Technical Field

The present invention relates to the synthesis of antimicrobial and/or antibacterial and/or antifungal and optionally antiviral powders comprising a carrier material and a functional dopant being a metal and/or a metal oxide, in particular a powder with a silica (Siθ2 ) carrier, said carrier having inclusions of said functional dopant, said dopant being silver and/or silver oxide and/or copper and/or copper oxide, and said powder being made using a Flame Spray Pyrolysis (FSP) process .

Background Art

It is known that silver metal (and silver ox- ide) exhibits antimicrobial and antibacterial activity yet is non-toxic to humans ^[1] . The antibacterial activity of silver is described to be due to the presence of Ag ⁺ cations which bind strongly to electron donor groups on constituent bacterial molecules containing sulphur, oxy- gen, or nitrogen ¹²¹ . The Ag ⁺ cation is also assumed to displace essential metal ions such as Ca ²⁺ and Zn ⁺ and to damage the bacterial cell membrane' ³¹ . The required concentration for antimicrobial activity has been found to be very low, with levels of 5 to 10 ppb in water capable of biocidal activity ¹⁴¹ . Silver ions can also destroy fungi , however copper metal (and copper oxide) also exhibits antifungal activity ^[5] .

Silicon dioxide particles that are doped with silver or silver oxides and their use for antibacterial applications are known from US 2003 /0235624 Al having the title "Bactericidal silicon dioxide doped with silver" . Said particles are prepared by vapor-phase synthesis of

silicon dioxide ( chloride process ) with introduction of an aqueous aerosol of dopant precursor into the flame gas .

The production method described in US 2003 / 0235624 Al for pyrogenically obtaining silica doped with silver or silver oxide by flame oxidation or flame hydrolysis has three key stages , namely 1 ) formation of dopant aerosol , 2 ) mixing the aerosol with gaseous Si- compound containing flame feed gases , and 3 ) feeding said , mixture of aerosol and feed gases into a flame .

Antimicrobial activity of Ag particles on metal oxide support particles other than silica particles and produced via a wet precipitation method is known from US 5 , 714 , 430 , and an antimicrobial glass is described in US 2004 / 0170700.

The flame spray pyrolysis method (FSP) has been demonstrated as a versatile process technology to produce a wide variety of materials for many applications ¹⁶¹ . Hitherto , however, said method has neither been suggested let alone used for the synthesis of silver and/or silver oxide and/or copper and/or copper oxide doped silica . Furthermore, there are no examples of FSP being used to specifically produce powders for antibacterial and/or antimicrobial and/or antifungal applications . Therefore there is still a need for antimicrobial and/or antibacterial and/or antifungal powders and a method for the manufacture of such powders .

Disclosure of the Invention

Hence , , it is a general obj ect of the invention to provide a method for the production of a doped silica ( Siθ2 ) in the form of particles having antimicrobial and/or antibacterial and/or antifungal effect .

It is a further obj ect of the present invention to provide a doped silica ( SiC>2 ) in the form of parti-

cles and having antimicrobial and/or antibacterial and/or antifungal effect .

It is still a further obj ect of the present invention to provide products comprising such doped sil- ica .

Now, in order to implement these and still further obj ects of the invention, which will become more readily apparent as the description proceeds , the method for the production of a flame made doped silica ( Siθ2 ) in the form of particles having antimicrobial and/or antibacterial and/or antifungal effect is a flame spray py- rolysis (FSP ) process , in particular a process manifested by the features that said flame made doped silica comprises at least one functional dopant , said functional dopant consisting of at least one antimicrobial and/or antibacterial and/or antifungal acting metal and/or metal comprising compound, in particular at least one antimicrobial and/or antibacterial and/or antifungal acting metal and/or at least one antimicrobial and/or antibacte- rial and/or antifungal acting metal-oxide , said method comprising

( i ) preparing a precursor solution comprising at least one functional dopant precursor and at least one silica precursor in an organic solvent , ( ii ) spraying said precursor solution into a flame fuelled by combustion of said precursor solution itself ,

( iii ) collecting the particulate doped silica . The term functional dopant as it is used in the scope of the present invention describes dopants having antimicrobial and/or antibacterial and/or antifungal effect .

Further dopants may be present , e . g . carrier dopants , wherein the term carrier dopant is used for dopants influencing the silica morphology .

The term additional dopant is used for dopants that are present for other purposes , e . g . for providing another function such as an antiviral effect .

