| United States Patent Application |
20040007539
|
| Kind Code
|
A1
|
|
Denes, Ferencz S.
; et al.
|
January 15, 2004
|
Method for disinfecting a dense fluid medium in a dense medium plasma
reactor
Abstract
A method for disinfecting water and other dense fluid media containing
microorganisms is carried out in a dense media plasma reactor. The plasma
reaction in the reactor produces reactive species, such as electrons,
ions, and free radicals that promote the inactivation of the
microorganisms. In various embodiments, the plasma reaction also sputters
off minute antimicrobial particles of the electrically conducting
material from which the electrodes are made.
| Inventors: |
Denes, Ferencz S.; (Madison, WI)
; Manolache, Sorin O.; (Madison, WI)
; Lee Wong, Amy C.; (Madison, WI)
; Somers, Eileen B.; (Madison, WI)
|
| Correspondence Name and Address:
|
FOLEY & LARDNER
150 EAST GILMAN STREET
P.O. BOX 1497
MADISON
WI
53701-1497
US
|
| Serial No.:
|
194751 |
| Series Code:
|
10
|
| Filed:
|
July 12, 2002 |
| U.S. Current Class: |
210/748; 210/764 |
| U.S. Class at Publication: |
210/748; 210/764 |
| Intern'l Class: |
C02F 001/46 |
Goverment Interests
[0001] This invention was made with United States government support
awarded by the following agencies: United States Department of
Agriculture/Forest Service 99-RCRA-3331. The United States government has
certain rights in this invention.
Claims
What is claimed is:
1. A method of disinfecting a dense fluid medium containing at least one
microorganism, the method comprising: (a) providing a reaction vessel for
containing a dense fluid medium, the dense fluid medium containing at
least one microorganism; (b) charging the dense fluid medium into the
reaction vessel; (c) providing a first electrode comprising a first
conductive material, the first electrode immersed within the dense fluid
medium; (d) providing a second electrode comprising a second conductive
material, the second electrode immersed within the dense fluid medium and
disposed opposite the first electrode; (e) stirring the dense fluid
medium between the first and second electrodes; (f) applying an electric
potential between the first electrode and the second electrode to create
a discharge zone comprising a plurality of discharges wherein the
electric potential is high enough to produce reactive species in the
dense fluid medium; and (g) exposing the microorganisms in the dense
liquid medium to the reactive species in the dense fluid medium for a
time sufficient to at least partially inactivate the microorganisms.
2. The method of claim 1 wherein the reactive species are selected from
the group consisting of electrons, ions, free radicals, and mixtures
thereof.
3. The method of claim 1 wherein the microorganism is a bacteria.
4. The method of claim 1 wherein the bacteria is a spore.
5. The method of claim 1 wherein the first electrode is a rotating
electrode and the second electrode is a stationary electrode.
6. The method of claim 1 wherein the second electrode is hollow and
includes at least one conduit for passage of the dense fluid medium.
7. The method of claim 5 wherein the first electrode rotates at a rate
sufficient to cause cavitation in the dense fluid medium.
8. The method of claim 5 wherein the first electrode rotates at a speed of
up to about 5000 RPM.
9. The method of claim 5 wherein the first electrode rotates at a speed of
at least about 1000 RPM.
10. The method of claim 1 wherein at least one of the first conductive
material or the second conductive material comprises an electrical
conductor selected from the group consisting of metals, carbon or
combinations thereof.
11. The method of claim 1 wherein at least one of the first conductive
material or the second conductive material comprises an electrical
conductor selected from the group consisting of aluminum, antimony,
bismuth, carbon, copper, gold, iron, lead, molybdenum, nickel, platinum,
silver, tin, tungsten, zinc, stainless steel, rare earths or combinations
thereof.
12. The method of claim 1 wherein at least one of the first conductive
material or the second conductive material comprises titanium.
13. The method of claim 1 wherein at least one of the first conductive
material or the second conductive material comprises iron.
14. The method of claim 1 wherein at least one of the first conductive
material or the second conductive material comprises stainless steel.
15. The method of claim 1 including the step of passing a gas through the
discharge zone.
16. The method of claim 1 wherein the first electrode and the second
electrode each comprise at least one planar surface, wherein the planar
surface of the first electrode is substantially parallel to the planar
surface of the second electrode.
17. The method of claim 16 wherein the substantially planar parallel
surfaces are separated by a gap of about 1 mm.
18. The method of claim 16 wherein the first electrode comprises at least
one pin made of the first conducting material, the pin projecting from
the planar surface of the first electrode towards the planar surface of
the second electrode.
19. The method of claim 16 wherein the first electrode comprises multiple
pins made of the first conducting material, the pins projecting from the
planar surface of the first electrode towards the planar surface of the
second electrode.
20. The method of claim 19 wherein the pins are arrayed in a spiral
pattern.
21. The method of claim 1 wherein the electric potential is between 100
and 800 volts.
22. The method of claim 1 wherein the electric potential is between 100
and 250 volts.
23. The method of claim 1 wherein the electric potential is about 200
volts.
24. The method of claim 1 wherein at least one of the first conductive
material or the second conductive material comprise a material having
antimicrobial properties and further wherein the electric potential
between the first electrode and the second electrode is sufficient to
dislocate antimicrobial nanoparticles from the electrode comprising the
material having antimicrobial properties.
25. The method of claim 24 wherein the material having antimicrobial
properties is silver.
26. The method of claim 1 further including mixing a colloidal suspension
of antimicrobial nanoparticles with the dense fluid medium.
27. A method of disinfecting a dense fluid medium containing at least one
microorganism, the method comprising: (a) providing a first reaction
container for containing a dense fluid medium, the dense fluid medium
containing at least one microorganism; (b) charging the dense fluid
medium into the first reaction container; (c) providing a first electrode
comprising a first conductive material, the first electrode immersed
within the dense fluid medium and housed in the first reaction container;
(d) providing a second electrode comprising a second conductive material,
the second electrode immersed within the dense fluid medium and disposed
opposite the first electrode in the first reaction container; (e)
applying an electric potential between the first electrode and the second
electrode to create a discharge zone comprising a plurality of discharges
wherein the electric potential is high enough to produce reactive species
in the dense fluid medium; (f) providing a second reaction container for
containing a dense fluid medium, the second reaction container connected
to and in fluid communication with the first reaction container through
an inlet port; (g) changing the dense fluid medium between the first
reaction container and the second reaction container; (h) providing a
third electrode comprising a third conductive material, the third
electrode immersed within the dense fluid medium and housed within the
second reaction container; (i) providing a fourth electrode comprising a
fourth conductive material, the fourth electrode immersed within the
dense fluid medium and disposed opposite the third electrode in the
second reaction container; wherein at least one of the third conductive
material or the fourth conductive material comprises a material having
antimicrobial properties and, further, wherein the electric potential
between the third electrode and the fourth electrode is high enough to
dislocate antimicrobial nanoparticles from the at least one conductive
material having antimicrobial properties.
Description
FIELD OF THE INVENTION
[0002] The present invention relates to methods for disinfecting water and
other dense fluid media in a dense medium plasma environment.
