Patent Application
20140326660
The present disclosure describes subject matter that relates to nanotechnology and, in particular, to filtration and detection of nano-materials.
Materials that arise from developments in nanotechnology (also “nano-materials” and/or “nano-particles”) are likely key to future technology in various applications and industries, e.g., energy, drug delivery, medicine, and environmental. However, the rapid advancement of nanotechnology and the increasing use of nano-materials or nano-materials-based products and processes present both opportunities and challenges. For example, some of the special properties that make nano-materials useful may also cause them to pose hazards to humans and the environment. Nano-particles are believed to be toxic when inhaled because they present a large surface area to the lung, and are able to bypass the blood-brain barrier through the olfactory bul. Other nano-particles such as ultra fine metal nano-particles have been reported to affect the inflammatory processes of the central nervous system. Moreover, a clear understanding of the potential impact of nano-materials on the environment has been limited by insufficient understanding of the risks associated with development, manipulation, and wide-ranging applications of nano-materials. The identification and characterization of these materials are important first-steps in assessing the potential risks of nano-materials and nano-particles.
Conventional membrane filters often have large pore sizes that cannot be used to filter submicron particles, nano-particles (NPs), or biological particles having sizes of 100 nm or below. One solution to address these pore size issue is to use micro-porous polypropylene filters with surface charge modification. These types of filters can filter NPs with sizes between 60 nm and 200 nm. However, during operation, the filters capture NPs by adsorption based on their surface charge. Another solution may utilize carbonaceous nano-fiber membranes made of carbon nano-fibers with little interaction with the filtered NPs. These types of filters have been shown to filter NPs with a wide range (5 nm 150 nm). In other examples, nano-porous membranes are used of increasing thicknesses (e.g., up to 45 μm) could separate smaller particles (CdTe quantum dots of 2-4 nm in size) and act as size-selective chromatography.
Other techniques may employ membranes more commonly associated with ultra-filtration (UF) for the filtration of NPs. These membranes may be polymeric and naturally hydrophobic. Examples include polysulfone, polyethersulfone, polypropylene, or polyvinylidenefluoride. Still other membranes comprised of inorganic aluminum oxide membrane, having a precise, nondeformable honeycomb pore structure with uniform pore size and extraordinarily high pore density can be used in micrometer and nanometer filtration. However, although these commercial or laboratory filters and membranes have been used for the isolation and separation of NPs, these convention membranes often suffer from inconsistent range in pore size that leads to inadequate in filtration efficiency. For example, some extremely small NPs could still penetrate the pores. Moreover, many conventional filters cannot be used for both detection and separation.
The present disclosure describes embodiments of devices and systems (and methods) that can capture, isolate, and detect nano-materials (e.g., engineered nano-materials) and, further, distinguish these nano-materials from naturally-occurring particular matter. These embodiments may enable size-selective and on-site detection of engineered nano-materials in the environment. These features facilitate new approaches that create materials to take advantage of enhanced catalytic, optical, and electrical properties of nano-materials.
As set forth herein, embodiments can comprise polymeric membranes that were tested using them as nano-filters to isolate and remove silver nano-particles, quantum dots, and titanium dioxide particles in food supplements and environmental samples. These embodiments exhibit filtration efficiencies over 99%. Because the porosity of the membranes can be controlled, discrimination of the NPs from bacteria, enzymes, and even soot and other hydrocarbons was possible. The sensor capabilities were tested on nano-materials in soil, sediment, and water matrices. These tests showed the potential for continuous and in-situ sensing.
Examples of the polymer have excellent physical and chemical properties: transparency, flexibility, electrical conductivity, and accessibility to forming large-area devices. The polymers can be modified for chemical and electrocatalytic applications. For example, the polymer can reduce chromium VI to chromium III, which implies the potential for use in remediation. Somewhat surprisingly, it was also found that the linked flavoinoids, which reduce chrome VI, inhibit enzymes that can lead to alleviation of pain in cancer patients. This is possible because examples of water soluable compounds developed in connection with embodiments set forth herein can degrade in the human body, unlike existing non-water soluable compounds that cannot biodegrade and, therefore, cannot be used in the human body.
This disclosure describes, in one embodiment, a filter device that comprises a substrate and a first layer disposed on the substrate, the first layer having a composition comprising a first component of poly(amic) acid, the first layer having a first porous structure with pores of a first pore size, wherein the first pore size is less than 100 nm.
This disclosure also describes, in one embodiment, an apparatus for filtering nano-particles from a solution, the apparatus comprising a filter media and a membrane disposed on the filter media. The membrane comprises a composition of poly(amic) acid and one or more additive components bonded with the poly(amic) acid, wherein the membrane is configured with at least one functional group that is configured to bond with biomolecules.
This disclosure further describes, in one embodiment, a membrane that comprises a porous structure with pores less than 100 nm, the porous structure comprising poly(amic) acid, a first additive cross-linked with the poly(amic) acid, and a second additive comprising nano-particles bonded to the porous structure.
The discussion that follows below provides information that quantifies and qualifies these and other exemplary embodiments of the membranes, devices, apparatus, and systems contemplated herein. This information is, for example, useful to illustrate the effectiveness of a membrane having a composition with a first component (e.g., of poly(amic) acid) and one or more second components, or additive components, that bond with the first component. This structure enhances the operative characteristics of the membranes, thus lending the embodiments to perform well to capture, isolate, and detect nano-particles and like particulates and contaminants. In some embodiments, the porous structure is configured to capture particulates on the nano-scale (e.g., less than 100 nm), which lends these embodiments to a wide range of applications that are available for membranes having the general structure discussed herein.
Where applicable, one or more of the following terms may be used throughout the discussion:
ODA—4,4′-oxydianiline; PMDA—pyromelitic dianhydride; DMAc—N,N-dimethylacetimide; PAA—poly(amic) acid; PS—PAA-silicone; PG—PAA-gold; PSG—PAA-gold-silicone composite; PI—Polyimide; PET—polyethylene terephthalate; NP—Nano-particle; SEM—Scanning electron microscopy; EDS—energy dispersive spectroscopy; TEM—Transmission electron microscopy; XRD—x-ray diffraction; SE—secondary electrons; CV—cyclic voltammetry; DPV—differential pulse voltammetry; ROS—reactive oxygen species; MF—microfiltration; UF—ultrafiltration; NF—nano-filtration; RO—reverse osmosis; SD—standard deviation.
Reference is now made briefly to the accompanying figures in which:
Where applicable like reference characters designate identical or corresponding components and units throughout the several views, which are not to scale unless otherwise indicated,
Broadly, the discussion that follows describes various embodiments of a filter device that utilizes a multi-layer structure. Examples of this multi-layer structure can comprise a substrate and a membrane, disposed on the substrate; although certain configurations may focus on the membrane (both as a singular layer and multi-layer structured embodiment. These examples can leverage certain advantages of poly(amic) acid that helps to formulate the layers (and/or layered structure) with a porous structure having pores with pore sizes of 100 nm or less. As further noted herein, these advantages further provide uniform and controllable pore size/structure/configuration to adapt the resulting membrane and/or filter device for use to capture, isolate, and/or detect nano-sized particulates,
At a relatively high level, embodiments of the filter device 100 can capture, isolate, and detect nano-particles, e.g., silver nano-particles (AgNPs). These embodiments incorporate pores and cavities, one or more of which may be interconnected to form three-dimensional pores and cavities. This construction traps and/or fixes nano-particles. Moreover, the resulting structure has sufficient porosity to avoid clogging that may reduce the efficacy of the filter device 100.