Preferably, said functional dopant comprises metals and/or metal oxides selected from the group consisting of silver, silver oxide , copper , copper oxide and mixtures thereof . For the antimicrobial and antibacterial effect , silver and silver oxide are preferred. For antifungal activity copper and/or silver or their oxides , re- spectively may be used, whereby copper is preferred in this respect due to its demonstrated antifungal performance .

Preferred functional dopant precursors are highly soluble in organic solvents and are combusted without generation of harmful byproducts . Suitable functional dopant precursors include but are not limited to e . g . AgNθ3 (Silver nitrate) , Cu (CH3COCHCOCH3 ) 2 (copper acetylacetonate) , and mixtures thereof .

The solubility of the silica precursor is less critical . A broad variety of organosilanes is suitable, said organosilanes having the additional advantage that the organic residue is combusted in the flame . Well suitable organosilanes include but are not limited to e . g . tetraethoxyorthosilane (TEOS) , Hexamethyldisiloxane (HMDSO) .

In the FSP method of the present invention any organic solvent providing sufficient solubility can be used, however, solvents with high combustion enthalpy are preferred . For e . g . silver nitrate suitable solubil- ity in combination with acceptable enthalpy has been found with alcohols , in particular alcohols or mixtures of alcohols with a mean carbon content of 1 to 3 carbon atoms per hydroxy group, such as methanol , ethanol , n- propanol , 2-propanol , ethane diol , propane diol and mix- tures thereof . At present 2-propanol is preferred . Alcohols with higher carbon content may be selected to achieve higher flame temperatures , providing sufficient

solubility of the precursors is maintained. Precursor solutions should be limited to sub-saturation concentrations to prevent precipitation prior to delivery to the flame . In general , the total silicon and functional dopant metal concentration is around 0.3 moles per liter .

In view of a good antimicrobial and/or antibacterial and/or antifungal effect it is preferred that the functional dopant is present in an amount of at least 2.5 at% based on silicon, preferably at least 3 at% , most preferred 4 to 5 at% . Higher concentrations than 5 at% in general are not needed, however, with higher concentrations the time of protection may be further extended if need be . For specific applications also less than 2.5 at% dopant may be sufficient . In the scope of the present invention, it has been found that silica has unexpected advantages if used as carrier material for functional dopants in the FSP production method of the present invention . Since it is non-crystalline, it forms an especially good matrix for the functional dopant . It assists in determining the particle size of the functional dopant, it provides a good porosity and it enables a good predetermination of the final particle size .

Thus , the system of the present invention can be varied in broad ranges thereby varying the ratio of the specific forms of functional dopants generated. By enhancing the silver to silicon ratio in the precursor solution while keeping the total concentration of metal + silicon and the production parameters constant , the aver- age diameter of the silver particles and therewith the amount of longtime active functional dopant can be increased with an almost constant final particle size .

The advantageous behavior of the system of the present invention is assumed to be due to the behav- ior of the specific system within the flame .

Without wanting to be bound by any theory, the combination of functional dopant-silica agglomerates

and silica-coated functional dopant morphologies suggests two formation pathways for the observed material . One pathway, leading to functional dopant-silica agglomerates , is consistent with gas-phase nucleation, surface- growth, and sintering for the growth of both silica and functional dopant particles followed by coagulation to form the large matrix structures . The second pathway, forming large silica-coated functional dopant particles , suggests the presence also of a droplet-reaction route with precursors reacting within the sprayed droplet ¹¹¹ - ¹ to give large functional dopant particles followed by condensation of gas-phase silica to give a surface coating on the functional dopant particles . Alternatively the presence of the large functional dopant particles might also derive from surface mobility and sintering of functional dopant particles nucleating on the surface of the silica agglomerates .

Since the morphology of the silica has been found to be of great importance to the features of the final product said features may be influenced by adding one or more carrier dopants that influence the morphology of the silica . Suitable carrier dopants are e . g . oxides of metals selected from the group consisting of titanium, zinc, aluminium and mixtures thereof . Such dopants will in general be present in amounts of at most 10 at% dopant metal based on Si , preferably in amounts of at most 5 at% , most preferred in amounts of at most about 2 at% .

Carrier dopant precursor compounds - if used - are also added to the precursor solution . Suitable car- rier dopant precursors are also organometallic compounds including but not limited to zinc acetylacetonate, titanium isopropoxide , and aluminium acetylacetonate .

