), AFM (Fig. 1(c)) observations and UV-Vis spectrum measure-ments (Fig. 1(c)). In the latter technique the band of absorbance at 400 nm shows that most particles are less than 50 nm (Fig. 1(e)), which is consistent with Prema and Raju and Sileikaite et al.22!23

3.2. Absorption Capacity and Retention of Silver Nanoparticles

The statistical analysis of the absorption capacity and retention of the solution expressed in g_Ag/m3

wood showed different groups: a first group of greater absorption capac-ity comprising G. meiantha, followed by C. alliodora

and V. ferruginea with significantly different absorption

capacity (Table I). Then comes a group consisting of

A. mangium,C. odorataandO. pyramidale, which showed

Figure 1. Silver nanoparticles utilized in wood and observed by means of a Transmission Electron Microscope-TEM (a and b), an Atomic Force Microscope-AFM (c) and the UV-Vis spectrum of silver nanoparticles (d).

species. Lastly,E. cyclocarpum and G. arboreaform the group of species with the lowest absorption capacity and retention values in mg/m3_{(Table I).}

As for retention in treated wood measured in mgAg/kgwood the behavior was different: the highest statis-tical retention was obtained withO. pyramidale, followed

byC. alliodoraandG. meianthawith statistically similar

values; then, a group consisting ofC. odorataandV.

fer-ruginea again with statistically similar values (Table I).

The species A. mangiumandE. cyclocarpumshow statis-tically equivalent values, and lower retention than the pre-vious species, although statistically greater thanT. grandis

Table I. Absorption capacity and retention of silver nanoparticles in treated and untreated wood of nine tropical species from Costa Rica. Absorption capacity Retention of nano-silver Retention of nano-silver Wood density

Type of wood (liters/m3

wood" (gAg/m3wood" (mgAg/kgwood" (Kg/m3"

Acacia mangium 377A _14$6A _27$2A ₅₃₃A

Cedrela odorata 345A _13$5A _34$9B ₃₉₁B

Cordia alliodora 546B _21$4B _62$5C ₃₄₆C

Enterolobium cyclocarpum 207C _8$1C _23$5AD ₃₄₆C

Gmelina arborea 194C _7$7C _15$9E ₄₉₅D

Goethalsia meiantha 640D _25$1D _63$3C ₃₉₉B

Ochroma pyramidale 346A _13$6A _112$0F ₁₂₄E

Tectona grandis 283E _11$1E _19$0DE ₅₉₆A

Vochysia ferruginea 487F _19$1F _36$1B ₅₄₄A

Notes: Average values identified with the letters A and B are statistically different at%₌99%.

and G. arborea, which are the species with statistically

lower retention (Table I).

As for the air dry density before impregnation with sil-ver nanoparticles, results showed that the species show-ing statistically higher density wasT. grandis, followed by

V. ferruginea and A. mangium and then by G. arborea,

with a density of 495 kg m−3_{. The following group} con-sists of C. odorata and G. meiantha with no statistical difference between them. Then,C. alliodoraandE.

cyclo-carpum form a group with no statistical differences and

With regard to the absorptions capacity of silver nanoparticles obtained—from 194 to 640 liters/m3

wood—it

appears that it is possible to use the pressure-vacuum method, commonly used in wood preservation,20!24 _with the dissolution of silver nanoparticles. However, it will be necessary to evaluate the effect of nano-particle solu-tions in metal and plastic components that are part of these preservation systems.25

The differences between the absorption capacity val-ues in the different species (Table I) must be analyzed in detail. Although the absorption capacity is associated with the specific gravity of wood (Stan 2010), this did not occur in the present study.O. pyramidale, with very low specific gravity (Table I), showed intermediate absorption capacity values among the species, similar toA. mangium

andC. odorata, which have much higher specific gravity

values. A. mangium did not show the lowest absorption capacity values, although it is a species with high den-sity. G. arborea and E. cyclocarpum, with intermediate values of specific gravity, show low absorption capacity values. The only case that seems to match isG. meiantha, which has low density and presents the highest absorption capacity values. This lack of relationship between specific gravity and absorption capacity is associated with other characteristics of wood, such as the presence of heartwood and anatomical structure (Stan 2010). As for the heart-wood of most species, liquid impregnation by the vacuum-pressure method is refractory,26 _{and for A. mangium,}