The membrane 108 affords the filter device 100 with features that facilitate removal of particulates, e.g., from the flow F. These features may include, for example, physical and/or chemical properties that prevent particles, e.g., nano-particles, from transiting through the layered structure 102. Examples of the membrane 108 can comprise poly(amic) acid (FAA) membranes and membranes of like composition, e.g., that include poly(amic) acid as a constituent component.
On the other hand, the second layer 106 may provide structural features that provide rigidity and support to the first layer 104. In one embodiment, the sublayers 110, 112 can exhibit finger-like (e.g., macrovoids) and sponge-like properties. Exemplary compositions for the sublayers 110, 112 can include porous materials (e.g., filter paper) as well as other materials compatible with filtration applications.
The devices and membranes disclosed herein may embody biosensors, biochips, nano-sensors, electrocatalysts and microelectronic devices. Fabrication of the membranes, membranes, and like polymeric materials (collectively, “membranes”) may exploit a combination of strategies including chemical, electrochemical, hot embossing, and imprinting techniques. Imprinting techniques, for example, can allow sub-micrometer patterning with dimensions smaller than 100 nm on hard imprint materials. The oxidation of a π-conjugated conducting polypyrrole with gold trichloride, silver nitrate, palladium ions, and copper sulfate following photochemical reaction can produce conducting membranes having metal clusters in 5-100 nm range.
The techniques that are available to fabricate membranes for sensing and other applications include solvent casting, spin coating, chemical polymerization, and electro-polymerization. Of these, immobilization of biomolecules in electro-polymerized membranes allows for electrochemical control of various parameters such as the thickness of the membrane, the biocomponent loading, and/or the biocomponent location. However, the extreme hydrophobicity and insolubility of the polypyrrole matrix can trigger protein adsorption, which might be less ideal for some sensing applications. In other embodiments, membranes may possess a hydrophilic surface for low unspecific binding. Moreover, membranes can possess one or more functional groups that are configured to attach biomolecules or can be easily functionalized, e.g., for purpose of bonding with biomolecules. Some membranes may be suitable for operation in harsh conditions and, in this case, this disclosure contemplates membranes that are configured with special mechanical and chemical resistance, e.g., almost inert, for these conditions.
The membranes herein may find use in polyfunctional materials because of the presence of amide and carbonyl functionalities. An average functionality in these membranes are found in a range from about 160 to about 600 depending upon molecular weight. Reactive materials having two or more functionalities can crosslink the membrane to produce a high molecular weight cross-linked polymeric framework. The membrane can act as a precursor of PIs with cation complexing properties. Complexing power of the membranes may be significantly higher than that of the imide form; thus rendering the membrane with carboxylic acid groups that exhibit polyfunctional behavior. Moreover, in some embodiments, the membrane can comprise mono-dispersed, nano-scale particles (e.g., of noble metals) that can be used to create a high density of anchor groups for directed immobilization of biomolecules. Conversion of PAA to PIs typically occurs via thermal imidization process involving the loss of water molecules.
As noted herein, embodiments of the membranes (and related films, layers, coatings, etc.) can be highly flexible, mechanically strong, and ternary polymeric blends of PAA, Si—O—Si framework, and metal NPs. In one embodiment, the resulting copolymer can retain the functional moieties of the membrane with enhanced mechanical properties compared to the parent PAA. This disclosure also contemplates a new approach for creating flexible, stand-alone PAA hybrid co-polymers. The carbonyl and amide functionalities in PAA act as anchors resulting, in one example, in the fabrication of flexible, nano-structured, PAA-silicone-gold membranes. Although structurally and mechanically different from the parent PAA, copolymerization with silanes can significantly improve the porosity and mechanical property of the PAA membrane. Membranes of this disclosure are found to have properties including one or more of flexible, rigid, brittle, transparent, and mechanically strong, often depending on the synthesis conditions and composition.
As noted more below, the discussion provides several implementations to quantify and/or qualify embodiments of the membranes. In one implementation,
All reagents are analytical grade unless otherwise stated. The following reagents were obtained from Sigma-Aldrich Co.: 4,4′-oxydianiline (ODA), pyromellitic dianhydride (PMDA), gold (III) chloride, N-[3-(trimethoxysilyl)-propyl]aniline (TMOSPA), 3-aminopropyl-trimethoxysilane (APTMOS), dichlorodimethylsilane (DCMS), N,N-dimethylacetimide (DMAc). Tetramethoxysilane (TMOS) was obtained from Thermo Fisher Scientific Inc. All water used was triply distilled de-ionized water with resistivity of 18 MΩ or better. Gold (III) chloride was dissolved in water to make 0.1M aqueous solution. Various silanes were dissolved in DMAc to make 150 mg/ml silane DMAc solutions. Thermal curing was achieved using a Fisher Scientific Isotemp programmable force-draft muffle furnace (Series 750, Model 126).
Copolymers were prepared using a fixed composition of PAA, gold, and silane while curing temperatures were varied. All PSG membranes in this series have similar weight ratio of 100:20:1 for PAA/silane/gold using two silanes, TMOS and TMOSPA (4:1 in weight ratio). The PSG membranes were heated at varying temperatures (75° C., 100° C., 150° C., 200° C., 250° C., 300° C.) in a temperature-controlled furnace.
In one example, copolymer membranes were prepared using a fixed composition of PAA and silanes with varying amounts of gold. The weight ratio of PAA/silanes/gold in these examples was 70:17:x (wherein, x is 1, 2, 3, etc.). The silanes added in this series are DCMS, TMOS and APTMOS (6:1:1 in weight ratio). The resulting polymer membranes and films containing different amount of gold were compared.
In another example, polymers have fixed composition of PAA and gold with various addition amounts of different silanes. The gold and PAA contents in PSG membranes has a weight ratio of 3:140 (gold:PAA). The polymer membranes resulting from various addition of silanes were compared.
Characterization of the membranes utilized proton NMR, 13C NMR, XRD, and FTIR techniques to obtain the chemical structure of the polymers. 1H NMR and 13C NMR spectra were recorded on a Bruker AM 360 spectroscopic system equipped with 8.45 T magnet and multinuclear and inverse detection capabilities at 360 MHz and 20° C. Samples were dissolved in DMSO-d6 and prepared into 25 mg/ml DMSO solution. Infrared spectra were recorded with a Bruker Equinox 55 spectrophotometer equipped with OpusNT software version 2.06. Samples were grounded into fine powder and then mixed with KBr (FUR grade) in a 1:99 ratio; 0.1 g powdered mixture were pressed into pellet for IR analysis; 0.1 g pure KBr salt (FTIR grade) was also made into pellet as blank sample. The crystallinity of gold NPs was assessed from thin film XRD patterns obtained on a Siemens D5000 X-ray diffractometer with a Cu Kα1 monochromatized radiation source (λ+1.540562 Å) operated at 40 kV and 30 mA.
A 1×2 cm PSG membrane slice was placed in a 3 ml quartz cuvette and measured by HP8453 UV-Vis spectrometer using empty quartz cuvette as blank. Two slices of different thickness (85 μm and 0.5 mm) were compared.