In general, a flame made doped silica in the form of particles of the present invention is character- ized by a functional dopant that is simultaneously present in form of embedded particles , surface exposed parti-

cles and large silica-coated particles . Such a doped silica is obtainable by a method of the present invention .

The doped silica in form of particles of the present invention comprises dopants as described above . The observed combination of embedded, surface exposed and large silica-coated functional dopant particles is quite desirable for antimicrobial and/or antibacterial and/or antifungal applications . Antimicrobial and/or antibacterial and/or antifungal activity is de- rived from the ^" delivery of functional dopant ions to bacteria and so the presence of surface exposed functional dopant in these powders allows ready availability of functional dopant ions and therefore high initial antimicrobial and/or antibacterial and/or antifungal activity. However, the presence of embedded functional dopant particles acts as a reservoir of functional dopant for long- term performance with functional dopant ions diffusing to the outside of the agglomerates to maintain activity of the powder over extended times . Furthermore, the presence of large silica-coated functional dopant particles also augments this reservoir effect . While the presence of the large functional dopant-particles is of benefit for antimicrobial and/or antibacterial and/or antifungal activity, a powder with a higher proportion of functional dopant-silica agglomerate structures may be more desirable for applications requiring transparency or minimal powder color .

At present , a preferred dopant is silver in predominantly or exclusively metallic form. However, if predominantly .or exclusively antifungal activity is desired, copper and/or copper oxide may be preferred.

For specific applications , the surface of the doped silica particles may be functionalized with organic groups for selective fixation to specific surfaces or within polymer matrices .

Such functionalization can be made using generally known methods/ substances , e . g . methods/substances

known from primer technology . Suitable are e . g . bi- functional molecules with one functional group having a high affinity or reactivity towards the silica carrier and one group having a high affinity or reactivity to- wards the material wherein the flame made doped silica shall be incorporated or that is intended to be treated or impregnated with the doped silica . Such material is also referred to as "material to be doped" . Examples for silica liking groups are silane compounds which can un- dergo silylation reactions with surface hydroxyl groups and groups having an affinity to the material to be doped can be tuned for hydrophobic or hydrophilic functionality. Other reactions capable of grafting to the silica surface include esterification and amidization . FSP is a process capable of synthesizing the flame made doped silica powders of the present invention in large quantity and at low cost . These powders can be incorporated in many applications to provide clean and sterile functionality to numerous goods . The doped silica of the present invention can e . g. be used as filler for polymers and/or polymer composites . If homogeneously mixed, it can in addition perform the function of mere silica fillers . In such applications high amounts can be added, in general up to 50 wt% based on the polymer/polymer composite . Such polymers/polymer composites can then be used to produce respectively doped woven and/or nonwoven textiles .

It is also possible to use the doped silica of the present invention in .combination with goods such as polymers and/or polymer composites and/or natural fibers and/or woven and/or nonwoven textiles, e . g . by at least partially, preferably fully, treating or impregnating such goods .

Doped materials or treated/impregnated mate- rials of the present invention can not only be used for producing textiles but also for the production of e . g .

food and/or beverage containers , for tooth brushes , other consumer products , and medical equipment .

Since it is possible to vary the ratio of the specific forms of functional dopant by varying the pro- duction parameters and/or the dopant/carrier ratio, it is also within the scope of the present invention to use mixtures of differently produced doped powders . This enables to produce a broad spectrum of behavior over time with a minimal number of different production processes needed .

Brief Description of the Drawings

The invention will be better understood and obj ects other than those set forth above will become apparent when consideration is given to the following detailed description thereof . Such description makes reference to the annexed drawings , wherein: Figure 1 is a schematic diagram of a suitable flame spray pyrolysis apparatus as used in the scope of the examples .

Figure 2 shows X-ray diffraction (XRD) patterns for silver-doped silica for Ag concentrations be- tween 0 and 5 atom% .

Figure 3 shows the results of particle size measurements using two different methods , wherein the BET equivalent diameter evaluated from specific surface area data is shown by the open circles and corresponds to the estimated diameter of the primary particles (SiC>2 and Ag) within the as-produced powder, and wherein the solid diamonds correspond to diameters estimated from the XRD patterns using the fundamental parameter approach .