BACKGROUND OF THE INVENTION
[0003] Decontamination and disinfection of potable water, water used in
food-processing industries, and water frequently in contact with human
beings (e.g. water in swimming pools and spa pools), are major health
issues currently under intense scrutiny due to heightened awareness.
Disinfection is defined as the killing or inactivation of disease-causing
organisms. The levels to which microbial colony forming units are
permitted in various waters fit for human contact is carefully regulated.
Conventional approaches employed for the inactivation of toxins, such as
hydrolysis, electrochemical oxidation, solvated electron technology,
plasma arcs, and chemical treatments are complex processes with
significant limitations related to the generation of toxic side-products
or low efficiencies for large scale applications.
[0004] Technologies based on atmospheric pressure plasma environments
present an alternative approach to the disinfection of water. However,
most of the processes available today were developed for low pressure
environments, which are plagued by the need for complex and expensive
vacuum systems, batch-type processing, and difficult robotics handling.
These characteristics make conventional plasma technologies economically
viable only for applications where the economies of scale processing are
targeted toward the creation of high value-added items.
[0005] Gas phase discharges have been studied extensively for their
ability to sterilize microorganism-contaminated solid surfaces. However,
technologies for decontaminating fluids, and water in particular, are
considerably less developed. The destruction of living cells, such as
Saccharomyces cerevisiae (yeast cells) and Bacillus natto, has been
studied in pulsed high voltage cylindrical discharge reactors in various
electrode configurations. These studies show that yeast cell populations
in deionized water can be destroyed using a wire-cylinder electrode
configuration under 20 kV/cm, 140 .mu.s pulse width, and 250 Hz pulse
frequency conditions.
[0006] The pulsed high-voltage discharge-mediated formation of chemical
species and their effects on microorganisms has also been studied. Using
a needle-plate electrode configuration, the formation of .OH and .H free
radicals has been monitored by Optical Emission Spectroscopy. The studies
indicated that .OH and .H free radicals generated in situ by a discharge
were not effective at killing yeast cells, although the H.sub.2O.sub.2
generated by the discharge added ex situ to a contaminated sample could
be used to kill the cells.
[0007] Unfortunately, these pulse discharge experiments for
decontaminating water employed a high voltage, pulsed discharge which
generated filamentary non-stationary discharge channels, resulting in
reactions having a very localized character, which tends to limit the
effectiveness of the reactions for inactivating microorganisms.
[0008] Another approach to the disinfection of microorganism-contaminated
water employs antimicrobial nanoparticles. Nanoparticles are important
components in the development of catalytic, sensor, aerosol, filter,
biomedical, magnetic, dielectric, optical, electronic, structural,
ceramic and metallurgical applications. Nanoscale metallic particles
exhibit volume and surface effects which are absent in the same material
with dimensions in the micron range (i.e., 0.1 micron<particle
diameter<1 micron).
[0009] The use of colloidal suspensions of silver as antimicrobial agents
is well known. Such use is resuming increased importance as antibiotic
resistant bacteria become more prolific. Minimizing the silver particle
sizes is believed to be important both from the stability of the
colloidal suspension and for the efficacy against microbes.
[0010] Various processes to produce nanoparticles are known in the prior
art. For example, U.S. Pat. No. 5,543,133, issued to Swanson et al.,
discloses a process of preparing nanoparticulate agents comprising the
steps of: (i) preparing a premix of the agent and a surface modifier;
and, (ii) subjecting the premix to mechanical means to reduce the
particle size of the agent, the mechanical means producing shear, impact,
cavitation and attrition.
[0011] Likewise, U.S. Pat. No. 5,585,020, issued to Becker et al., teaches
a process of producing nanoparticles having a narrow size distribution by
exposing microparticles to an energy beam such as a beam of laser light,
above the ablation threshold of the microparticles.
[0012] Also, U.S. Pat. No. 5,879,750, issued to Higgins et al., teaches a
process for producing inorganic nanoparticles by precipitating the
inorganic nanoparticles by a precipitating agent for a microemulsion with
a continuous and a non-continuous phase and concentrating the
precipitated nanoparticles employing an ultrafiltration membrane.
[0013] Additionally, U.S. Pat. No. 6,540,495, issued to Markowicz et al.,
teaches a process for making a powder containing metallic particles
comprising the steps of: (i) forming a dispersion of surfactant vesicles
in the presence of catalytic metal ions; (ii) adjusting the pH to between
5.0 and 7.0; (iii) mixing the dispersion with a bath containing second
metal ions; and; and, (iv) incubating the mixed dispersion at a
temperature sufficient to reduce the second metal ions to metal particles
having an average diameter between 1 to 100 nm.
[0014] CS Pro Systems advertises a high voltage AC processor producing
nanoparticles of
colloidal silver. The HVAC process is claimed to produce
particle sizes between 0.002 to 0.007-9 microns by imposing an AC
potential of 10,000 volts across two silver electrodes in a distilled
water medium.
[0015] The production of large quantities of
colloidal silver solutions
required for industrial applications, such as water treatment or
treatment of biological fluids, are not economical by using the
electrolytic approach.
[0016] The prior art methods do not provide simple, convenient, low-cost
methods for disinfecting water, and other dense media, contaminated with
undesirable microorganisms.
SUMMARY OF THE INVENTION
[0017] One aspect of the invention provides a method for disinfecting a
dense fluid medium, such as water, containing at least one undesirable
microorganism. The method uses multiple spark discharges to inactivate
the microorganisms in an intensely stirred liquid medium. The method
comprises the steps of: providing a reaction vessel for containing a
dense fluid medium containing at least one microorganism; charging the
dense fluid medium into the reaction vessel; providing a first electrode
comprising a first conductive material, the first electrode immersed
within the dense fluid medium; providing a second electrode comprising a
second conductive material, the second electrode immersed within the
dense fluid medium and disposed opposite the first electrode; stirring
the dense fluid medium between the first and second electrodes; applying
an electric potential between the first electrode and the second
electrode to create a discharge zone comprising a plurality of discharges
to produce reactive species in the dense fluid medium; and exposing the
microorganisms in the dense fluid medium to the reactive species in the
dense fluid medium for a time sufficient to at least partially inactivate
the microorganisms. The reactive species include electrons, ions, free
radicals, and mixtures thereof which are capable of interacting with the
microorganism to promote the inactivation of the microorganism. In a
preferred embodiment, the first electrode is a rotating electrode and the
second electrode is a static electrode. In this embodiment the dense
fluid medium is stirred by the rotating motion of the first electrode.
[0018] Another aspect of the invention provides a method for disinfecting
a dense fluid medium containing at least one microorganism using
antimicrobial colloidal nanoparticles generated in a dense medium plasma
(DMP) environment through multiple spark discharges in an intensely
stirred liquid medium. The steps in this method are substantially the
same as those described above, however, in this aspect of the invention
at least one of the first conductive material or the second conductive
material comprise a material having antimicrobial properties and the
electric potential between the first electrode and the second electrode
is sufficient to dislodge or dislocate antimicrobial nanoparticles from
that material. A particularly preferred material having antimicrobial
properties is silver.