The flexibility and fatigue characterizations were tested on an open air fatigue tester. PAA, PG, and PSG solutions were dropped on a substrate (e.g., polyethylene terephthalate (PET) strips) to form thin membranes. The samples were wrapped on fatigue tester for bending test with bending radius of 3 mm. Images of sample surfaces were taken by Zeiss Axio Imager M1M Advanced Upright Compound Microscope before and after each bending test which bend these membranes for 1000 and 5000 times separately. Information regarding the membrane's thermal stabilities and degradation properties were collected using TGA and DSC techniques respectively. Thermogravimetric analysis was conducted using TA Instruments TGA-Q50 equipped with Q series Explorer software. The membranes were heated from room temperature to 800° C. with an increase of 20° C. per minute using ramp method. DSC was performed (TA Instruments DSC-Q200) using the ramp method with hermetic sample holders for the analytes and an empty sample holder as reference. The polymer samples were heated from −10° C. to 400° C. at the rate of 5° C. per minute.
In solubility tests, about 0.42 mg PAA series membranes were dissolved in 5 ml various solvents. Two sets of solvents were used. The first set were aqueous solutions of HCl and NaOH with various concentration and at different pH values. The concentrations of HCl solutions were from 1M to 10−6M with pH values from 0-6. The concentrations of NaOH solutions were from 1M to 10−6M with pH values from 8-14. Another set includes various organic solvents ranging from non-polar to polar; namely toluene, hexane, ethyl acetate, carbon tetrachloride, acetone, chloroform, methanol, ethanol, acetonitrile, dimethylformamide (DMF). DMAc and dimethylsulfoxide (DMSO). All chemicals were purchased from Fisher Scientific, Inc. (USA).
Electrochemical measurements were performed using an EG&G Princeton Applied Research 263A potentiostat/galvanostat equipped with M398 software. A conventional three-electrode system was employed in electrochemical measurements, which consists of a glassy carbon electrode (GCE) (with a geometrical area of S=0.11 cm2) as working electrodes, a Ag/AgCl reference electrode (RAS), and a platinum wire as auxiliary electrode. The working electrode was polished with alumina, sonicated for five minutes, and copiously rinsed with triply-distilled de-ionized water followed by methanol rinse. Cyclic voltammetry (CV) was used to characterize the electrochemical properties of the PSG membrane. In one example, PSG membranes are deposited onto Glassy Carbon Electrodes (GCE) by thermal curing process. The modified GCE electrode was used as working electrode. The bare GCE electrode was tested as the blank, PG membranes modified electrode was tested as the control or reference materials. To check the electroactivity of all these polymer membranes, the CV experiments were performed in 0.1M pH6 PBS solution24 by scanning the potential between −400 mv to 1200 mv at a scan rate of 150 mV/s. Differential pulse voltammetry (DPV) was used to further investigate the electrochemistry of PSG on electrode using similar conditions as the CV. All electrochemical measurements were repeated more than three times using different GCE electrodes. NaH2PO4.H2O and Na2HPO4.7H2O were dissolved in water and adjusted with NaOH to prepare 0.1M pH 6.0 PBS buffer.
SEM and transmission TEM were employed for surface morphology and particle size characterization. EDS analysis was conducted to provide surface elementary information. SEM and EDS analyses were conducted on a Zeiss Supra 55 VP, analytical ultra high resolution FESEM+EDAX Pegasus EDS+EBSD, equipped with SmartSEM™. The acceleration voltage of SEM analysis was 5 kV with maximum magnification of 5×106. The sample membranes were fixed on a 45°/90° aluminum SEM mount using carbon conductive tape. Elemental composition information of membranes was obtained at the same time using EDS. The acceleration voltage was 20 kV with magnification of 20000 and working distance about 10 mm. For TEM imaging, one drop (˜5 μl) of 8.4 mg/ml PSG DMAc solution was dropped on 300 mesh copper TEM grid purchased for Ted Pella, Inc., and then dried at room temperature before was imaged by a TEM microscope manufactured by Toshiba.
As shown in
19-33a
115b
aRange of pore size (nm).
bNumber of pores in 1 μm2
Increases in the gold concentration can prevent the PSG solutions to evenly disperse on the surface to form thin membranes due to increasing surface tension of the gold in the solution. In turn, this feature can also influence the mechanical properties of the resulting PSG membrane. TABLE 2 describes weight and molar ratios of PAA, gold, and silanes in PSG membranes. This information shows that PSG membranes with higher gold content were found to be rigid and opaque, while PSG membranes with lower gold content are flexible.
aWeight and molar ratio of PAA and gold, as calculated from the initial amount of ODA, PDMA and gold salt, assuming complete reaction;
bWeight of PAA and silicone, assuming complete reaction;
cWeight ratio of silane;
dT, transparent; F, flexible; B, brittle; S, strong
Observations of this set of membranes are summarized in TABLE 3 below, which indicates weight and molar ratios of PAA, gold and silanes in PSG. By varying the amount and types of silanes in PSG copolymer, the membranes can be prepared with certain properties (e.g., rigid or flexible; opaque or transparent; good or poor mechanical property, etc.). Among the silanes tested, DCMS did not produce desirable effect for thin membranes and membranes, while silanes such as APTMOS, TMOS and TMOSPA which were usually used as linkers and condensers, worked very well to produce thin PSG membranes. Also, it is found that it was easier to disperse the PSG solution containing high gold content when TMOS or TMOSPA were added than when they were absent. So APTMOS, TMOS and TMOSPA were used in further synthesis of PSG with a weight ratio of 4:3:1.
aWeight and molar ratio of PAA and gold, as calculated from the initial amount of ODA, PDMA and gold salt, assuming completely reaction;
bWeight of PAA and silicone, assuming complete reaction;
cWeight ratio of silanes;
dO, opaque; T, transparent; R; rigid; F, flexible; S, strong; B, brittle.
According to the results of these optimization tests of copolymers, further studies were performed on PSG membranes using the composition with PAA/silanes/gold (70:17:1 in weight ratio) because it has desirable mechanical property including flexibility and transparency. The silane composition used was APTMOS/TMOS/TMOSPA (4:3:1 in weight ratio). In this PSG membrane, the molar ratio of PAA/gold was 16:1 and the molar ratio of PAA/APTMOS/TMOS/TMOSPA was 20:5:5:1.
The properties of PSG membranes were compared with membranes comprising pure PAA and/or PAA with or without addition of silicone component.
13C NMR (ppm)
B=Kλ/L cos θ Equation (1)
wherein B is the width of the peak at half maximum intensity in radians, K is a constant between 0.89 and 1 (0.9 was used in this calculation), λ is the wavelength of incident x-rays which is 1.540562 Å, θ is Bragg angle, and L is the crystallite length.
With reference to
With reference to
HCl solution, NaOH solution and pure water were used as solvents for solubility test. In one example, only the high pH (≧12) NaOH solution was found to “dissolve” PSG membranes. PSG membranes did not change or swell in other lower pH solutions. However, no PSG membranes recovered after exposed to high pH NaOH solutions. This observation implies that PSG may actually hydrolyze in this high pH NaOH solution instead of simple dissolution. Other PAA membranes showed the similar solubility behavior. TABLE 6 summarizes the solubility of PSG in aqueous solution at various pH values.
Several common organic solvents were tested for solubility and most PAA membranes only dissolved in highly polar organic solvents such as DMF, DMSO and DMAc. TABLE 7 summarizes solubility of PAA series membranes in organic solvents. In one example, the dissolved membranes can be recovered by evaporating the solvents.
DPV was used to further investigate the electrochemistry of PSG on the solid electrodes. All electrochemical potentials were measured vs. silver/silver chloride.