Figure 4 shows transmission electron micro- scope (TEM) images for 5 at . % Ag doped Siθ2 , wherein the left hand image a shows metallic silver particles ( dark gray, indicated also by arrows ) within an agglomerate ma-

trix of amorphous silica ( light gray particles ) and wherein right hand image b shows that a range of silver particle sizes is present in the sample including some large silica-coated particles . Figure 5 shows on the left a scanning transmission electron microscope ( STEM) image of a silver- doped silica matrix, wherein the bright spots are metallic silver particles , and on the right graphs representing the EDXS elemental analysis of points indicated in the STEM image .

Modes for Carrying out the Invention

The present invention is now further described for a method starting from a binary precursor system and the respectively produced products .

The FSP method for producing a flame made doped silica ( Siθ2 ) in the form of particles having antimi- crobial and/or antibacterial and/or antifungal effect , wherein said particles comprise at least one functional dopant consisting of at least one antimicrobial and/or antibacterial and/or antifungal acting metal and/or metal comprising compound, comprises the steps of ( i ) preparing a precursor solution comprising a functional dopant precursor and a silica precursor in an organic solvent ,

( ii ) spraying said precursor solution into a flame fuelled by combustion of said precursor solution itself ,

( iii ) collecting the particulate doped silica .

In a much preferred embodiment , the functional dopant consists of at least one antimicrobial and/or antibacterial and/or antifungal acting metal and/or at least one antimicrobial and/or antibacterial and/or antifungal acting metal-oxide .

Surprisingly it was found that by using the method of the present invention not predominantly the expected dopant metal oxides are formed but pure metal particles , in particular in the case of silver as functional dopant .

Starting from a precursor solution comprising AgNO3 , tetraethoxyorthosilane and iso-propanol as solvent particles of (within the detection limits) pure metallic silver were obtained. The total (Si+Ag) concentration for each precursor solution should be in the range of 0.1 to 0.5 mol/L and preferably about 0.3 mol/L . The liquid precursors may be exposed to ultrasonication for a time and with a power suitable to aid in dissolution of the AgN03 solids . All sample preparation and FSP synthesis steps may be performed in low light conditions together with suitable light shielding devices to avoid alteration of light sensitive precursors .

As-produced powders showed at least one, preferably at least the first 3 , much preferred all of the following characteristics : simultaneous presence of embedded and surface exposed functional dopant, in particular silver particles , - matrix of amorphous silica, embedded and surface exposed functional dopant particles , in particular silver particles , in general with a diameter <20 nm, large silica-coated particles in general with diameters >50nm, antibacterial and antimicrobial effect up to at least 3 days , preferably at least 5 days , much preferred at least about 7 days, if measured by dispersing 1 mg/ml of the doped silica within a liquid culture of Es- cherichia coli . (The antimicrobial and/or antibacterial effect can be and was measured by a test, wherein the initial concentration of bacteria was a colony of approxi-

mately 1000 bacteria (E. CoIi ) dispersed in 5 ml of culture medium, and lmg of each powder was placed in ImI of this bacteria culture . In this test , an effective powder in general leads to the death of the bacteria with asso- ciated decrease in visual turbidity and a conservation of the culture medium in this state for the above mentioned time . )

By using other precursors and/or other dopants such as copper and/or other solvents and/or other concentrations and/or other feeding rates , the particle size and/or the particle type ( embedded, surface exposed, silica coated) and/or the metal to metal oxide ratio of the functional dopant may be varied .

Examples

Example 1 : Production of silver doped silica particles

Silver doped silica particles were produced by flame spray pyrolysis using a flame spray pyrolysis (FSP) nozzle ¹⁷¹ that had a radially symmetric configuration with a stainless-steel capillary tube ( ID 0.41 mm; OD 0.71 mm) at the central axis serving as the liquid feed nozzle ( see Figure 1 ) . Immediately surrounding the capillary tube was a narrow annular gap of adjustable cross-sectional area, that issued 5 L/min of oxygen for spray atomization of the liquid feed. The pressure drop across the nozzle was maintained at 1.5 bar during FSP operation . A narrow concentric orifice ring ( 0 -.15 mm spacing, 6 mm radius from nozzle axis ) was supplied with a mixture of CH4 ( 1.5 L/min) and O2 ( 3.2 L/min) to serve as a premixed pilot flame for ignition and support of the spray flame . A sheath gas flow of 5 L/min of oxygen was issued through an annular sintered metal frit ( 8 mm width, inner radius 9 mm from the nozzle axis) to stabilize and contain the spray flame . The precursor liquid feed was supplied at 5 ml/min using a rate-controlled sy-

ringe pump ( Inotech R232 ) and all gas flows (Pan Gas , >99.95% ) were metered using mass flow controllers (Bronk- horst) . A water-cooled, stainless-steel filter housing supported a glassfiber sheet (Whatman GF/D; 25.7 cm di- ameter) for collection of the flame-produced powder with the aid of a vacuum pump (BUSCH) .