[0019] Yet another aspect of the invention provides a two-step method for
disinfecting a dense fluid medium containing at least one undesirable
microorganism. In the first step of the two step method the dense fluid
medium containing the at least one microorganism is exposed to reactive
species created by multiple spark discharges in an intensely stirred
medium. The reactive species are allowed to react with the microorganism
to at least partially inactive the microorganism. The method for carrying
out this first step has been described above. Briefly, a dense fluid
medium containing at least one microorganism is disposed between two
electrodes. The medium is stirred between the electrodes and an electric
field sufficient to produce multiple spark discharges is applied between
the electrodes to produce reactive species that interact with the at
least one microorganism to promote its inactivation. In the second step
of the two step method, the dense fluid medium containing the at least
one microorganism is exposed to antimicrobial colloidal nanoparticles.
[0020] The second step of the process may be accomplished by mechanically
mixing a solution containing antimicrobial nanoparticles, which may be a
colloidal suspension, into the dense fluid medium. Such a solution may be
produced by conventional means well known in the art or may be produced
using a dense medium plasma reactor, as described in greater detail
below. The mixing may take place before, during, or after the dense fluid
medium has been exposed to the reactive species created by the multiple
spark discharges in the first step of the process. Alternatively, the
antimicrobial nanoparticles can be formed within the dense fluid medium
by exposing the dense fluid medium to multiple spark discharges between a
first and a second electrode, at least one of which is comprised of a
material having antimicrobial properties.
[0021] In the two-step method, the first step and the second step may take
place simultaneously or in tandem. For example, where at least one of the
first or second electrodes is made from a material having antimicrobial
properties and the voltage between the first and the second electrodes is
high enough to dislocate antimicrobial nanoparticles from that electrode,
reactive species and antimicrobial nanoparticles will be formed
simultaneously in the same reaction vessel. Alternatively, the two step
process may be carried out in a dual-stage dense medium plasma reactor
having separate reaction stages, or containers, housed within a single
reaction vessel which may be connected in parallel or, preferably, in
series, to facilitate continuous production of the colloidal dispersion.
[0022] When carried out in a dual-stage dense plasma reactor, the method
includes the steps of: providing a first reaction container for
containing a dense fluid medium containing at least one microorganism;
charging the dense fluid medium into the first reaction container,
providing a first electrode comprising a first conductive material, the
first electrode immersed within the dense fluid medium and housed within
the first reaction container; providing a second electrode comprising a
second conductive material, the second electrode immersed within the
dense fluid medium and disposed opposite the first electrode within the
first reaction container; applying an electric potential between the
first electrode and the second electrode to create a discharge zone
comprising a plurality of discharges, wherein the electric potential
between the first and the second electrodes is high enough to produce
reactive species in the dense fluid medium; providing a second reaction
container for containing a dense fluid medium containing at least one
microorganism the second reaction container connected to and in fluid
communication with the first reaction container through an inlet port;
charging the dense fluid medium between the first reaction container and
the second reaction container; providing a third electrode comprising a
third conductive material, the third electrode immersed within the dense
fluid medium and housed within the second reaction container; providing a
fourth electrode comprising a fourth conductive material, the fourth
electrode immersed within the dense fluid medium and disposed opposite
the third electrode within the second reaction container; applying an
electric potential between the third electrode and the fourth electrode
to create a discharge zone, wherein at least one of the third conductive
material or the fourth conductive material comprises a material having
antimicrobial properties; and further wherein the electric potential
between the third and the fourth electrodes is high enough to dislocate
antimicrobial nanoparticles from the electrode comprising the material
having antimicrobial properties.
[0023] It should be understood that in the two-step process described
above, the flow of the dense fluid medium may be from the first container
(i.e. the container wherein reactive species are created) to the second
container (i.e. the container wherein the antimicrobial species are
created) or vice versa. Thus the phrase "charging the dense fluid medium
between the first reaction container and the second reaction container"
does not limit the flow of the dense fluid medium to one direction or the
other, but merely indicates that the fluid is moving or circulating
between the containers.
[0024] Still another aspect of the invention provides a method for
producing a colloidal dispersion of nanoparticles of at least one
conductive material in a dense fluid medium. The production of
nanoparticles, and in particular antimicrobial nanoparticles, in this
manner is well-suited for use with applications for disinfecting water,
and other dense fluid media, contaminated with microorganisms. The method
is based on the operation of a modified dense medium plasma reactor,
which allows the initiation of multiple spark discharges in a very
intensely stirred liquid medium. The method comprises the steps of:
providing a reaction vessel for containing the dense fluid medium;
charging the dense fluid medium into the reaction vessel; providing a
first electrode comprising a first conductive material, the first
electrode immersed within the dense fluid medium; providing a second
electrode comprising a second conductive material, the second electrode
immersed within the dense fluid medium and being near to the first
electrode; stirring the dense fluid medium between the first and second
electrodes; and imposing an electric potential between the first
electrode and the second electrode to create a discharge zone, the
electric field between the electrodes being sufficiently high to
dislocate nanoparticles of at least one of the first conductive material
or second conductive material from the respective electrode. Preferably,
the electrodes are easily interchanged to facilitate changeover between
dispersions. In a preferred embodiment, the first electrode is a rotating
electrode and the second electrode is a static electrode. In this
embodiment the dense fluid medium is stirred by the rotating motion of
the first electrode.
[0025] An exemplary dense phase plasma discharge apparatus suitable for
use with the invention may include a chamber forming a reaction vessel
for the dense medium. A first electrode is mounted for a rotation about
an axis in the chamber and has an end piece which is formed of conductive
material with a planar surface. A plurality of pins are mounted in an
array projecting from the planar surface. A second electrode is mounted
in the chamber and has an end piece of conductive material with a planar
surface, with the planar surfaces of the end pieces of the first and
second electrodes separated from each other by a gap. The end pieces of
the first and second electrodes, including the pins on the one end piece,
may be formed of silver for efficiently producing
colloidal silver. A
motor may be coupled to the first electrode to selectively drive the
first electrode in rotation. A magnetic coupling system may be utilized
to couple the drive from the motor to the rotating electrode. The pins in
the electrode are preferably formed in a spiral array. Rapid rotation of
the electrode with the pins therein creates vigorous mixing and
cavitation of the dense medium, such as water, between the upper and
lower electrodes to enhance the action of the discharges taking place
between the electrodes and thereby enhance the production of
nanoparticles dislodged from the electrodes from the discharge.
[0026] Utilization of the method of the invention with silver electrodes
may be used to produce
colloidal silver which is highly effective as a
bactericide and can be used for controlling viruses, spores, and other
undesirable microorganisms.
[0027] Further objects, features and advantages of the invention will be
apparent from the following detailed description when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 shows a schematic representation of a dense medium plasma
reactor suitable for use in this invention.
[0029] FIG. 2 shows a multistage dense medium plasma reactor suitable for
use in the continuous disinfection of a contaminated dense fluid medium.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0030] As used herein, the term "nanoparticle" refers to particles having
an average diameter of less than about 100 nm, preferably less than about
50 nm, more preferably less than about 20 nm, most preferably less than
about 10 nm.
[0031] As used herein, the terms "electrically conducting material,"
"conductive material" or "conductor" are interchangeable and refer to a
class of bodies incapable of supporting electric strain such that a
charge given to a conductor spreads to all parts of the body of the
conductor.