PG membranes with various gold concentrations were also tested with DPV method. The ratio of PAA and gold was the same as noted above. As shown in
Referring now to
PG membranes with constant PAA/gold molar ratio (1:20) were heated at various temperatures. Generally, with increasing thermal curing temperature, the color of membrane changed from brownish purple to purple and then reddish purple as shown in
Other concentration that commonly used for PSG membrane fabrication also imaged, but they did not give a clear image due to their high concentration leading to a thick membrane on TEM grid. 8.4 mg/ml is a ten times diluted PSG solution and it gave a brighter background because it formed a thinner membrane than the others. In
In view of the foregoing, a new class of flexible nano-structured materials encompassing a ternary PAA-silane-gold nano-composite has been successfully synthesized. Solutions of copolymers that were synthesized from these composites have been used to fabricate a range of mechanically and optically distinct stand-alone membranes using thermal curing technique. This approach avoids the cyclization of PAA into PI at low temperature and utilizes the unique reactivity of PAA to form designed polymer-assisted nano-structured materials. In one implementation, by way of an appropriate selection of the experimental variables (temperature, gold and silane composition), it is possible to create PSG nano-structured membranes with controlled morphology, particle size, particle distribution and mechanical property. The characterization of this material also shows they are electroactive with unique morphology. These materials could find a wide range of uses including sensors, bioelectronics and interconnect applications. For example, the presence of free carboxylic acid groups in the PSG membranes may enable their functionalization for the immobilization of biomolecules in immunoassays, molecular bioelectronics and biosensor devices. The presence of gold NPs could allow the PG and PSG membranes to be employed in surface enhanced Raman spectroscopy. In addition, their flexibility makes them compatible with flexible electronics and interconnects technologies.
The discussion that follows describes fabrication and morphology studies of novel phase-inverted PAA membranes. This discussion also describes the possible conversion between phase-inverted and thermally-cured PAA membranes as well as their method of storage.
The reagents and synthesis procedures are as described in Section A (Implementation 1) above. However, after the solution based PAA and its derivatives polymer were synthesized, the examples of Section A were casted onto hard substrates or flexible substrates with subsequent phase inversion process instead of heating into solid membranes. The hard substrates used to fabricate PAA and its derivatives membranes include glass slides (e.g., from Thermo Fisher Scientific, Inc.) and gold working electrodes (e.g., from Bioanalytical Systems, Inc.). The flexible substrates include PI and PET sheet (e.g., from Endicott Interconnect Technologies, Inc.) and filter papers (e.g., from Whatman Ltd. (USA)). The discussion hereinbelow identifies examples and embodiments derived from the fabrication of phase-inverted PAA membranes, wherein the fabrication process has been divided into two parts (1) stand-alone membrane and (2) membrane coated filter paper, respectively.
In one implementation, 20 μl PAA solution was dropped and dispersed on a piece of glass slide to form an even thin layer. The glass slide was immersed in water and DMAc diffused into water. The pale yellow PAA membrane began to show off immediately on the glass slide. After about 2 minutes PAA membrane slowly peeled off by itself from glass slide. The resulting membrane was then taken out and immersed in a clear water trough for another 10 minutes in order to thoroughly remove the DMAc solvent. The membrane was then exposed in air for 15 minutes to dry. Other PAA membrane derivatives were made utilizing a similar method and/or procedure.
The amount of casting solution applied on glass slide was optimized. Various amounts (5 μl, 10 μl, 15 μl, 20 μl, 25 μl and 30 μl) 0.2M PAA solutions were pipetted on round glass slides with 18 mm diameter and the pores size of resulting PAA membranes were compared
Grade 1 (11 μm pore size) qualitative filter paper (e.g., purchased from Whatman Ltd.) was applied in this section. Since filter paper can absorb PAA casting solution, 25 μl instead of 20 μl casting solution was used to coat the filter paper surface. First, 25 μl was dispersed on 15 mm dia. round glass slides. The slide was used as a stamp to transfer PAA casting solution to the filter paper. After filter paper absorbed PAA casting solution, it was placed into water until the yellow PAA coating layer began to show off on its surface. Then the filter paper was transferred into fresh water and immersed for another 10 minutes followed by drying in air.
In optimization of the amount of casting solution dropped on substrate, PAA solution was used.
This fabrication step gives the PAA membrane a durable support without affecting the functional surface. The resulting PAA coating layer on filter paper coated with PAA membranes were smoother than PAA stand-alone membranes.
Unlike thermally-cured membranes, examples of the phase-inverted membranes are opaque, thus their fluorescence properties were studied instead of UV-Vis absorption characteristics.
Although PAA, PS and PG, PSG have different color because of gold NPs (
The emission intensities of PAA series membranes increased with increasing excitation wavelength until they reached their maximum with excitation wavelength at 460 nm, and then decreased at longer excitation wavelength (
Although the emission of PAA series membranes at single excitation wavelength is quite similar as fluorescent emission, several results were inconsistent with the principles of fluorescence spectroscopy. First of all, the fluorophore's absorption and excitation spectra, in most cases, should be symmetric with each other, which is the mirror image rule196. This rule is applicable to the peaks of PAA series membranes which can be considered as a Stokes' shift as shown in
As shown in
b exhibits the emission of PAA/DMAc solution at various excitation wavelengths. Although PAA solution still showed highest emission when it was excited at 450 nm as did the membrane, the emission peak had a big red shift, almost 50 nm. Also, the broad peak disappeared (
If these PAA series are non-fluorescent, the question remains: what is the source of their emission peaks as recorded in
As for the intensity change for each polymer membranes, the spectrum of Xenon lamp should be discussed. As shown in
Casting solutions of different polymers can be easily dispersed on PET surface to form phase-inverted membranes by immersing them in water. The results are stand-alone membranes. However, these phase-inverted membranes did not peel off from the PET substrate as they did on glass substrates. After drying out in open air, a thin phase-inverted membrane of each polymer attached very firmly to PET surface.
PAA series phase-inverted membranes showed different thermal degradation properties compared to their corresponding thermally-cured membranes. As shown in
Examples and embodiments of phase-inverted membranes may have unique porous structures. These embodiments may comprises a thin dense top layer and thick supporting layer with macrovoids. In
PAA coated filter paper has similar surface structures as PAA stand-alone membrane. However, the surface morphology of filter paper is significant modified because of the addition of PAA coating layer. Due to the absorption of casting solution by the filter paper, the sublayer in the PAA stand-alone membrane could be readily observed. The filter paper's structure mixed with this sublayer and became the support for the top PAA layer.
As shown in
The surface pore size of phase-inverted membrane is greatly depended on it's the concentration of the corresponding casting solution. As shown in
Membranes made from casting solutions with concentrations between 0.2M and 0.47M were imaged. Casting solution with concentration lower than 0.2M was too diluted to form stand-alone membrane, while with increasing concentration, casting solution became too viscous to be dispersed on the substrates. The casting solution with higher concentration than 0.47M was not discussed here. Between 0.2M and 0.23M, the pore size decreased dramatically. However, the pore size changed slowly leveling off with increasing concentration after 0.23M. Generally, PAA coating membranes on filter paper had a smoother surface than the PAA stand-alone membrane. At the low concentration range (0.2M-0.22M), PAA coating layer may have bigger pore size than these PAA stand-alone membrane. This is attributed to the absorption of filter paper and hence the presence of less PAA casting solution on its surface. In middle concentration range (0.23M-0.30M), PAA coating layer generally had a smaller pore size range and more uniform pores than the stand-alone membranes. At the concentration range (0.31M-0.47M), the pore sizes are quite similar for both PAA coating layer and PAA stand-alone membrane. The surface pores of the high concentration membranes were more uniform and in a smaller size range than the pores at low concentrations. Utilizing statistical analysis software SigmaPlot 12.0 developed by Systat Software Inc., indicates that single component exponential decay fitting (
Y=a+b*exp(−cX) Equation (2)
wherein, X is the concentration in M, Y is the corresponding pore size in nm, a, b and c are the coefficients. For PAA stand alone membranes, these constants are 11.77, 237063.59 and 38.77 respectively. The R value of 0.9748 and R2 value of 0.9502 were recorded. For PAA coated filter paper, these constants were 16.06, 901330.28 and 45.97, with R value of 0.9271 and R2 value of 0.8594.
