Controlled Deposition of Gold and Silver on a Porous Silicone Matrixs

Research article

Controlled Deposition of Gold and Silver on a Porous Silicone Matrixs

Corresponding author: Dr. Solano-Umaña Victor, Hologic Surgical Products, Senior Chemist, DOCINADE’s Ph.D. student, phone. (506) 24362739, E-mail: Victor.solano@hologic.com

Abstract

During the last decade and nowadays an enormous research effort has been deployed to the porous materials. Design, pores sizes, shapes, morphology and density are crucial features to increase the surface material, which helps to improve adsorption and absorption properties and helps the interaction of living cells with the porous material. To achieve this goal, the use of coatings with gold and silver nanoparticles or nanofilms can increase the level of biocompatibility or biotoxicity, especially due to their effect on cell adhesion and proliferation. The biocompatibility and other properties obtained for nanoparticles application coatings are related to the particle size. Gold and silver nanoparticles are size controlled when a colloid is form and later this particles are immobilized on a porous silicone matrix, the pH, temperature, concentration, and proportion or ratio between the metal compound and the stabilizer or reducing agent are factors to consider for a gold and silver particles control on the colloid formation, an lather on the immobilization of these particles over a porous silicone matrix to obtain hybrid nanostructures that combine the best properties from porous polymers and metal particles based on the application.

Keywords: Gold nanoparticles; Silver nanoparticles; Porous materials; Silicone Matrix; Nanofilmm

 

Introduction

Nanotechnology is an emerging new technology that opens the possibility to create new products in a different science areas with an unimaginable applications, as industrial uses, biology, biotechnology, and medicine [1]. Nanotechnologycould be defined as the gate to design, characterize, produce and apply materials, structures, devicesand systems withsizes and shapes at the nanoscale level. It provides especial characteristics that can mimic the biological molecules andtheir systems, for this reason nanotechnology has a tremendous potential use for biomedical applications [2].

The new nanoscience and nanotechnology field applied to health sciences is called Nanomedicine, and its objective isto develop new devices and diagnostic methods to improve the human life quality. Nanomedicine is a branch of nanoscienceand nanotechnology that would allow the ability to cure diseases in the cellular or molecular habitat [3]. Nanomedicinestudies nanoscale interactions and it uses devices, systems, and technologies that include nanostructures that caninteract at those levels. The common objective from this new applied field (nanomedicine) is to solve medical problems,and improved human healthy, nanomedicine embraces a big number of miniaturized technology [4].

Micro, Meso and Macro porous materials can be manufactured using different substances base on the application.Attached to the advantage of porous materials, micro, meso and macro channels surface can be modify with metallic nanoparticles or nanofilms to further improve their properties [5]. Most new researches used different polymers likethe silicone to address the requirement of porous materials with surface functionality, where channels and walls play akey for a successful medical applications. The potential of silicone polymers implanted in the human body are countless because they can be easily manufactured in different shapes such as fibers, fabrics, films and blocks. Silicone or polysiloxaneis a hybrid compound with chemical formula (R2SiO) n, where R could be methyl, ethyl or phenyl (organic group), attached to an inorganic network of Silicon and Oxygen [6].

Surface modification can be achieved by applying coatings, or using metal nanoparticles to increase biocompatibility properties, prevent biofilm formation and favor cells adhesion and proliferation. Along with the size pore control, the application of metal nanoparticles improves the adsorption and absorption process, that are very important during vascularization and cells growing [7].

To obtain gold nanoparticle dispersion on a water base solution from a redox chemical reaction of tetrachloroauric acid with a reducing agent is important to control the pH, to ensure stability and control the reactivity that can achieve the formation of a ruby color red solution. Typical changes in gold nanoparticles size or shape distributions results from a change  in pH [8].