[0032] The term "dense medium" refers to materials that are liquid at the
operating conditions in the plasma reactor.
[0033] The term "antimicrobial nanoparticle" refers to nanoparticles that
play a role in the inactivation of microorganisms. The nanoparticles may
be capable of reacting directly with the microorganisms to promote the
inactivation of the microorganisms, or they may act as catalysts or
photo-catalysts for reactions between other reactive species and the
microorganisms.
[0034] The term "plasma" is used to identify gaseous complexes which may
comprise electrons, positive or negative ions, gaseous atoms and
molecules in the ground state or any higher state of excitation including
light quanta. The most common method for achieving a plasma state is
through an electrical discharge. Electrical discharge plasmas are either
"hot," i.e., thermal, or "cold," i.e., nonthermal.
[0035] "Hot" (thermal plasma) plasmas comprise gas atoms and electrons
which are essentially close to thermal equilibrium with each other. "Hot"
plasmas are produced from electrical arcs, plasma jets, and magnetic
fields. "Hot" plasmas produced from electrical arcs and plasma jets
require equilibrium conditions in which the gas and electron temperatures
are very high (5.times.10.sup.3K) and nearly identical. As a result, most
organic molecules and polymers cannot be treated under these conditions
because they would be rapidly degraded.
[0036] There are also several disadvantages associated with hot plasma
synthesis. One main disadvantage is the presence of elevated temperatures
of the gas phase and substrate. The elevated temperature requirement
limits plasma synthesis to organic reactions, limits high molecular
weight depositions, and creates a requirement for costly handling
equipment due to the high temperatures generated. Other disadvantages
include the predominance of decomposition reactions and the production of
non-recyclable gas compositions generated from undesired secondary gas
phase recombination reactions.
[0037] "Cold" plasmas, which are not at thermal equilibrium, comprise gas
atoms at room temperature and electrons at much higher temperatures. This
plasma state provides an ambient gas temperature along with electrons
which have sufficient kinetic energy to cause the cleavage of chemical
bonds. As a result, "cold" plasmas are highly suitable for chemical
reactions, such as organic synthesis, polymerizations, surface
treatments, and grafting reactions, which involve thermally sensitive
materials. "Cold" plasmas are characterized by average electron energies
of 1-20 electron Volts and electron densities of 10.sup.9 to 10.sup.12
cm.sup.-3. There are two types of "cold" plasmas: 1) the low pressure
glow types which are produced by alternating current, direct current, or
radio frequency discharges; and 2) the high pressure corona type and
barrier discharges produced at electrodes during a high-voltage
discharge.
[0038] The inventors have found that water, and other dense fluid media,
contaminated with undesirable microorganisms can be disinfected by
exposing the dense and intensely stirred fluid medium to a plasma
reaction between two electrodes comprising electrically conductive
materials. Moreover, the inventors have discovered that the disinfection
is optimized when the fluid medium contains antimicrobial nanoparticles.
[0039] In this method, two electrodes comprising conductive materials are
submerged in a dense fluid that is contaminated with at least one
microorganism and the dense fluid is stirred vigorously between the
electrodes. An electric potential is applied between the two electrodes
to produce multiple plasma discharges. The microorganisms in the dense
fluid are inactivated by the exposure to the discharges and the reactive
species created by the discharges. Optionally, the microorganisms may
also be exposed to antimicrobial nanoparticles. These nanoparticles may
be mechanically mixed into the dense fluid medium or they may be produced
in the medium itself if at least one of the two electrodes comprises a
material having antimicrobial properties and the electric potential
applied across the electrodes is sufficiently high to dislocate
nanoparticles from the at least one electrode comprising a antimicrobial
material. The method may also be carried out in two steps wherein the
dense medium containing the microorganisms is exposed to multiple plasma
discharges and the reactive species that are created by those discharges
in a first step, and exposed to antimicrobial nanoparticles in a second
step. The two steps preferably take place in separate reaction containers
that are connected either in series or in parallel in a single reaction
vessel, such that the dense fluid medium may be circulated from one
container to the other. Similarly, the process may take place in a
multistage systems having three or more reaction containers connected in
series or parallel, each of which may contain two electrodes comprising
materials chosen for their specific conductive or antimicrobial
properties.
[0040] Without wishing to be bound to any particular theory, the inventors
believe that the inactivation of the microorganisms is due, at least in
part, to the interaction between the microorganisms and reactive species
created by the discharge, such as electrons, ions, and free radicals,
including .O, .OH, and .OOH radicals. In addition, the inactivation may
be promoted by the physical effects of the discharge itself on the
microorganisms, for example, the application of an electric field or
shock waves created by a discharge may cause cell membranes to breakdown.
When antimicrobial nanoparticles are present in the dense medium, the
inactivation is further promoted by the interaction or reaction between
the particles and the microorganisms.
[0041] The methods according to the present invention are particularly
well suited for the disinfection of dense fluids containing bacteria, and
more specifically bacteria in the form of spores. Certain bacteria can
exist both in the vegetative state and as spores. Spores are dormant
states that bacteria assume during nutritionally unfavorable conditions.
Spores are widespread in nature because they can survive for long periods
and can be dispersed readily by water, wind, and hosts such as animals
and insects. In the dormant state, spores undergo no detectable
metabolism and exhibit a higher degree of resistance to inactivation by
wet and dry heat, freezing, ultraviolet and gamma radiation, extreme
desiccation, and oxidizing agents. As a result, harsher processes are
required to inactivate spores in food and water in comparison to
vegetative microorganisms.
[0042] It is suggested that the longevity of spores is related to their
capacity to protect their critical cellular components from damage and to
repair or replace damaged components during germination. This incredible
viability can be attributed to the specific structures, characteristics
and reaction mechanisms associated with the genetic makeup of sporulating
species, including spore coat and coat structure.
[0043] The protection of the nucleoid of a spore by a multilayer coat,
cortex, and a low permeability core make access difficult for germicidal
agents and other wet chemical disinfection agents. To circumvent this
problem, conventional techniques have employed strong penetrating
electromagnetic radiation, including ultraviolet and gamma radiation, to
break down the spore's protective shell prior to attack by inactivating
chemicals. The inventors believe that the present invention overcomes the
difficulties of spore inactivation by using a multistage attack wherein
plasma discharges are used to physically breakdown the protective outer
layers of the spore while reactive species, and optionally, antimicrobial
nanoparticles are used to attack the critical cellular components of the
spore.
[0044] The inventors have also found that colloidal suspensions of
antimicrobial nanoparticles of electrically conductive materials can be
produced by generating a plasma reaction between two electrodes,
comprising the desired electrically conductive material(s), which are
immersed within a dense fluid medium. Preferably, the dense medium is
rapidly recirculated between the two electrodes.
[0045] The colloidal nanoparticle dispersion is produced by fine
sputtering particles of the electrically conducting material from the
electrodes, by the multitudes of (DC or AC) discharges initiated and
sustained between the rotating and the stationary electrodes, into the
intensely stirred dense medium, which is preferably water.