Unlike PI, PAA is not stable when exposed in light, heat and moisture. PAA casting solution usually became darker over time when exposed to light. Generally, fresh PAA solution is light yellow and it eventually turns to light orange. The color change of the casting solution indicates the gradual onset of imidization and hence the formation of PI. In addition to the color changes, the homogeneous PAA solution also changes with time. A gel-like light orange material precipitates from the PAA solution finally when left standing over time. Both casting solution and membrane can absorb moisture slowly from air, and the PAA content in the casting solution will gradually become hydrolyzed by the moisture. If the casting solution is exposed under light or air, it will expire within a week. If it is covered by aluminum foil and stored in a dark place, it can remain stable and could be used for fabrication over one or two months. Phase-inverted PAA membrane has a much short life than its casting solution. Once the membrane becomes totally dry in air, it will be brittle and lose flexibility in just one day. So a method must be developed to keep PAA membrane stable. One easy way is to store the casting solution and fabricate PAA membrane whenever it is needed. However, this is not convenient when there are no fabrication tools and conditions. Another way is to store PAA membrane directly under stipulated conditions without exposure to air, light or moisture.
To store PAA membranes, light and heat should be prohibited. But the most important thing is to maintain the equilibrium between wetness and hydrolysis. Without moisture, PAA membrane will become brittle; however, it may also be hydrolyzed with too much moisture. In one example, the storage solution was a mixture of several common solvents. Tests for the present implementation utilized water, ethanol, acetone, DMAc, and DMF. One piece of PAA membrane was immersed in each 2 ml storage solution which was kept in a drawer and was physically observed after 2 weeks. The storage solution was sealed with Parafilm to prevent evaporation and exposure to air.
However, results showed that no combination provided the suitable storage solution for phase-inverted membranes. Among all the solvent mixtures tested, a combination of polar nonsolvents and solvents shows better result than that containing only polar nonsolvents. For instance, the combinations involving ethanol/DMF, water/DMAc and ethanol/DMAc retained the shape, size, color and some flexibility of PAA membranes. The best case out of all these imperfect combination is ethanol/DMF with a volume ratio of 90/10. However, the combinations made of only nonsolvents usually result in a more brittle membrane, and eventually hydrolysis. These preliminary data may eventually lead to the development of permanent storage solution for the phase-inverted membranes. The ideal storage solution should have both the nonsolvent and the solvent in order to maintain the equilibrium of the phase-inverted process. In that respect, polar nonsolvent other than water can be used. In this way, it can keep the wetness of the membrane, and also prevent hydrolysis.
In view of the foregoing, the method for fabrication of PAA stand-alone phase-inverted membrane and PAA coating filter paper were described. The surface morphology shows that these new classes of PAA membranes have similar surface morphology, while PAA coating membranes are smoother than their stand-alone membranes. The amount of casting solution applied did not affect the surface pore size. The pore size was greatly depended on the concentration of the casting solution. The trend in the pore size was the same for both PAA coated layer and stand-alone membranes. Increasing concentration produced a decrease in pore size and size range.
Although phase-inverted PAA membranes have similar chemical composition as their thermally-cured membranes, and they can converted between each other, these two PAA forms have different physical properties. The phase-inverted membranes were not transparent and were found to exhibit unique enhanced Raman scattering emissions. This is a significant finding as it could lead to the development of novel materials for sensing and filtration.
The results show that PAA series phase-inverted membranes are a class of flexible, opaque, and porous sponge-like membranes. They can find wide applications in membrane filtration because of their special nano-size pores and their anisotropic structures. Moreover, because of their optical and electrochemical properties, these membranes may also find applications as sensing materials.
As noted above, the nano-porous surface and isotropic sublayer structure of PAA membranes are especially promising for application in NPs filtration. The surface pore size range can be easily adjusted by its casting solution concentration, and hence generate a series of membranes with various size range for separation purpose. The discussion below describe in various examples and implementations the performance of PAA membrane applied in UF and continuous separation.
Broadly, PAA membranes were fabricated as described in herein (see, e.g., Section A) using related reagents for synthesis. Several NPs and nano-powder samples are used in this implementation including: QSH620 (CdSe/ZnS QDs in H2O—Carboxyl) purchased, e.g., from Ocean NanoTech, LLC. (Springdale, Ark., USA) with the majority of QDs being 20 nm in size. The original solution was diluted using deionized water into various concentrations ranging from 500 nM to 0.5 nM. Aqueous dispersion of TiO2 NPs (<150 nm (DLS), 33-37 wt %) were purchased from Sigma Aldrich (USA). Aqueous dispersion of TiO2 NPs was diluted into 0.1 mg/ml for filtration test. 10 nm, 20 nm, 50 nm and 200 nm gold NPs, and 40 nm and 60 nm silver NPs (10 ppm) aqueous samples were purchased, e.g., from Ted Pella Inc (Redding, Calif., USA). MesoSilver (>20 ppm) was purchased, e.g., from Purest Colloids Inc (Westampton, N.J., USA). Colloidal Silver (35-45 ppm) was purchased, e.g., from Golden Touch Mfg./Ultra Pure (Benton, Ky., USA). Sovereign Silver (10 ppm) was purchased from Natural Immunogenics Corp (Pompano Beach, Fla., USA)211. 13 mm Anodisc™ anopore aluminum oxide membranes (0.02 μm) were purchased from Whatman Ltd (USA). Qualitative filter papers No. 1 was purchased from Whatman International Ltd. (England). 13 mm nylon filter membranes were purchased from Grace Davison Discovery Science (USA). 61 nm and 118 nm polystyrene beads aqueous solutions were purchased from Phosphorex, Inc. (Fall River, Mass., USA).
The surface of PAA membranes (before and after each filtration) were imaged by a Zeiss Supra 55 VP analytical ultra high resolution SEM with inlens and second electron detectors, equipped with software SmartSEM™. Elementary information was obtained by an EDAX detector integrated with SEM. All the image samples were coated with a 5 nm gold layer for SEM. The samples for EDX were coated with about 8 nm carbon. The accelerating voltage for SEM was 5 kV and the one for EDX was 10 kV. Fluorescence emission was recorded by Panorama Fluorat-02 Fluorimeter (Analytical Instruments LUMEX Ltd.) equipped with Panorama Pro, and CaryEclipse (Varian) Fluorescence Spectrophotometer equipped with CaryEclipse Scan application. UV absorbance was measured on a HP Hewlett Packard 8453 UV-Visible spectrometer was equipped with Chemstation software 845X UV-Visible System, and with a integration time 0.5 s, interval 1 nm.
In order to distinguish cascaded separation of various NPs, the filtration may be defined as the isolation of single type of NPs with one nominal particle size from liquid matrices.
The filtration or capture efficiency is defined as the percentage of NPs captured on the filter. Since the volume of sample is not changed before and after filtration, concentration was used instead of direct NPs number. Equation (3) below shows the filtration efficiency (or capture efficiency) based on the change of NPs number and their concentration. Where η is filtration efficiency, N is number of NPs, C is concentration.