Gold nanoparticles with size at nanometers level are of importance in bio-nanotechnology. There are great opportunities todesign medical devices and detection methods on nanomedicine field, as these materials were used in pre-clinical biomedicalresearch to study the biological effects of their application on different materials and medical devices [9]. The effective application of gold and silver nanoparticles on medicine dependsfrom the particle size, therefore size particle controlled process is a key on this synthesis [10:11]. Gold and silvernanoparticles offer different physical and chemical properties from the bulk materials, and these properties are related totheir size, shape, and surface. Biomedical applications of the gold nanoparticles based on their biocompatibility, dependfrom the size and shape, combined with surface functionalization. [12].

Colloidal gold nanoparticles with size controllable distribution and stability could be prepared with the use of amino compoundsas stabilizer agents. These compounds form covalent bounds that give stability to the nanoparticles. One popularprocedure for nanoparticles production is based on the tetrachloroauric acid reduction by reduced agents, in presence ofprotective or stabilizer agents as amino or thiol compounds [13]. Nanoparticles stability is always an issue, nanoparticleswill always be created with a size distribution of particles, and the changes in size distribution of a colloid over time is a criticalinterest factor in biomedical applications [14].

Porous gold films have attracted interest over the last ten years due to possible applications, and their used in diverse fields,for example, as electronica, sensors, and solar cells [15]. With the control of different variables that effect the metal particlesynthesis is possible to obtain an uniform metal film or a porous film bot  of them are very important and have different applications on industrial, scientific and medical areas, and uniform film or uniform particle deposition on porous siliconmatrix have a tremendous potential on implants and scaffolds for biomedical applications.

2- Experimental

2.1- Preparation

2.1.1- Chemicals

All chemicals were used as received. Porous Silicon Matrix was prepared in the laboratory with poly-dimethylsiloxane(PDMS) product of Nusil Silicon technology (MED-4860), the porous size are between 100 and 300 micro meters, AgNO3(99.99%, Sigma-Aldrich), HAuCl4•3H2O (99.99%, Sigma-Aldrich), L-Ascorbic acid (99%, Sigma-Aldrich), Disodium ethylenediaminetetra-acetate (EDTA-2Na ) ACS reagent (99.4 %, powder, Sigma-Aldrich), Dextrose monohydrate (USP grade, Sigma-Aldrich), Sodium hydroxide (ACS reagent, ≥97.0%,pellets, Sigma-Aldrich), Ammonium hydroxide solution (ACS reagent, 28.0-30.0%, Sigma-Aldrich), and anhydrous SodiumSulfite (≥98%, Sigma-Aldrich).

To prepare a chemical solution and perform a silver and gold deposition treatment over the porous silicon matrix, the conditionson the following table were followed:

Table 1. Chemical solution parameters.


2.1.2 Silver solution

Based on Table 1, AgNO3 was weighted and dissolved in DI-water and concentrated ammonium hydroxide was addedto the aqueous silver nitrate solution. In those cases where a precipitation of silver oxide occurred the addition of moreammonium hydroxide re-dissolved the precipitated silver oxide by forming an ammoniac silver complex (Ag(NH3)2]OH),later a EDTA-2Na solution was added, molar ratio 1:1 respect to AgNO3, and the pH was adjusted where required (using ammonia),later the porous silicone matrix was added, dextrose monohydrate solution was added and finally the required temperaturewas reached. The molar ratio from silver nitrate to Dextrose monohydrate was 1:5.

The porous silicone matrix was removed after one hour to reach the required temperature.

2.1.3 Gold solution

Based on Table 1, HAuCl4•3H2O was weighted and dissolved in DI-water, added to a Sodium Sulfite (Na2SO3) solution, molarratio from HAuCl4•3H2O to Na2SO3 was 1:10. EDTA was dissolved in DI-water and added to the mixed solution, molarratio from HAuCl4•3H2O to EDTA was 1:1, the pH was adjusted with NaOH, the porous silicone matrix was added and thenthe L-Ascorbic acid solution was added, molar ratio from HAu- Cl4•3H2O to L-Ascorbic acid was 1:10, which finally reachedthe required temperature.