[0046] Advantageously, the electrodes can comprise any desired
electrically conductive material. Typically, electrically conducting
materials usable in this invention include metals, carbon or combinations
thereof. Specific examples of suitable metals include aluminum, antimony,
bismuth, copper, gold, iron, lead, molybdenum, nickel, platinum, silver,
tin, tungsten, zinc or the rare earths (group IIIB (lanthanide series) of
the periodic table as published in Hawley's Condensed Chemical
Dictionary, 12.sup.th ed., front cover), such as titanium. Combinations
or alloys of the conductive materials, such as stainless steel, are
encompassed within the scope of the current invention. In various
embodiments the conductive material is substantially pure. "Substantially
pure" means that the resulting nanoparticles have sufficient purity for
the desired use. In these embodiments, the electrically conducting
material should be at least about 90, preferably at least about 95, more
preferably at least about 99 percent pure.
[0047] In embodiments wherein the plasma discharge system is to be used to
produce antimicrobial nanoparticles within the dense fluid medium during
the disinfection process, the electrodes are made from a metal having
antimicrobial properties. Silver electrodes are particularly well-suited
for this application, as it is well-known that silver nanoparticles
possess bactericidal properties.
[0048] In embodiments wherein disinfection of a dense fluid medium is
conducted in multiple steps including the step of exposing the dense
fluid medium to reactive species created by multiple spark discharges and
the step of exposing the dense fluid medium to antimicrobial
nanoparticles created by multiple spark discharges, the electrodes used
to carry out each step should be selected according to their ability to
produce reactive species and antimicrobial nanoparticles, respectively.
Electrode materials that are well suited for use in inactivating
microorganisms through the production of reactive species include iron,
titanium, and stainless steel. Electrode materials that are well suited
for use in inactivating microorganisms through the production of
antimicrobial nanoparticles include silver.
[0049] Preferably, the electrodes are constructed of an electrically
conducting material which is substantially inert to the dense medium.
"Substantially inert" means that the electrically conducting material
does not react with or dissolve at undesirably high rates under the
conditions present during the plasma reaction. If desired, the electrodes
may be made from different materials in order to produce a colloidal
suspension of more than one electrically conducting material.
[0050] Preferably, the electrodes are constructed so as to be easily
removed and installed. This easy interchangeability facilitates replacing
worn electrodes or changing electrodes to accommodate the production of
different colloidal dispersions.
[0051] The dense fluid medium may be any liquid having a viscosity low
enough to permit rapid circulation of the fluid between the two
electrodes. The present invention has important implications in the field
of water disinfection, and therefore the dense medium will typically be
water. However, the invention may also be used disinfect other
microorganism-contaminated fluids where desirable.
[0052] The plasma reaction will decompose the molecules of the dense
medium into highly reactive free radicals. As such, the reaction products
formed from the dense medium free radicals may be final reaction products
or undesirable contaminants to the colloidal solution. An undesirable
by-product is any compound that must be removed, due to technical,
practical or aesthetic reasons, from the colloidal dispersion prior to
use. In most cases, when inorganic/organic hybrid nanoparticle systems
are prepared, liquid phase organometallic compounds can be used. In a
preferred embodiment, the conductive materials of the electrodes, the
dense fluid medium, and the plasma reactor parameters (i.e., voltage,
current, angular speed of any rotating electrode, and gas flow rate) can
be selected such that metallic, metal-oxide- and inorganic/organic
hybrid-nanoparticles are produced simultaneously in a controlled manner.
The dense medium can be typically selected to avoid the production of
undesirable by-products. In the preferred embodiment, the decomposition
reaction products of water (H.sup.+ and OH.sup.-) readily react with each
other to reform the water molecule. In contrast, the decomposition
reaction products of other dense media, e.g. benzene, are free radicals
which may initiate polymerization reactions.
[0053] When sole (non-hybrid) nanoparticle systems are required, the
plasma should not generate byproducts from dense medium. Preferably, any
reaction between the dense medium and the electrically conductive
material is slow enough that the nanoparticles in the colloidal
dispersion have the desired shelf-life. Most preferably, the dense
medium, and any minute quantity of by-product, is non-reactive with the
electrically conducting material. "Non-reactive" means that the dense
medium and the nanoparticle material do not combine to form a new
compound under the operating conditions of the plasma reactor. Examples
of usable dense medium liquids include organic solvents, silicon
tetrachloride, isobutylene, etc. (in applications where by-products are
not undesirable), and water, preferably water.
[0054] The electrodes and the dense fluid medium are located within any
suitable containment means. The containment means may be open or closed,
preferably closed, more preferably a closed pressure vessel. The
containment means optionally has means, such as a vacuum pump, to
evacuate the containment means. Preferably, the containment means may be
pressurized, more preferably pressurized by charging the containment
means with an overpressure of inert gas. Preferably, the containment
means has means, such as ports, valve, pumps, etc., to charge and
discharge the dense medium. The dimensions of the containment means
should be sufficient to prevent loss of the dense medium or colloidal
dispersion and to provide the volume required for the volume of the dense
medium and electrodes. Preferably, the containment means has a size and
shape to accommodate the desired batch size/throughput rate of the dense
medium such that the desired circulation pattern of the dense medium may
be obtained without dead spots.
[0055] Several containment means may be connected in parallel or,
preferably, in series, to facilitate continuous production of the
colloidal dispersion or to facilitate a multistage disinfection process,
as described above. The containment means may be connected through ports
such that the output port of one containment means is connected to the
input port of another containment means, allowing the dense fluid to
circulate between the containment means. Optionally, valves and pumps may
be employed to help circulate the dense fluid and to charge and discharge
the fluid from each containment means.
[0056] The methods of disinfecting dense fluid media and producing
colloidal dispersions according to this invention are conveniently
produced in dense medium plasma reactors. An example of such a reactor is
disclosed in U.S. Pat. No. 5,534,232 which is incorporated herein by
reference. Another example of such a dense medium plasma reactor is shown
in FIG. 1. However, one skilled in the art will recognize that any dense
medium plasma reactor which comprises at least two electrodes, means for
rapidly recirculating the dense medium between the two electrodes and,
optionally, means to provide bubbles within the dense medium circulating
between the electrodes is suitable for use in this invention.
[0057] A preferred apparatus for disinfecting a dense medium and producing
colloidal nanoparticles in a dense medium is illustrated generally in
FIG. 1. The apparatus of FIG. 1 includes a DC power supply 1, an
evacuation tube 2 for gasses, a coolant exit tube 3 and an inlet tube 26,
outer and inner glass cylinders 4 and 7 which form part of the chamber
for the reaction vessel, an electrical brush contactor 5, and coolant 6
which is enclosed within the volume defined between the outer glass
cylinder 4 and the inner glass cylinder 7. An upper electrode may include
an end piece 8 having an array of conductive pins 23 in a ceramic holder.