The basis liquid precursor solution was composed of 2-propanol (Aldrich, 99.9% ) , tetraethoxyorthosi- lane (TEOS , Aldrich, >98% ) , and silver nitrate (AgN03 , Fluka, >99% ) . Silver concentrations ranged between 0 and 5 atom percent (at . % ) in proportion to silicon . The total ( Si+Ag) concentration for each precursor solution was 0.3 mol/L . The liquid precursors were exposed to ultrasonica- tion ( Sonics Vibra-Cell) for 3 minutes at 75% probe power with 1.0s/0.5s on/off configuration to aid in dissolution of the AgNO3 solids . No solution alteration or precipitation was observed from the sonication step . All sample preparation and FSP synthesis steps were performed in low light conditions together with suitable light shielding devices to avoid alteration of light sensitive precursors .

As-produced powders were characterized using (i) powder X-ray diffraction (XRD) (Example 2 ; Fig . 2 ; estimated silver particle diameter Example 3 ; Fig. 3 ) ,

( ii ) BET adsorption isotherms and specific surface area (SSA) analysis (Example 3 ; Fig. 3 ) ,

( iii ) high resolution electron microscopy (HRTEM) (Example 4 ; Fig . 4 ) , ( iv) scanning transmission electron microscopy (STEM) (Example 5 ; Fig . 5 ) , and

(v) energy dispersive x-ray spectroscopy (EDXS) analysis (Example 6 ; Fig . 5 ) .

Example 2 : Powder X-ray diffraction

The powder X-ray diffraction (XRD) was performed with, a Bruker AXS D8 Advance spectrometer at 2θ (Cu-Ka) 10 to 70° , a step size of 0.03 ° , and a scan speed of 0.6 ° /min ( source 40 kV, 40 mA) . XRD patterns were analyzed using the Fundamental Parameter (FP) method to match the profile of individual peaks within each XRD pattern, allowing extraction of crystallite size information ¹⁸³ . The XRD patterns for Ag-doped silica for silver concentrations between 0 and 5 at . % are shown in Figure 2. The presence of amorphous silica ( a-SiC>2 ) in the sample is clearly reflected in the broad peak appearing in the baseline between 15 and 35 ° . Peaks corresponding to metallic silver are indicated with asterixes at 38.1 , 44.3 and 64.5 ° . These peaks correspond to the [ 111 ] ,

[200] and [220 ] silver crystal planes respectively . The indicated peaks are consistent with reference pattern PDF 87-0717 ^[9] and no peaks were observed for silver oxides .

The silver XRD peaks , and most noticeably the peak at 38.1° , increased in size as the Ag concentration was increased, consistent with increasing particle size . The silver peaks were largely indistinguishable for dopant concentrations below 3 at% indicating that silver crystals were only present at the higher concentrations with only small ( or atomic clusters ) present at lower dopant concentrations .

Example 3 : Particle size determination BET adsorption isotherms and specific surface area analysis were performed using a MicroMeritics TriStar 3000 system after degassing in nitrogen for 1.5 hours at 150 ⁰ C . The specific surface area ( SSA) was measured using 5-point nitrogen adsorption at 77 K . The BET equivalent diameter was evaluated from the measured SSA for each sample , assuming a spherical primary particle geometry and a composition-corrected density . The

such obtained BET equivalent diameter (dg^T^ ^was compared with the diameters estimated from the XRD patterns using the fundamental parameter approach .

The results obtained with said two methods of particle size analysis as a function of silver concentration are shown in Figure 3. The open circles show the BET equivalent diameter (dg^T) °f the as-produced powder where the diameter was evaluated based on the measured specific surface area of the silver-doped silica powder . This BET equivalent diameter was observed to decrease from 11 inn for no silver dopant to 8 nm at 3 at . % Ag and then to remain at 8 nm for concentrations up to 5 at . % Ag . The observed decrease of the dggiji was consistent with the observations of Tani and coworkers ¹¹⁰ ' who found that the addition of even low dopant concentrations can induce a marked reduction of the silica primary particle size and largely attributed this influence to the influence of dopants on the silica sintering properties .