C. odorata,G. arborea, E. cyclocarpum and T. grandis,

although efforts were made to consider sapwood alone, these species show a region of transition in which the heartwood cannot be distinguished from the sapwood,27 where liquid penetration into the wood is affected. With regard to the anatomical structure, the ability to absorb or not a liquid is related to the abundance and size of points of communication between the different wood cells, as well as the abundance and quantity of wood extractives, which in many cases block the passage of fluids.26

The absorption capacity of the solution of silver nanoparticles obtained with these species (Table I) is sim-ilar in other tropical species. For example, Moya et al.12 also found variability in the values of absorption in a study on silver nanoparticle absorption capacity in A. magium,

C. odorataandV. guatemalensis. Furthermore, absorptions

of wood preservative between 121 and 417 liters/m3 wood were found in a group of 8 species,27 _{similar values to} those found in the present work (Table I).

Because of the high absorption capacity found in tropical woods (except for G. arborea and E.

cyclo-carpum, which have low levels of absorption capacity)

high amounts of silver nanoparticles can be found in wood treated with a solution of such particles. High absorption capacity ofG. meiantha, for example, result in the highest values of retention, either by wood volume (gAg/m3wood"or by wood weight (mg_Ag/kgwood" (Table I). The absorption

capacity values found are similar to those reported by Taghiyari13 _{forFagus orientalis,Populus nigra,Platanus}

orientalis, Alnus spp.and Abies alba, but lower than the

values obtained by Liu et al.9–11

3.3. Effect of the Density on the Retention

The effect of the density on the retention measured in g_Ag/m3

wood and mgAg/kgwood showed variation among the species. In the species C. alliodora, G. arborea,

G. meiantha, T. grandis and V. ferruginea, the retention

measured in g_Ag/m3

wooddecreased statistically as the density increased (Fig. 2(a)), while in C. odorata and E.

cyclo-carpum the retention in g_Ag/m3

wood is not affected by the variation of the density (Fig. 2(b)). On the other hand, in

A. mangium and O. pyramidale, an increasing retention

(g_Ag/m3

wood"with increasing density occurs, although with

low correlation coefficients (Fig. 2(b)).

The correlation analysis of the retention (mg_Ag/kgWood" of the various species showed again that the retention decreases with the density in C. alliodora, G. arborea,

G. meiantha, T. grandis and V. ferruginea (Fig. 2(c)).

Furthermore, the species O. pyramidale and C. odorata

present this same decrease, whereas inA. mangiumandE.

cyclocarpumthe retention measured in mg_Ag/kgWood was

statistically unaffected by the density (Fig. 2(d)).

The internal variation of wood density has a negative effect in the values of the retention of nanoparticles for

C. alliodora, G. arborea, G. meiantha, T. grandis and

V. ferruginea, since the higher the density, the lower the

retention of the nanoparticles (Figs. 2(a and c)). As regards

toA. mangiumand O. pyramidale, there is an increment

in the retention of the nanoparticles measured in g_Ag/m3 wood (Fig. 2(b)), since the durability of the species is influ-enced by the increment in the density. No effects have been observed inC. odorataandE. cyclocarpumregarding variation of the density as a result of nanoparticle retention (Fig. 2(b)).

3.4. Decay Resistance

Weight loss due to the attack of brown-rot fungus and white-rot fungus (L. acutaandT. versicolor) is shown in Figure 3. In the nanoparticle-untreated wood, weight loss percentage ranged from 15% to 75% inL. acutaand from 8% to 55% inT. versicolor, while in wood treated with sil-ver nanoparticles weight loss percentage ranged from 8% to 35% inL. acutaand 7% to 11% inT. versicolor. In both types of fungi, treatment with the nanoparticles statistically decreased weight loss in most species except inT. grandis

withL. acuta(Fig. 3(a)) andC. odoratawithT. versicolor

(Fig. 3(b)). As for the species V. ferruginea, treatment with nanoparticles increased weight loss in T. versicolor

(Fig. 3(b)). Another important aspect to highlight is that the biggest differences between nanoparticle-treated and -untreated wood were observed in greater magnitude in

Figure 2. Relationship between retention measured in cubic meters of timber (a and b) and weight in kilograms (c and d) in nine commercial species of Costa Rica.