The chamber of the reaction vessel is further enclosed by a lower end cap
9 and an upper end cap 17 which are engaged with the glass cylinders 4
and 7 to form an enclosed space for the coolant 6 and an inner chamber
defining the reaction vessel. The lower electrode 10 is a non-rotating
electrode and is mounted with its end piece adjacent to the end piece of
the upper electrode. The lower electrode 10 is electrically connected to
a ground 11. A gas inlet tube 12 formed through the lower electrode
allows introduction of gas through the electrode into the gas space
between the planar faces of the end pieces of the upper and lower
electrodes. A motor 13, e.g., an electric motor, hydraulic motor, etc. is
controlled by a controller 14 (e.g., digital motor controller) and is
coupled via magnetic couplers 15 and 18, which form a magnetic coupling
system to the rotating upper electrode 19. The electrode 19 is conductive
to conduct power from the power supply 1, as transferred thereto by the
brush 5, to the electrically conductive end piece 8 and the pins mounted
thereon. The dense medium is supplied to the reaction vessel through an
inlet tube 16. A quartz enclosure 21 is mounted to the upper electrode as
an isolator to isolate the conductive shaft 19 of the upper electrode
within an enclosed space 20 and seal it off from the liquid medium within
the reaction vessel. A recirculating pump 22 is connected to the inlet
tube 16 to drive recirculation of the liquid medium from the bottom of
the chamber of the reaction vessel to the top. The pins 23 are mounted in
the planar surface of the end piece 8 of the upper electrode and,
preferably in a spiral pattern array as discussed further below, and are
formed of an electrically conductive material such that electrical
discharges (shown for illustration at 24 in FIG. 1) occur between the
pins and the adjacent planar surface of the end piece of the lower
electrode. Channels or conduits 25 are formed in the end piece of the
lower electrode to allow recirculation of fluid through the end piece
into the space between the upper and lower electrodes. A valve 27 is
connected to the supply line 16 to allow discharge of the fluid medium
within the chamber after treatment has been completed.
[0058] As noted above, the reactor of FIG. 1 is composed of a cylindrical
glass chamber 7, which functions as the reaction vessel, provided with
two, stainless steel, upper and bottom caps 9, 17, and a cooling jacket
4. The rotating, cylindrical stainless steel, upper electrode 19 is
equipped with the quartz jacket 21 for avoiding the penetration of the
reaction media to the electrode sustaining central shaft and bearings.
The upper electrode has a cylindrically-shaped, disc cross-section end
piece, which is terminated in an interchangeable ceramic pin-array holder
8 for the pins 23. Preferably, the pins are spirally located in the
pin-array. As used herein, "pin" refers to any type or shape of
protuberance extending from the face of the end piece of the electrode.
The lower electrode is hollow, and has also an interchangeable conical
cross-section end piece, and in addition it is provided with channels 25
for the recirculation of the reaction media. Both the pin-array and the
interchangeable metallic part of the lower electrode may be made of
various conductive materials, including pure silver. The distance between
the pin-array and the lower electrode can conveniently be modified by a
micrometric (thimble) screw-system. A typical gap distance is at least
about 0.2, preferably at least about 0.5 mm, up to about 3, preferably up
to about 1 mm. The distance is selected according to the dielectric
constant of the liquid medium. The reactor is vacuum-tight and the
rotation of the upper electrode is assured through a magnetic coupling
system 15, 18 that couples a motor 13 (e.g., an electric motor, hydraulic
motor, etc.) to the upper electrode 19 to selectively drive it in
rotation. The reactor can be operated in batch-type or continuous-flow
modes, depending on the specific application. Reactive or inert gases can
also be introduced into the dense medium during the plasma processes
through the hollow lower electrode. These gases provide bubbles within
the planar gap between the electrodes, thereby facilitating the plasma
reaction. The rotation of the upper electrode is digitally controllable
in the range of 0-5,000 rpm. The rotation of the upper electrode causes
further bubble formation through cavitation of the liquid medium. The
bubble formation (cavitation) is very important to the efficiency of the
dense medium reactor in that the bubbles render a volume-reaction (i.e.,
the reaction occurs within the volume of the bubble) rather than an
interphase reaction between the plasma-generated species and the
microorganisms.
[0059] The contaminated dense fluid media are disinfected as follows.
References are to FIG. 1.
[0060] First, the reaction vessel 7 of the dense medium plasma reactor is
optionally cooled by recirculating cooling agents 6 such as gas or liquid
nitrogen, or cooled alcohol, through the mantle area located between the
glass cylinders 4 and 7 of the double-walled reaction vessel. Next, the
reaction vessel 7 is charged with a dense fluid medium containing at
least one microorganism. Upon charging the reaction vessel 7, a positive
supply of gas is introduced into the reaction vessel 7 through the lower
port 12 contained in the lower static electrode 10. The gas may be
various inert gases, one example of which is argon, or reactive gases,
examples of which are oxygen, ammonia and CF.sub.4. Many other gases may
be used depending on the desired reactions between the electrodes. The
gas travels through the lower hollow shaft of the lower static electrode
10, through the small central opening located at the upper end of the
lower static electrode 10, and then through the dense medium. The gas
then forms a gas blanket over the dense medium. The gas blanket may be
vented to the atmosphere to accommodate the positive supply of gas that
is being introduced through the lower static electrode 10, thereby
maintaining the system at the desired pressure, preferably atmospheric
pressure. Higher pressure conditions combined with low temperatures can
also be used for more volatile dense media such as silicon tetrachloride
and isobutylene.
[0061] Once the system has achieved low temperature and atmospheric
pressure conditions, the upper rotatable electrode 19 is rotated at a
high speed, e.g., at least about 50, preferably at least about 500, more
preferably at least about 1000 up to about 5,000, preferably up to about
2000 rpm, which results in the recirculation 25 of the dense medium. A
direct electric current is then established between the upper planar
electrode face of the upper rotatable electrode 19 and the lower planar
electrode face of the lower static electrode 10. The direct electric
current ignites the plasma reaction. More importantly for this invention
the direct electric current, coupled with the rotation of the upper
electrode, produces multiple electric pulses. These electric pulses
provide the energy to generate reactive species such as electrons, ions,
and free radicals, and to sputter off small particles of the conductive
material from the electrodes. The sputtered particles initially may have
an electric charge but are believed to rapidly lose that charge to
surrounding materials.
[0062] The intense stirring of the dense fluid within the plasma reactor
concentrates the electric field, and allows the generation of multiple
discharges at low voltage and current value. Preferably, the voltage
applied across the electrode faces is in the range of 100 to 800 Volts,
more preferably about 100 to about 250, most preferably about 200 V. The
higher voltage peaks (e.g., 250-300 V) applied to the electrodes at the
starting point (i.e., the first moment of voltage application) decrease
to 100-250 V during the reaction, which is determined by the conductivity
of newly synthesized compounds, and the DC current stabilizes between the
limits of 0.1-4 Amps. This results in a power range of 10-1000 Watts. By
establishing a low electric current to the electrode faces and rapidly
rotating the upper planar electrode face relative to the lower planar
electrode face without touching the stationary lower planar electrode
face, the electric discharge or discharges are initiated in different
locations of the plasma zone, i.e. different locations within the planar
gap, thereby eluding the creation of local caloric energy concentrations.
As a result, the reaction mechanisms produced from the inventive method
and apparatus for forming nanoparticles of conducting particles are
controlled by electron flux intensity rather than thermal energy.