The porous silicone matrix was removed after one hour to reach the required temperature.

Several complexing agents have been investigated such as sulfite, thiosulfate, thiomalate, chloride, and phosphate with thegold source with ascorbic acid, as the reducing agents to made gold nanoparticles. The gold sulfite reduction proceeds followingthis equation:

Gold sulfite solution has a relatively high stability, particularly under alkaline conditions, as long as the pH is maintained below10 the solution compatibility and stability is good, most commercial gold sulfite solutions are operated in the pH rangefrom 9 to 10 [16:17].

2.2 Characterization method

2.2.1 Optical inspection

AA visual inspection was performed on a SmartScope Flash 200, model CNC200, serial SVW2003849. SmartScope Flash isa full-featured automatic dimensional measurement system, optical metrology with a large capacity video measuring systemfor dimensional verification of manufacturing parts.

2.2.2 Fluorescence X-Ray Spectrophotometer

A thickness measurement from a silver and gold deposition over the porous silicone matrix was performed on x-ray equipment.The fluorescence (XRF) spectrometer is an x-ray instrument used on non-destructive chemical analyses of differentmaterials and metal plating. XRF is an analytical technique used to determine the elemental composition of materials, it determines the chemistry of a sample by measuring the fluorescent ray emitted from a sample when it is excited by x-raysource. Each element present a characteristic fluorescent pattern signs that are unique for that specific element, XRF spectroscopy is used for qualitative and quantitative analysis of material composition and can measure the thickness of themetal film deposit over different surfaces.

2.2.2 Scanning electron microscopy (SEM)

SEM images were obtained, using a JEOL JSM-6390LV Scanning Electron Microscope.

3- Results and Discussion


Figure 1. Porous silicone matrix optical micrographs, at different magnifications (A;40X, B:83X).

During the optical inspection, a non-uniform deposition was found on the Porous Silicone samples from lots 1 and 2. Someareas don’t have a metal deposition and have a white color (the color from the original material, as seen in Figure 1) and other areas have metal particles lumps, as seen in Figure 2, the only difference between lot 1 and lot 2 are the temperature, lot 1 ran at 35 ᵒC and lot 2 at 50 ᵒC. The lot 2 had more white color areas (areas without metal nanoparticles deposition) and lumps inside the silicone matrix pores; the deposition is more irregular than lot 1.

Figure 2. Optical micrographs, deposition of gold and silver nanoparticles over a porous silicone matrix, lots 1 and 2, at different magnifications (A;40X, B:83X).

TThe samples from lot number 3 and 4 ran at the same metal concentration and temperature than lots 1 and 2, but the pH was increased in one unit, lot 3 corresponds to lot 1 and lot 4 to lot 2, see table 1, in both cases the pH value increase produced a particle agglomeration to form a big lump inside the silicone matrix pores, like a solid rocks inside the holes, also present a white surface areas that are the silicone surface without metal particles deposition. This effect produces the clogging of the pores, as seen in Figure 3, it was confirmed by a rigid consistency on the porous matrix and also after the cutting of the matrix, on the cross section area, the pores were fill-up of solid particles. Consequently to this, at low pH, the ascorbic acid solutions are quite stable, however, as the pH is raised the stability of the ascorbate solution decreases, and the oxidation rate increases, since the pH has an extreme importance of metals in controlling nanoparticles size [18].

On the silver nanoparticles reaction the pH is also a critical factor. Chemical reduction route to obtain silver colloid withcontrollable particles sizes ranging from 45 to 380 nm is the reduction of [Ag(NH3)2]+ complex with reducing sugars as dextrose, fructose and sucrose, where ammonia or sodium hydroxide concentration in the reaction is the key parameter to control the particles size. Also sodium hydroxide could be used  o enhance the reaction velocity [19:20:21].


Figure 3. Optical micrographs, deposition of gold and silver nanoparticles over a porous silicone matrix, lots 3 and 4, images at different magnification (A:40X, B:83X).