The evaluation of the effect of the retention measured in g_Ag/m3

wood or mgAg/kgWood on weight loss showed that the variation of the retention does not affect statistically the weight loss percentage inA. mangium nor in O. pyrami-dale, with any of the two fungi tested. RegardingL. acuta, weight loss percentage was not affected statistically by the retention in C. odorata, C. alliodora, G. meiantha,

T. grandis and V. ferruginea (Table II). On the other

hand, in E. cyclocarpum and G. arborea, retention is statistically related with weight loss percentage in both types of fungi. With T. versicolor a positive relationship was found in C. odorata, E. cyclocarpum, G. arborea,

T. grandisandV. ferruginea(Table II). Lastly, a significant

Figure 3. Weight loss percentage caused byL. acuta(a) andT. versicolor(b) in nine tropical species treated and untreated with nano-silver particles in Costa Rica. The bars represent the confidence intervals at%₌99%.

correlation was observed in T. versicolor between the retention expressed in mg_Ag/kgwood in C. alliodora and

G. meiantha; however, this correlation was not significant

when retention was expressed in g_Ag/m3

wood.

Several works have shown the potential of the use of silver nanoparticles in wood protection,9–11 _behavior in case of the fire, improvement of the properties of the wood or already known wood treatments such as heat treatment, densification of wood or particleboard manufacturing.14!28–30

Table II. Correlation between weight loss caused by L. acuta and

T. versicolorand retention of nano-silver particles in wood treated with nano-silver in nine tropical species in Costa Rica.

Retention of Retention of

Ochroma pyramidale L. acuta 0$14NS ₀

$09NS Notes:∗∗_{Correlation statistically significant at%=99%,}∗_{correlation statistically}

significant at%₌95%, NS: correlation not statistically significant at%₌95%.

borate (Na₂B₄O₇" at 22 "C contains 94.75 grams of H3BO3 L−1, while the system used with nanoparticles is 7.7 to 25.1 mg_Ag/m3

wood, an amount of active ingredient much smaller than that used in the combination of H3BO3 and Na2B4O7. Also, the effectiveness of boric acid varies from 0.7 to 3.0 kg/m3

wood,31while the concentration of the nanoparticles ranges from 7.7 to 25.1 g_Ag/m3

wood, equiva-lent to a decrease of about 100 times less active element. One advantage noted by Schultz et al.,20 _{is that silver} nanoparticles in low concentrations, in conjunction with traditional wood preservatives, in addition to protect the wood, can also be added to wood preservatives in order to promote non-decay microorganisms degrading organic preservatives. Moreover, the low levels of metal employed could bring some advantages at the moment of deposit-ing or discarddeposit-ing the treated wood.20 _{However, the same} authors point out that even this type of preservatives are uneconomical.20

In relation with weight loss percentage with different fungi and according to the ASTM D2017-05 standard,17 most of the protection obtained is against the white-rot fungusT. versicolor (Fig. 3(b)); less effective is the pro-tection against the brown-rot fungusL. acuta (Fig. 3(a)). Impregnation with nanoparticles does not achieve pro-tection against L. acuta in T. grandis (Fig. 3(a)), but it does for the rest of the species; however, this protec-tion is classified as moderate or resistant according to the ASTM D2017-05 standard,17_{since weight losses} percent-age between 10 and 40% occur. High resistance in the

wood by means of protection with nanoparticles is only achieved forG. arborea.

Regarding behavior of the nanoparticles againstT.

ver-sicolor, the nine species were classified as highly

resistant (A) to T. versicolor (Fig. 3(b)), except for

C. cyclocarpum because of low levels of retention

(Table I). Contrastingly, untreated wood of most species was classified as non-resistant (D), as weight loss percent-ages were close to 50% (Fig. 3(b)).