In efficiency test of PAA membrane for filtration of QDs, the QDs solution were diluted into 500 nM, 160 nM, 80 nM, 50 nM, 20 nM, 10 nM, 5 nM, 1 nM, and 0.5 nM. The fluorescence emissions of their solutions were measured before filtration. The excitation wavelength was set at 570 nm while the measuring range for emission was 585-670 nm. A calibration equation was generated by comparing the concentration with the corresponding emission intensity. Following filtration using 0.42M PAA membrane, the filtrate fluorescence emissions were measured repeated and the filtrate concentrations determined by calibration equation.
The efficiency of PAA membrane for silver NPs was calibrated using MesoSilver sample. MesoSilver was mixed with deionized water into a series of diluted samples with different percentage concentrations (from 5% to 80%) compared to its original concentration (100%). The filtrate from PAA membrane was measured and concentration was compared with its original concentration.
The separation is based on continuous filtration. Solution of NPs mixture made of three different particle sizes was filtered three times in sequence using PAA membranes of various pore sizes. Each filtration used the same setup as shown in
Three sets of NPs mixture were tested for separation by PAA membranes. TABLES 13, 14, AND 15 below provide information related to these tests. In TABLE 13, set 1 was made of 1.5 ppm 200 nm, 50 nm and 20 nm gold NPs. The membranes used for set 1 were 0.2M PAA, 0.23M PAA and 0.36M PAA with pore sizes of 110 nm, 42.1 nm and 11.5 nm respectively. Set 2 (TABLE 14) was made of 0.004 mg/ml TiO2 NPs, 4 ppm 60 nm silver NPs and 0.8 ppm 10 nm gold NPs. Set 3 (TABLE 15) was made of 1.2×1010 particles/ml 118 nm polystyrene beads, 8.0×1011 particles/ml 61 nm polystyrene beads and 5.7×1012 particles/ml 10 nm gold NPs. The membranes used for sets 2 and 3 were 0.21M PAA, 0.23M PAA and 0.44M PAA with pore sizes of 95.8 nm, 42.1 nm and 8.2 nm respectively.
QSH620 QDs have an orange-red color under normal light. After filtration using 0.42M PAA membrane, the resulting solution became colorless, irrespective of the original concentration. It was observed that the light yellow PAA membrane turned into orange-red because of the captured QDs on its surface.
The filtration results using PAA membranes are found in TABLE 16 below. Their filtration efficiencies for different concentrations were calculated according to Equation (3) and calibration plots are shown in
PAA membranes exhibited stable performance. Except for some experiments at 10 nM, all the filtration efficiencies were above 80% with one single experiment reaching 99.97%. Although the efficiencies generally decreased with increasing concentration, the concentrations of filtrates were quite consistent in most experiments regardless of the original filtered concentrations. Majority of the concentrations of filtrates determined using the PAA membrane were between 2-5 nM. Because of the defects of membrane filters, NPs bigger than the nominal pore size can still remain in the filtrate. So a high total amount of filtered NPs or increasing original concentration will lead to a higher amount of NPs in filtrate or increasing concentration of filtrate. However, the concentration of filtrate will remain constant when the surface of the filter has been saturated or clogged by NPs. The consistent efficiencies recorded in the majority of the experiments suggest that the pores have been clogged at the specific concentration although water can still be pushed through and no obvious pressure increase was observed.
In all the filtration experiments, PAA membranes are easy to handle because they are flexible, soft, and durable under pressure. PAA membranes were also found to have a higher total average efficiency (87.46%) with lower SD (6.87%).
The PAA membrane tested were used to filter 1 ml 200 nm QSH620 solution. The signal recorded for PAA membranes was about 772.7863A.U. This high emission could be explained by solid state fluorescence emission and the amount of quantum dots accumulated on the PAA surface. As best shown in
The original silver NPs samples have light yellow to greenish colors. After being filtered by 0.36M PAA membrane, the solutions of silver NPs found colorless and the yellow PAA membranes were coated with a thin layer of black shining material.
EDS technique and mapping were used to identify the dark materials captured on PAA membranes as silver NPs.
PAA coated filter was tested for its filtration property instead of PAA stand-alone membrane. Three PAA coated filter paper from three casting solutions at various concentrations were compared for filtration of same TiO2 samples. The three concentrations tested were 0.2M, 0.26M and 0.32M respectively. It was noted that the diluted 0.1 mg/ml TiO2 NPs have no uniform size. According to the manufacturer's description, the particles are smaller than 150 nm. After 1 ml TiO2 NPs aqueous dispersion was filtered by each PAA coated filter paper, the change of milky white TiO2 NPs dispersion was obvious but not much color change was noted on PAA coated filter membranes because TiO2 NPs are white.
As summarized in TABLE 13 above, the NPs tested in this set were same kind but with various sizes. The SEM images in
The final filtration step utilized 0.44M PAA membranes to capture all the NPs in the mixture. As shown in
It was noted in this work that the separation is not absolute because of the defect of PAA membranes. Although the irregular shape and size in the TiO2 NPs posed difficulty to identify each kind of NPs according to their appearances, EDS mapping provided solid support for identification. TiO2 NPs were found in both first and second filtration but the trapped TiO2 NPs have different size range when used. This result indicates that PAA membranes for separation were not based on the chemical composition of the NPs but according to their sizes
As shown in
The separation results were shown in
Embodiments of the PAA membrane shows superior filtration efficiency and performance compared to qualitative filter papers, nylon analytical filter membrane with a single efficiency up to 99.7%. These embodiments also show great potential for application for NPs separation. Although it does not show much selectivity according to the NPs' chemical composition, it shows its ability to separate efficiently based on NPs' size. Due to the limitation of membrane filtration, PAA membranes cannot separate the NPs' absolutely and hence some NPs bigger than the nominal pore size were not filtered. However, majority of NPs trapped on PAA membrane were still bigger than its nominal pore size. It was observed that due to NP nature of aggregation and “caking”, some difficulties in separation were recorded. In summary, PAA membranes can be applied in UF and NF for NPs' isolation and separation.
Silver is a non-essential toxic element while silver NPs are increasingly used in a variety of applications including medical devices, water treatment, nutraceuticals, food colorants, food storage containers, baby pacifiers and antimicrobial agents. Bactericidal activity of silver NPs is dependent on their shape and size, with particles of sizes less than 100 nm showing optimal antibacterial activity. The knowledge of the ability of silver to kill harmful bacteria has made it popular in creating various consumer products. In spite of these useful applications, silver NPs have been reported to be toxic as introduced. Besides the on-going debate about the safety and potential risks of engineered silver NPs are already being used in the food industries as food additives and packaging materials. Although the use of silver NPs may bring about a range of benefits to the food sector, such as new taste, textures and sensations, improved packaging, and traceability of food products, the presence of silver NPs in food, beverages and storage containers may also cause unintended harm to human health that may be difficult to trace. Furthermore, there is currently no standard analytical method for monitoring these nano-sized analytes in food samples.
As noted above, PAA membrane show filtration properties for NPs with a filtration efficiency as high as 99.97%. The discussion below describes the quantitative detection application of examples of PAA membranes for, e.g., silver NPs. In this implementation, four food supplement samples containing silver NPs purchased from various manufacturers were evaluated using examples of the PAA membrane. The resulting concentrations were compared with result of atomic absorbance spectroscopy. Moreover, this disclosure presents optical method and electrochemical method without PAA membrane, as well as methods for silver NPs detection with and without PAA membrane.