The pH plays an important role in silver nanoparticles (AgNPs) production from silver nitrate reduction with dextrose, and it

can be studied using surface plasmon resonance (SPR), and the  different SPR behavior is explained in terms of size distribution
of AgNPs, if the pH is increased (more ammonia or sodium hydroxide) the particles size increase [22].

Lots 5, 6 and 7 show a better distribution of the metal nanoparticles compared to lots 1,2,3, and 4, the deposition is uniform and non-clogged up pores were detected, as seen in Figure 4, 5 and 9.


Figure 4. Optical micrographs, deposition of gold and silver nanoparticles over a porous silicone matrix, lots 5 and 6, different magnification (A;40X, B:83X).


Figure 5. Optical micrographs, deposition of gold and silver particles over a porous silicone matrix, lot 7, different magnification (A;40X, B:83X).

The lots 5, 6 and 7 show a uniform bright color which it is indicative from their particle size, and the lot 7 shows a ruby color, on this samples the pores are not clogging and it was confirm by a transversal section cutting and inspection, alsoa SEM imagine confirm this, Figure 10 . Noble metal nanoparticles like gold presents a characteristic strong color of their colloidal solutions, caused by surface plasmon absorption properties [23].

The results of the optical inspection were validated with a metal thickness measurement, performed with the FluorescenceX-Ray Spectrophotometer XDLM model, which used WinFTM software (Fisher Thickness Management Software for Windows) to control the measurement head of the x-ray and handles the evaluation of the signal. Later the thickness values were analyzed with statistical software called Minitab 17 to obtain the boxplot graphs from the different lots.


Figure 6. Thickness of silver nanoparticles deposited over a porous silicone matrix, thickness boxplot.

The silver and the gold thickness from lots 1, 2,3 and 4 have a huge standard deviation, therefore the boxplot shows a hugevariability. As seen in Figures 6 and 7, lots 5, 6 and 7 have a lower standard deviation or lower variability than lots 1,2,3and 4. This effect is due to the pH reduction, but also lot number 7 has the lowest variation (lowest standard deviation), on this lot the metal concentration for silver and gold was reducedby 50%, and this reduction from the initial concentration impacts the size distribution of nanoparticles over the porous silicone matrix. This difference is seen in lots 5, 6 and 7 detailed in Figure 8. The size of colloidal nanoparticles during the gold reduction could be easily tuned in the nanometer range by adjusting initial concentration of HAuCl4 solution [24:25]. The agglomeration of nanoparticles from a solution is practically unavoidable in the absence of a stabilizer, stabilization has two approaches: a- steric repulsion caused by surfactants, or other
electrostatic (van der Waals) repulsion of organic species bound to nanoparticles’ surface, for this study the EDTA was us as stabilizer. The growth process of the nanoparticles start as nucleation, experimental evidence suggests, that concentration and temperature become predominant factors determining growth rates from nanoparticles [26]. In Figure 6 the temperature effect over the resulting thickness is evidenced when lots 1 and 2, or lots 3 and 4 are compared, as seen in Table 1, the temperature increase on lots 2 and 4 produces an increase on the main silver thickness. Also the variability and standard deviation from these lots increase respect to lot 1 and 3. In Figure 7 the same situation is present for the gold thickness, when lots 3 and 4 are compared, lot 4 has the biggest thickness, standard deviation and variation, however the comparison from lots 1 and 2 does not match this criteria, lot 2 was produced at bigger temperature than lot 1, and on this lot the variation for the gold thickness is smaller than the variation from lot 1, but it is contrary on the silver thickness, as seen in Figure 6 and 7, it could be explained for the non-uniform deposition of the particles on lot 2.


Figure 7. Deposition of gold nanoparticles over a porous silicone matrix, thickness boxplot.