The reduction of decay in nine tropical species agrees with other studies with nano-silver particles.12 _For exam-ple, Velmururan et al.32 _{with mycelial growth of the stain} fungiOphiostoma flexuosum,O. tetropii,O. polonicumand

O. ipswere reduced on media amended with different

con-centrations of silver nano-sized particles synthesized from silver nitrate and sodium borohydride. They found that mycelial growth decrees with nano-silver concentration. The mycelial growth was the highest with a concentration of 1 ppm and the lowest growth was found with a con-centration of 100 ppm. On the other hand, Liu et al.1!9–11 found also a reduction of wood decay with different sta-bilizing agents or adding different fungicides to the nano-silver particles solution.

According to Dorau33 _{wood decay fungi have been} shown to be susceptible to silver nanoparticles because sil-ver ions, in solution, inhibited the activity of their cellulase enzymes.34 _{Wood decay fungi feeding on a block treated} with silver halide would likely release more silver as a result of oxidative Fenton reactions during the breakdown of cellulose.35 _{Besides, silver nanoparticles constitute a} reservoir for the antimicrobial effect; in the presence of moisture, metallic silver oxidizes, which results in the release of the silver ions. Silver ions are the species that are responsible for microbial inhibition. Because silver oxidation is a slow reaction, the size of silver particles is critical to achieve microorganism growth inhibition. The smaller the particle size, the higher the surface area, and the greater the area available for oxidation. Particles with diameter less than 100 nm are required to have the surface area necessary to allow a continuous release of silver ions. The main advantages of nano-silver particles over organic biocides are:

(1) non-volatile and non-degradable over time, (2) odorless and

(3) long term efficacy.

Lastly, nanoparticle concentration is one more aspect influencing the values of weight loss percentage due to fungal attack;20 _{moreover, a correlation between retention} and weight loss percentage was found in most of the species (Table II). Although in the present work greater effectivity was observed against the attack ofT. versicolor

with a concentration of silver nanoparticles of 50 ppm (Fig. 3(b)), no values of weight loss were observed for

L. acuta as to enable classifying the species as highly

concentration is probably not suitable is the correlation between retention and weight loss (Table II), because the resistance to fungal attack is related to the concentration of the active element in the wood preservative.26 There-fore, in order to achieve better performance against fungal attack it is desirable to further increase the concentration of nanoparticles. For example, Velmurugan et al.32 _and Liu et al.9–11 _{worked in their studies with concentrations} between 100 and 200 ppm, much higher values than the 50 ppm used in this study.

4. CONCLUSIONS

Synthesized silver nanoparticles have dimensions from 40 to 100 nm. When applied to wood, retentions of 16 to 112 mg of silver per kilogram of wood or 7.7 to 25.1 g of silver per cubic meter of wood are achieved, varying with the species. It was also observed that the effect of den-sity on retention varies between species. InC. alliodora,

G. arborea, G. meiantha, T. grandis and V. ferruginea

the retention decreased statistically with increasing spe-cific gravity. While inC. odorataandE. cyclocarpumthe retention in g_Ag/m3

woodis not affected by the variation of the density. Furthermore, in O. pyramidale and A. mangium

increasing retention occurs as the density increases, but with low correlation coefficients.

Applying silver nanoparticles to the tropical species studied, improves their durability and classifies them as Class A or highly resistant to white-rot fungus T.

versi-color. However, resistance of the species to the brown-rot

fungusL. acutais lower; this can be attributed to low par-ticle concentration. Also, it was observed that the effect of nanoparticle retention was not significant in all species.

In A. mangium and O. pyramidale the variation of the

retention with these two methods did not affect statisti-cally the weight loss in the two types of fungus, whereas

inE. cyclocarpumandG. arborea,the retention is

statis-tically related with weight loss percentage in both types of fungi. In the rest of the species this relation varies with the type of fungus.

Acknowledgment: The authors wish to thank the Vicer-rectoría de Investigación y Extensión at the Instituto Tec-nológico de Costa Rica (ITCR) for financial support for the study.

References and Notes

1. Y. Liu, P. Laks, and P. Heiden,J. Appl. Poly Sci.79, 458(2001).

2. T.P. Schultz, D.D. Nicholas, and A. F. Preston,Pest. Manag. Sci.

63, 784(2007).

3. N. Scott and H. Chen,Ind. Biotech.9, 17(2013).

4. R. Mie, M. W. Samsudin, and L. B. Din,Adv. Mat. Res.667, 251

(2013).