[0063] The dense medium is preferably circulated between the upper and
lower planar electrode faces because the formation of reactive species
and sputtering only occur in the planar gap located between the electrode
faces. The circulation of the dense medium results from the centrifugal
force created by the rotation of the upper planar electrode face relative
to the lower planar electrode face. This centrifugal force causes the
dense medium located between the electrode faces to move radially outward
(away from electrode faces). The radial outward movement of the dense
medium creates vacuum and cavitation effects which draw more dense medium
from within the reaction vessel 7 in the direction of arrows 25 through a
plurality of ports 28 located in the lower static electrode 10, into the
hollow shaft of the electrode 10, and through the openings of the lower
static electrode 10 to the planar gap located between the electrode
faces.
[0064] The rotation of the upper rotatable electrode 19 also aids in
circulating the dense medium contained within the reaction vessel 7. The
same centrifugal force created by rotating the upper planar electrode
face in relation to the lower planar electrode face causes some of the
dense medium located in the planar gap between the electrode faces to
gravitate into the lower portion of the reaction vessel 7. This
gravitation of the dense medium subsequently forces the dense medium to
recirculate from a lower site within the reaction vessel 7 to an upper
site within the reaction vessel 7 via the reactant recirculation line 16
and peristaltic pump 22 which comprises part of the dense medium plasma
reactor. In summary, the centrifugal force created by rotating the upper
rotatable electrode 19 induces a very intense movement and mixing of the
dense medium.
[0065] The rotation of the upper rotatable electrode 19 permits the fast
removal of active species from the plasma zone, i.e., that area which
constitutes the planar gap located between the electrode faces, thereby
inhibiting the development of extensive decomposition reactions. The
rapid circulation also removes the nanoparticles from the area between
the electrodes thereby decreasing the possibility of the nanoparticles
reattaching to the electrodes. In addition, the rotation of the upper
rotatable electrode 19 aids in the achievement of a caloric energy
equilibrium of the dense medium.
[0066] The dense fluid medium may be discharged from the dense medium
plasma reactor by any convenient means. For example, for batch dense
medium reactors, a drain port and valve may be located on the bottom of
the reaction vessel 7 to allow the dense fluid medium to be drained into
any convenient collection vessel. In another example, for a continuous
dense medium plasma reactor, the discharge port may be located on the
upper surface of reaction vessel 7 and the dense fluid medium removed as
overflow from the reactor vessel.
[0067] The temperature of the system and, therefore, the temperature of
the material contained within the reaction vessel 7, may be monitored and
controlled by a thermostat.
[0068] The methods for disinfecting a dense fluid medium or producing
nanoparticles of electrically conducting materials by means of an induced
plasma state may also be carried out as a continuous flow-system
reaction. This can be achieved by selecting the proper residence times of
dense media in the reactor and employing circulation means, e.g., a
peristaltic pump, to circulate the dense medium in and out of the
reaction vessel via input and output lines which are connected to the
reaction vessel.
[0069] The continuous method for producing nanoparticles of electrically
conducting materials may also be carried out in a multistage plasma
reactor. Such a multistage plasma reactor comprises multiple dense medium
plasma reactors each housed in a separate reaction container with a
single reaction vessel, connected either in series or parallel,
preferably in series. In the preferred series multistage reactor, the
output of the upstream reactors is connected to the input of the
downstream reactors. An example of such a multistage reactor is shown in
FIG. 2.
[0070] In FIG. 2, six dense medium plasma reactors 40, each housed with a
separate reaction container, are stacked one above the other to form six
stages enclosed within an enclosed reaction vessel 41. The rotating
electrodes 43 are all attached to a common shaft 44 and therefore rotate
at the same speed with respect to stationary electrodes 46 which have
openings 48 therein to allow the fluid medium to flow into the space
between the upper and lower electrodes. The dense medium is introduced to
this multistage reactor through an inlet on the bottom of the reactor. In
the process for inactivating microorganisms, the dense medium containing
at least one microorganism flows through the conduits 48 in the first
stage static electrode 48 into the center area of the planar gap between
the first stage static electrode 48 and the first stage rotating
electrode 43. The dense medium from the first stage of the plasma reactor
exits the first stage and enters the second stage of the reactor through
conduits in the second stage static electrode. The conduits in the second
stage static electrode also introduce the dense medium to the center of
the planar gap between the second stage static electrode and the second
stage rotating electrode. This process is repeated for each stage of the
multistage reactor. In each of the various stages, reactive species,
antimicrobial nanoparticles, or both are produced. The disinfected dense
fluid medium, as well as any colloidal dispersion of nanoparticles, may
be collected from the final stage of the multistage reactor.
Conveniently, such collection is by means of an overflow discharge from
the top of the last stage of the multistage reactor.
[0071] The following examples are offered by way of illustration and not
by way of limitation.
EXAMPLES
Example 1
Inactivation of Spores with Multiple Plasma Discharges
[0072] This example describes the results of experiments on
microorganism-containing water in a plasma discharge reactor.
[0073] Spores for Water Contamination.
[0074] Bacterial spore strains used were Bacillus stearothermophilus ATCC
7953, a heat resistant spore, and Bacillus cereus MGBC 145, a common
pathogenic spore. The spores were prepared as follows: 100 .mu.L of an
overnight TSB culture of B. Cereus or B. stearothermophilus were spread
onto the surface of a sporulation agar plate (nutrient agar with 50 mg
MnSO.sub.4/L). The plates were incubated at 37.degree. C. for up to 5
days and checked daily for sporulation. Five milliliters of cold saline
were added to each plate, and the surface was scraped to suspend the
spores. The spores were washed by centrifuging and resuspending the
pellet in 10 ml of distilled water three times. After the final wash, the
spore preparation was frozen at -20.degree. C. Before each experiment, an
aliquot of the spore preparation was thawed and heat shocked at
80.degree. C. for 10 minutes to inactivate any vegetative cells.
[0075] Procedure for Inactivating Spores.
[0076] Spore contaminated water samples (200 ml) were treated in the
reactor shown in FIG. 1. The reactor parameters and the results are shown
in table 1 for different time intervals. The plasma reactor was
pressurized with oxygen (O.sub.2) gas in each experiment.
1TABLE 1
Plate Percent
Counts of
(%)
Surviving Decrease
DC DC Bacteria in
Electrode voltage current Time (log Bacteria
# Sample Material (V)
(A) (min) CFU/ml) Counts
1 Bacillus Stainless 200
1.5 0 4.53 0
stearothermophilus Steel (control)
ATCC
7953
3 4.02 69
5 3.83 80
7 3.69 86
10 2.97 97
20 2.93 98
2 Bacillus Titanium 200
1.5 0 4.94 0
Stearothermophilus (control)
ATCC 7953
5 4.69 44
7 4.69 44
10 4.57 57
20 4.08 86
30 3.34 98
3 Bacillus Silver 200 1.5 0
4.90 0
Stearothermophilus (control)
ATCC 7953
5 3.65 94
7 3.69 94
10 4.72 34
20
4.70 37
30 3.55 96
4 Bacillus cereus Stainless 200 1.5
0 5.25 0
MGBC 145 Steel (control)
0.5 5.18 15
1 5.14 22
2 5.25 0
3 5.05 37
5
4.96 49
[0077] In all of the spore-contaminated solutions, at least 98% of the
spores were inactivated after being exposed to the plasma for 30 minutes.