All reagents were analytical grade unless otherwise stated. Stock solutions were prepared using triply distilled deionized Nanopure water with resistivity of 18 MΩ or better. The following reagents were obtained from Sigma-Aldrich Co. These include: sodium ethylenediamine (EDTA), silver nitrate (99.99%), ODA, PMDA, and DMAc. Zinc oxide (ZnO) nano-powder (<50 nm) was obtained from Aldrich, ZnO 6% doped with Al. Silver nano-powder (100 nm), hydrogen peroxide aqueous solution, sodium chloride were purchased from Fisher Chemical. Nitric acid was purchased from J. T. Baker. Gold-coated glass slides with 200 Å continuous gold coating layer were purchased from Asylum Research (USA). Aqueous colloidal solutions of silver NPs (Standard 40 nm) and gold NPs (standard 50 nm) were purchased from Ted Pella Inc. (USA). Silver NPs food supplement samples were purchased from various sources including MesoSilver (>20 ppm), Purest Colloids Inc (USA); Colloidal Silver (35-45 ppm), Golden Touch Mfg./Ultra Pure (USA); and Sovereign Silver (10 ppm), Natural Immunogenics Corp (USA). Other reagents were purchased from Thermo Fisher Scientific Inc. (USA).
The 40 nm silver NPs standard aqueous solution (Ted Pella Inc.) was diluted with Nanopure water in various concentrations. These solutions were used as silver NPs standards for electrochemical measurements. 10 ppm silver nitrate and 0.01M EDTA (adjusted to pH=7) aqueous solutions were prepared by dissolving corresponding salt with deionized water. ZnO NPs suspension (5 mg/ml) was made by dissolving ZnO NPs into water and sonicating for 10 minutes. Two phosphate buffer solutions were prepared. One is 0.01M Na2HPO4 with 0.25M NaCl adjusted to pH7.0, and the other without NaCl. All the aqueous solutions of NPs were sonicated for 5-10 minutes before use.
1 mg silver nano-powder was first dissolved in 1 ml solution of nitric acid and H2O2 with a v/v ratio of 1:10. Then the resulting solution was then reacted with 4M NaCl solution. The reaction ratio is shown in TABLE 19 below. Two silver nano-powder samples with concentration of 13.18 μg/ml and 18.67 μg/ml were prepared separately. The solutions were measured using UV-Vis spectrometer immediately after reacting with NaCl solution. A solution mixed with water and NaCl solution was used as blank.
Examples of phase-inverted PAA membranes were fabricated as described above. 1 ml various samples were filtered with each piece of membrane. Silver NPs were captured onto PAA membranes and applied to the gold-coated glass slides having surface area of 15 mm×15 mm. This PAA/Au glass slides was used as a working electrode in a three electrodes system, with Ag/AgCl as reference and Pt as auxiliary electrodes. Phosphate buffer solution with NaCl (pH 7.0) was used as the supporting electrolyte. All electrochemical experiments were conducted using BAS 100B potentiostat. Both Cyclic voltammetry (CV) and Differential Pulse Voltammetry (DPV) techniques were employed in the detection measurements. The PAA membranes having various amounts of standard silver NPs were first tested and the peak currents resulting from the oxidation and reduction of the silver NPs were used to generate calibration plots. The PAA membranes with silver NPs from food samples were subsequently tested the same way and their concentrations were estimated using standard silver NPs.
In tests for interfering NPs, gold and ZnO NPs were filtered separately and were captured on PAA membrane. The PAA membranes with these NPs were then applied on gold electrodes and tested electrochemically under same condition as tests for silver NPs. Also ZnO and silver NPs mixture, in which ZnO was 250 times abundant than silver NPs, was tested as well. Silver nitrate solution in which silver ions was same concentration of silver NPs was filtered by PAA membrane and then tested on gold electrode. In order to estimate the effect of silver ions on the electrochemical detection of silver NPs, EDTA solution was filtered through PAA membrane after silver ions or silver NPs were filtered.
This experiment was carried out in a three electrodes system which included a gold working electrode, a platinum auxiliary electrode and a Ag/AgCl reference electrode purchased from Bioanalytical Systems, Inc. Two buffer solutions were used including phosphate buffer solutions with and without NaCl.
During the experiments, 4 ml buffer solutions were used as blank. 1 ml MesoSilver solution was then added to the buffer solution each time until the total addition reached 8 ml. By using two buffer solutions, the effect of NaCl was compared.
In test of reversibility of this method, 4 ml MesoSilver solution was mixed with 4 ml buffer solution with NaCl and tested with various scan rates from 20 mV to 250 mV.
AgNO3 stock solutions (from 50 ppb to 100 ppm) were measured by Perkin Elmer Model AAnalyst 300 atomic absorption spectrometer. The source of light was Fisher Scientific Au/Ag cathode lamp having slit width setting of 0.7 nm at measuring time of 5 sec. The oxidant rate setting was 101/min while the fuel rate was 31/min using time average measurement method. The absorption line at 328 nm was used to generate a calibration line of Ag ions. 5 ml each of the food supplement samples was mixed separately with 5 ml acid mixture consisting of concentrated nitric acid and sulfuric acid at 3:1 ratio. This acid mixture was used to oxidize silver NPs to silver ions in food supplement samples. The oxidized solutions were diluted into 50 ml solution using 3-times distilled water. The concentrations of silver NPs were estimated using AAS calibration line of Ag ions at 1:1 ratio.
Silver nano-powder dissolved in acidic hydrogen peroxide was oxidized into silver ions by hydrogen peroxide. The resulting silver ions were then reacted with NaCl to form AgCl. The chemistry for the reactions of silver nano-powder is shown below.
2Ag+H2O2+2HNO3→2AgNO3+2H2O
AgNO3+NaCl→AgCl↓+NaNO3
The final solution changed from clear colorless solution to white milky solution because of AgCl particles suspended in water. This suspension has an UV absorbance at 255 nm as shown in
40 nm silver NPs aqueous solution was oxidized in same way and reacted with NaCl solution. However, there was no color change in solution. The solution remained clear and colorless, and no white cloudy AgCl suspension formed. The reason for different results to silver nano-powder and silver NPs in aqueous solution may be due to their structure. As described by its manufacturer, silver nano-powder was made by thermal plasma without addition of capping layer. While silver NPs solution were synthesized by recovery of silver nitrate with sodium citrate, and the resulted silver NPs have citrate ions as capping layer. The capping layer on silver NPs may protect silver from oxidization by hydrogen peroxide, and hence no silver ions formed to react with NaCl.
Electrochemical techniques provide significant advantages for in-situ monitoring applications due to their rapid response, remarkable sensitivity, selectivity, inherent miniaturization, low cost and independence of sample turbidity. The electrochemical oxidation of silver metal produces silver ions and electrons accompanied by a reversible redox signal224,225. Upon reduction, the silver ions return to the surface and a reduction current is measured. As shown in
Moreover, all the three food supplement samples exhibited these redox peaks at similar potentials with a slight shift that may be due to the interaction between the silver NPs, the sizes and shapes of silver NPs226 and their capping organic citric acid molecules. This interaction had been well demonstrated by the voltammetric signals recorded for Colloidal Silver in
In electrochemical detection, DPV technique was also used to monitor the electrochemical oxidation of silver NPs following its isolation from the food supplements samples. DPV has higher sensitivity than CV and could provide a better resolution as well as quantitative information related to trace amounts of silver NPs concentrations.