5. A. Tarmian, A. Sepehr, and H. Gholamiyan, Special. Top. Rev. Porous Media Int. J.3, 149(2012).

6. I. Mahapatra, J. Clark, P. J. Dobson, R. Owen, and J. R. Lead,Env. Sci. Proc. Impacts15, 123(2013).

7. N. A. Anjum, S. S. Gill, A. C. Duarte, E. Pereira, and I. Ahmad,

J. Nanoparticle. Res.15, 1(2013).

8. B. Reidy, A. Haase, A. Luch, K. A. Dawson, and I. Lynch,Materials

6, 2295(2013).

9. Y. Liu, P. Laks, and P. Heiden,J. Appl. Poly. Sci.86, 596(2002).

10. Y. Liu, P. Laks, and P. Heiden,J. Appl. Poly. Sci.86, 608(2002).

11. Y. Liu, P. Laks, and P. Heiden,J. Appl. Poly. Sci.86, 615(2002).

12. R. Moya, A. Berrocal, A. Rodriguez-Zuñiga, J. Vega-Baudrit, and S. Chaves-Noguera,Wood Fiber Sci.46, 527(2014).

13. H. R. Taghiyari,Wood Sci. Tech.46, 939(2012).

14. G. Rassam, M. Ghofrani, H.R. Taghiyari, B. Jamnani, and M. A. Khajeh,Eur. J. Wood Prod.70, 595(2012).

15. K. S. Tan and K. Y. Cheong,J. Nanoparticle Res.15, 1(2013).

16. S. Kheybari, N. Samadi, S. V. Hosseini, A. Fazeli, and M. R. F. Daru,

J. Faculty Pharmacy, Tehran Univ. Medical Sci.18, 168(2010).

17. K. Mavani and M. Shah.Int. J. Eng. Res. Tech.2, 1(2013).

18. M. Akhtari, H. R. Taghiyari, and M. G. Kokandeh,Eur. J. Wood Prod.71, 283(2013).

19. ASTM D-2017, American Society for Testing and Materials, West Conshohocken, PA(2003).

20. T. P. Schultz, D. D. Nicholas, and C. R. McIntyre,Recent Pat. Mat. Sci.1, 128(2008).

21. J. H. Crabtree, R. J. Burchette, R. A. Siddiqi, I. T. Huen, L. L. H. Dott, and A. Fishman,Perit. Dial. Int.23, 368(2003).

22. P. Prema and R. R. Aju,Biotech. Bioprocess Eng.14, 842(2009).

23. A. Sileikaite, I. Prosycevas, J. Puiso, A. Juraitis, and A. Guobiene,

Mat. Sci.12, 287(2006).

24. H. M. Barnes, ACS Symposium Series Oxford University Press

(2008), Vol. 982, pp. 583–597).

25. R. D. Glover, J. M. Miller, and J. E. Hutchison,Acs. Nano5, 8950

(2011).

26. L. Stan, Wood Handbook, General Technical Report FPL-GTR-190, U.S. Department of Agriculture, Forest Service, Forest Products Lab-oratory, Madison, WI(2010), pp. 15–15.

27. R. Moya and F. Muñoz,J. Trop. For. Sci.22, 317(2010).

28. H. R. Taghiyari and O. F. Bibalan,Eur. J. Wood and Wood Prod.

71, 69(2013).

29. H. R. Taghiyari, A. Enayati, and H. Gholamiyan,Wood Sci. Tech.

47, 467(2013).

30. H. R. Taghiyari, H. Rangavar, and O. F. Bibalan, Bio. Resources

6, 4067(2011).

31. J. D. Lloyd, Inteernational Research Group on Wood Preservation, IRG/WP 98-30178, IRG Secretariat, Stockholm, Sweden (1998), p. 17.

32. N. Velmurugan, G. G. Kumar, S. S. Han, K. S. Nahm, and Y. S. Lee,Iranian Polym. J.18, 383(2009).

33. B. Dorau, R. Arango, and F., USA. Forest Products Society, Madison, WI(2004), p. 15.

34. T. L. Highley, Res. Pap. FPL-247. U.S. Department of Agriculture, Forest Service, Forest Prod. Lab., Madisson, WI(1975), p. 9.

35. G. Xu and B. Godell,J. Biotechnology87, 43(2001).