The solid diamonds in Figure 3 represent the diameter (dxpj}) of the silver crystallites as evaluated using the fundamental parameter method ¹⁸¹ on the 38.1° silver peak from the XRD patterns . Estimates for the diameter of the silver crystallites were obtained only for dopant concentrations between 2 and 5 at . % with the Ag size increasing from 22 to 34 nm over this range . These crystallite sizes were larger than the BET estimate for silica primary particle size suggesting that the silver crystallites were surrounded by an agglomerate matrix of the more numerous silica particles .

Eκample 4 : Transmission electron microscope (TEM) images

Transmission electron microscope (TEM) images of the silver-doped silica powder ( 5 at . % Ag) are shown in Figure 4. The left hand image marked a shows a typical morphology observed for the silver-doped silica powder .

The material consisted of metallic silver particles with a range of observed sizes embedded within a matrix of amorphous silica . The silver particles were found to be randomly dispersed within the silica matrix with most particles surrounded by silica, however some silver particles appeared at the edge of the silica agglomerate indicating that they were exposed at the surface .

Right hand image marked b shows a view of the as-produced powder that features a large (>50 run) silver particle coated in a silica shell with a thickness of approximately 20 inn. This large feature is shown in close association with a silver-silica agglomerate similar to that observed in image a. This combination of silver- silica agglomerates and silica-coated silver morpholo- gies, i . e . the simultaneous presence of embedded, ^' surface exposed and large silica-coated silver particles , is desirable and suggests two formation pathways ( see above) .

Example 5 : Scanning transmission electron microscope (STEM) image

Figure 5 shows a representative scanning transmission electron microscope (STEM) image of silver- doped silica powder (5 at . % Ag) . The STEM image once again illustrates the morphology of silver particles embedded within the amorphous silica matrix in agreement with the TEM images shown in Figure 4. The silver particles in the STEM image appear with much higher contrast compared to TEM, however with the bright features in the image being the metallic silver particles . The diffuse light gray areas of the image are indicative of the amorphous silica surrounding the silver particles .

Eκample 6 : Energy dispersive x-ray spectroscopy (EDXS ) analysis

Energy dispersive x-ray spectroscopy (EDXS ) analysis of specific spots was carried out with a Phil- lips CM30ST microscope (LaB5 cathode , 300 kV) . Said specific spots were points a , b and c indicated within the STEM image . The spectra associated with each point are given in the inset graph in Figure 5. Point a is dominated by silver ( signal at ca . 3 keV) consistent with a surface exposed particle . Points b and c show silver together with silica ( signal at ca . 1.7 keV) and oxygen ( signal at ca . 0.5 keV) indicating that these silver particles are embedded within the silica matrix . The high signal for silver, together with the absence of oxygen in the EDXS signal for point a , indicates the formation of metallic silver rather than oxides , which is also in agreement with the XRD analysis .

Example 7 : Antibacterial Performance

The antibacterial performance of the silver- doped silica powders was evaluated in a qualitative manner by dispersing 1 mg/ml of each powder ( 0 to 5 at . % Ag) within a liquid culture of Escherichia coli ( E. coli is a gram-negative bacteria) and visually monitoring the turbidity ( turbid appearance indicates presence of bacterial population) . After 24 hours at room temperature the 0 , 1 and 2 at . % Ag samples had insufficient strength to inhibit the growth of the bacteria colonies . The 3 at . % Ag sample inhibited growth in the initial 24 hours followed by a slow increase in bacteria population . The samples with 4 and 5 at . % Ag completely prevented the further growth of the bacterial population . Although qualitative in nature , these observations clearly indicate the anti- microbial activity of the silver-doped silica powders produced using FSP .

Thus , the observed combination of embedded, surface exposed and large silica-coated silver particles is quite desirable for antimicrobial applications . Antimicrobial activity is assumed to be derived from the de- livery of silver ions to bacteria and so the presence of surface exposed silver in these powders allows ready availability of silver ions and therefore high initial antimicrobial activity . However, the presence of embedded silver particles acts as a reservoir of silver for long- term performance with silver ions diffusing to the outside of the agglomerates to maintain activity of the powder over extended times . Furthermore, the presence of large silica-coated silver particles also augments this reservoir effect . While the presence of the large silver- particles is of benefit for antimicrobial activity, for other applications a powder with a higher proportion of silver-silica agglomerate structures may be more desirable, e . g . for applications requiring transparency or minimal powder color .

While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and prac- ticed within the scope of the following claims .

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