In some cases, the inactivation time was considerably shorter, and for
the silver electrodes 94% of the spores were inactivated in just 5
minutes. The enhanced rate of inactivation for the silver electrodes is
likely due to the antimicrobial properties of the silver nanoparticles
that are produced during the plasma reactions. (The production of silver
nanoparticles is discussed in greater detail in Example 2, below.)
[0078] Silver electrodes are particularly well suited for spore
inactivation. Table 2 shows the results of additional experiments
conducted using silver electrodes.
2TABLE 2
Plate Percent
Counts of
(%)
Surviving Decrease
DC DC Bacteria in
Electrode voltage current Time (log Bacteria
# Sample Material (V)
(A) (min) CFU/ml) Counts
1 Bacillus Silver 200 1.5
0 4.79 0
stearothermophilus
ATCC 7953
0.5
4.87 0
1 4.64 29
2 4.44 55
3 4.69 21
5 3.59 94
2 Bacillus Silver 200 1.5 5 3.91 91
stearothermophilus
ATCC 7953
3 Bacillus Silver 200 1.5 5
4.03 88
stearothermophilus
ATCC 7953
4 Bacillus
First 200 1.5 5 4.56 59
stearothermophilus Electrode =
ATCC 7953 Silver;
Second
Electrode =
Titanium
Example 2
Production of Antimicrobial Silver Nanoparticles
[0079] This example describes a method for producing a suspension of
bactericidal silver nanoparticles. The suspension and nanoparticles
described herein may be added to a liquid containing microorganisms in
order to inactivate the microorganisms.
[0080] Colloidal dispersions of silver were made in a reactor as shown in
FIG. 1. Both electrodes were made of 99.9% pure silver. In a typical
"stationary"-experiment 0.5 liter of ACS grade water (available from
Aldrich Chemicals) is introduced into the system, then an about 1 mm
distance between the electrodes is established, and the rotation of the
upper electrodes is initiated and sustained at about 1000 rpm. An argon
gas flow rate (0.2 slm) is passed through the liquid medium during the
process to degas the water. In the next step, the discharge was started
and sustained at the 200 DC voltage, 2.5A electric current and various
treatment times between 5 seconds and 5 minutes. The temperature of the
dense medium in the reaction system was 15.degree. C. At the end of the
reaction the power supply is disconnected from the system and the liquid
is removed and stored until analytical work is initiated.
[0081] Gravimetric concentration evaluation of the
colloidal silver
indicates that the silver concentration is about 200 ppm for a 1 minute
treatment time.
[0082] The colloidal dispersions produced above were evaporated and the
nanoparticles analyzed by scanning electron microscopy and energy
dispersion spectroscopy. The photomicrographs showed particles with
dimensions less than 10 nm.
Example 3
Antimicrobial Activity
[0083] The antimicrobial activity of colloidal dispersions of silver as
made by the current invention were measured. Bacterial contaminated water
solutions were prepared and treated either by processing through the
dense media plasma reactor or by adding a solution that was processed
through the dense media plasma reactor.
[0084] The bacterial contaminated water solutions and the plate counts of
surviving bacteria have been carried out at the Food Research
Institute-UW according to the following procedure:
[0085] Inoculum for Water Contamination.
[0086] Four bacterial strains were grown overnight in trypticase soy broth
(TSB) at room temperature. The next day they were transferred to TSB
(diluted 1:100 in distilled water) and grown overnight at room
temperature. The stains were pooled and inoculated into water which was
run through a MilliQ system (Millipore Corp., Bedford, Mass.) and
sterilized before use.
[0087] Bacterial stains used were two Pseudomonas fluorescens; a
Salmonella typhimurium and an Enterobacter agglomerans.
[0088] Procedure for Testing Survival of Bacteria.
[0089] Bacteria-contaminated water solutions were treated under the
DMP-plasma conditions for different time intervals (Table 3).
[0090] After plasma-treatment, the samples were directly plated or diluted
in phosphate buffered saline and then plated on trypticase soy agar. The
plated samples were incubated at room temperature for 72 hours. To test
for injured bacteria, 0.5 ml of each sample was added to 4.5 ml of TSB
and incubated at room temperature. The results are shown in table 3.
3TABLE 3
Antimicrobial Activity of
Colloidal Silver
Dispersion
DC DC Plate Counts of Plate counts of
voltage current Time surviving bacteria surviving bacteria
#
Sample (V) (A) (s) (log CFU/ml) (CFU/ml)
C.S. 1 Initial --
-- -- 5.73 537,032
inoculum
of water
C.S. 2 Water
held -- -- -- 5.41 257,040
until
treated
samples
were
plated
Ex. A Bacteria 200 0.4 5 <1.0 0.
samples
treated for
5 s
Ex. B Bacteria 200
0.4 10 <1.0 0.
sample
treated for
10 s
Ex. C Bacterial 200 0.4 60 <1.0 0.
sample
treated for
1 min
Ex. D Water ACS 200 0.4 60 <1.0 0.
treated
and added
1:1 to
bacteria
sample
Ex. E 1 ml of -- -- -- 3.69 4,898
bacteria
sample
treated for
10s added
to 200 ml
untreated
bacteria
sample
[0091] In all solutions resulting from the plasma-treatments the bacteria
were killed completely. Even solutions prepared from 200 ml of solution
containing the inoculum and 1 ml of the plasma-treated bacterial sample
that was treated in the plasma reactor for 10 seconds, exhibited a 99%
reduction of the living bacterial content. The only samples that were
positive for growth upon enrichment were the untreated water and
untreated water with 1 ml of 10 seconds treated bacterial sample.
[0092] Treatment of the samples, even for 5 seconds, killed the bacterial
inoculum. No bacteria were recovered either by direct plate count or by
enrichment. Addition of the 10 second treated bacterial solution to the
untreated inoculated water efficiently reduced the bacterial count by 99%
[0093] This high efficiency of
colloidal silver and silver oxide
production, and the extremely strong bactericide character of the
plasma-generated solutions, allow the development of technologies in a
continuous-flow-system mode, and the generation of silver-based solutions
in high volume liquid media. Such solutions may also be used to kill or
control other micro-organic matter such as viruses and spores.
[0094] Examples of technologies that are enabled by the inventive method
include industrial cooling water applications, air conditioning
(especially swamp coolers), and swimming pools. Industrial cooling water,
in particular cooling water used in the food processing industry, may
contain harmful microbes which could contaminate the food. Likewise, air
conditioners and swamp coolers, have been linked to various diseases such
as Legionnaires' disease. The results shown in the microbial example
indicate that a colloidal dispersion produced by 10 seconds treatment in
the dense medium plasma reactor is effective in killing bacteria even
after a 200:1 dilution. Therefore, the reactor described above, which
processes one-half liter of water into a colloidal dispersion of silver
in 10 seconds would, in continuous flow operation, provide 180 liter/hr
of
colloidal silver which at a 200:1 dilution could sterilize 36,000
liter/hr of contaminated water. This processing rate can easily be
increased by building larger dense media plasma reactors or by building
multistage plasma reactors.
[0095] It is understood that the invention is not confined to the
particular embodiments described herein, but embraces all such forms
thereof as come within the scope of the following claims.