On the chemical reaction, disodium ethylenediaminetetra-acetate (EDTA-2Na) that is a hexadentate chelating agent which could be administered intravenously to the human body and makes its way through the blood stream to pull toxic heavy metals because these attach themselves to it, and the attachment forms a compound which can be excreted through urine, EDTA-2Na could solvate metal cations, also serve as stabilizing agents once the metal nanoparticles are made. On this study EDTA-2Na ratio respect to metals was constant to avoid this interference on the particle’s size.Gold nanoparticles can be made from  chloroauric acid (HAu- Cl4) through redox chemical reaction with ascorbic acid and the particles can be immobilized on a silicon compound [27]. One way is to use a desired quantity of stable additive agent and ascorbic acid, a weak reducing agent; on the wet chemical synthesis to reduce aqueous HAuCl4 solution. If ascorbic acid content was little, the nanoparticles can grow up because the system will be unstable, more gold atoms move to the surface, the chemical reaction is slower and Au seed can grow up [28]. On these reactions the molar ratio from the metal to reducing agent was constant, 1:5 for silver and 1:10 for gold in order to prevent the reducer concentration effect over the particle’s size.

Figure 8. Thickness of nanoparticles deposited over a porous silicone matrix, boxplot for lots 5, 6 and 7, A: silver thickness, B: gold thickness.

Figure 9. Optical micrographs, gold and silver nanoparticles deposited over a porous silicone matrix, lot 5, 6 and 7 pore details.

Gold colloid color changes are visible to the naked eye and are associated to plasmon resonances (SPR) that may occur by changes on particle size, surface chemistry or inter-particles distance [29:30].

Figure 8 shows the different particle size measurement between lots 5, 6 and 7 which is an evidence for the difference color obtained on the silicone porous matrix, shown in Figure 4 and 5. On lot 7 the porous matrices show a ruby color and lot 6 a light golden color, as seen in Figure 9.

Figure 10 shows more detail imagine respect Figure 9 (lot 7), the metal film deposited over the porous silicone matrix has a thickness measure in the nanometer order. The silver film thickness is below 150 nm and gold film thickness is below 100 nm, see Figure 8, these measures evidence the deposition or immobilization of gold and silver nanoparticles to form a nanofilm, also Figure 10 shows the pores from the silicone matrix, initial pore size from 100 to 300 micro meters free and open. The silver and gold film immobilized over the porous silicone matrix do no block the channels.

Conclusions

Nowadays, gold and silver are more than noble metals, they have an enormous potential on different fields such as industrial and biomedical.

Based on their nanoparticle’s properties, many applications were developed and more are coming. Applications that will be revolutionize the medicine and strengthen the nanomedicine name.

Figure 10. SEM image of silver and gold particles deposition over a porous silicone matrix, lot number 7.

The green chemistry is an excellent alternative to prepare nanostructures with potential used on medicine, because these chemicals like dextrose or sodium ascorbate are not toxic or dangerous for human health. After the colloid formation or metal (silver, gold) nanoparticles stabilization on water, these particles would be immobilized on a porous silicone matrix, and obtain a hybrid nanocomposite, see Figure 9.

Many factors and their interactions are present on the metal nanoparticles synthesis, on this research the pH, temperature and concentration effects on gold and silver particles size were evidenced, but also, the proportion or ratio between the metal compound and the stabilizer or reducing agent affects the particle size. Therefore many factors must be control to obtain a particle’s size reproducibility.

On this research, one goal was to maintain the original porous free of obstructions, and this goal was met with lots number 5, 6 and 7, as show Figure 9 and 10, also base on the thickness measure from these lots (Figure 8) the gold and silver particlesdeposition do not affect the original pore size (100 to 300 micrometers), because  on these lots the gold and silver thickness are a few nanometers that do not affect the silicone matrix porous size, this is important for a future medical applications because the pore size is related to the cellular growth and the biocompatibility. On the other hand, the samples from lots 3 and 4 could be very useful to explore another application for a porous metal film immobilization (see Figure 3), on silicone matrix holes.

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