The DPV data for the standard 40 nm silver NPs produced an oxidation peak which was quite stable as shown in
In order to confirm the selectivity of this method for detection of silver NPs, some common NPs such as gold NPs and ZnO NPs that are also easily obtained from the market and widely used in consumer products were tested with electrochemical method as silver NPs. As shown in
A successful detection method should not only identify silver NPs from other NPs, but also tell the difference between various silver elemental status, from non-oxide to oxidated status. In this case, silver ion is the major concern which may interfere with the detection of silver NPs. In order to mask the effect of silver ions in the detection of silver NPs, EDTA was used to wash off silver ions from PAA membrane surface. Although we assumed that the major mechanism of trapping silver NPs on PAA membrane surface was due to the surface pore size and hence ions should flow through membrane with solution, we still observed silver ions redox peaks when we only filtered silver nitrate solution by PAA membrane (
Silver NPs were also washed with EDTA after they were captured by PAA membrane. However, unlike silver ions, the height of redox peaks assigned to silver NPs did not change much (
AAS analysis for silver is based on the absorbance of silver ions at 328 nm and where the silver ions have the highest absorbance. Solutions of AgNO3 were used to develop these calibration lines.
In
E=(Epn+Epc)/2 Equation (4)
where E is the half redox reaction potential, Epa is potential for oxidation peak and Epc is potential for reduction peak. Usually, Ag/AgCl reference electrode filled with saturated KCl solution has a standard electrode potential of 0.197V compared with standard hydrogen electrode (SHE). And Ag/AgCl reference electrode filled with 3M KCl solution has a standard electrode potential of 0.210V compared with SHE. The RE 5B reference electrode used in our experiment is filled with 3M NaCl solution according to the description of manufacturer. So 0.210V was chosen to convert the observed redox potential into standard potential compared with SHE. And the standard potential for the first pair is 0.403V while the standard potential for the second pair is 0.658V. Reaction (1) to (7) are the possible redox reactions related to silver element. According to Pourbaix diagram of silver, the redox reactions and corresponding potentials vary at different pH values229. The theoretical potentials of Reaction (3) and (4) at pH 7.0 are 0.341V and 0.569V respectively. So the first redox peak pair can be assigned to Reaction (3) and the second redox peak pair can be assigned to redox reaction of AgO (Reaction 4).
AgCl(s)+e−⇄Ag(s)+Cl−(aq) E0=0.222V Reaction (1)
Ag+(aq)+e−⇄Ag(s) E0=0.8V Reaction (2)
Ag2O(s)+2H+(aq)+2e−⇄2Ag(s)+H2O E0=1.17V Reaction (3)
2AgO(s)+2H+(aq)+2e−⇄Ag2O(s)+H2O E0=1.40V Reaction (4)
Ag2O3(s)+6H+(aq)+4e−⇄2Ag+(aq)+3H2O E0=1.67V Reaction (5)
AgO(s)+2H+(aq)+e−⇄Ag+(aq)+H2O E0=1.77V Reaction (6)
Ag2+(aq)+e−⇄Ag+(aq) E0=1.98V Reaction (7)
In
The two sharp reduction peaks for low addition amount of MesoSilver in
In summary, the discussion above presents three methods for detection of silver NPs. The electrochemical method using PAA membrane can successfully capture and detect silver NPs quantitatively. This method with PAA membranes also showed unique advantage compared to the optical method and the electrochemical method without PAA membrane. The PAA membranes provided a simple approach to concentrate and isolate the NPs sample by varying filtration volume with minimal sample preparation. And the electrochemical detection was fast, requiring only few minutes. With PAA membrane and EDTA masking method, this electrochemical detection technique targeted silver NPs merely without silver ions. The optical method can provide quantitative measurement only for non-protected silver NPs while electrochemical method without PAA membrane cannot eliminate the effect of silver ions.
In view of the foregoing, the discussion above describes, in varying detail, information and data to quantify and qualify various embodiments of filter devices, membranes, and implementations related thereto.
Implementation I presents the synthesis and fabrication of embodiments of PAA thermally-cured membrane and its derivatives. The polymers were synthesized using ODA and PMDA via thermal curing process. In order to prevent imidization and formation of PI, 75° C. was chosen for the thermal curing of PAA and a range of PAA derivatives were synthesized. The NMR and FTIR results confirmed that the functional carboxyl and amide groups were retained in the PAA series synthesized. Results also show that low temperature (75° C.) synthesis can successfully prevent the formation of PI from PAA. After comparing the mechanical properties of PSG resulting from the various compositions and different ratios of gold and silane, the molar ratio of PAA/gold in 16:1 and molar ratio of PAA/APTMOS/TMOS/TMOSPA in 20:5:5:1 were found to be the best conditions for these polymeric derivatives. The resulting membranes were flexible, transparent, conducting, and electroactive. The effect of thermal curing temperature to the quantity and size of gold NPs was discussed. With increasing thermal curing temperature, more gold NPs appeared on the surface of PG membranes. However, few gold NPs were observed on the surfaces of PSG membranes. The silicone content in PSG membranes contained gold NPs inside the membranes and it was noticed that the particles shifted to the surface at higher temperatures.
Implementation II presents the synthesis of embodiments of phase-inverted PAA membranes and its derivatives. Although the chemical compositions were the same as the thermally-cured membranes, the phase-inverted membranes were found to exhibit a totally different morphology and physical structure from the thermally-cured membranes. Phase-inverted PAA membrane and its derivatives are flexible, opaque, sponge-like and nano-porous membranes. Their structures are anisotropic, consisting of a dense nano-porous thin top layer and a thick supporting sublayer with micro size pores. Embodiments can comprise surface nano-pores of size that can be adjusted by the concentration of the casting PAA solutions. The change of surface pore size versus the corresponding concentration of casting solution was found to be described by a single component exponential decay. Generally, the mean pore size of PAA membrane decreases with increasing concentration of the casting solution. At low concentration range (smaller than 0.25M), the mean pore sizes of membranes decreased dramatically with increasing concentrations. From 0.25M to higher concentrations, the resulting pore sizes change in small range. Embodiments of the PAA membranes were also coated onto the surfaces of filter papers to create robust PAA-filter paper membrane and filter devices. The coated filter papers have a thin layer of PAA with similar pore size as its corresponding stand-alone PAA membranes. Moreover, the PAA-coated layer was observed to have smoother surfaces than the stand-alone membranes, and they also have smaller pore size range. Finally, the PAA-coated filter papers were more durable than PAA membranes alone because of the supporting filter paper.
In Implementation II, the casting solution could not be evenly spread onto the substrate, because the hand dispersion of PAA casting solution was not perfect. This led to some defects in the membranes, including uneven thickness, cracks on surface and holes resulting from trapped air bubble during fabrication. Automatic dispersion may remedy this deficiency for PAA fabrication.
Embodiments of the phase-inverted PAA membranes also exhibit unique optical properties which are different from its thermally-cured membranes. These embodiments generate some special fluorescent emissions after they were excited at various excitation wavelengths. This feature may indicate that the shift of emission wavelength compared with excitation wavelength can be better explained by enhanced Raman scattering and not by conventional fluoresecence spectroscopic principles. The results of enhanced Raman scattering spectroscopy confirmed that embodiments of phase-inverted PAA membrane can promote enhance Raman scattering on its surface, which could be attributed to the membrane's unique porous surface and conducting polymer nature. The last part of Implementation II presents the results of storage solution and stability of phase-inverted membranes. These results are not conclusive because the variety of the solvents tested for storage solution was quite limited. The results are limited to solvent combination of water, ethanol, acetone, DMAc and DMF; however, these results could be enhanced using more solvents, multi-phase solutions, and double-phase solutions for storage of PAA membrane.
Implementation III presents nano-filtration (NF) using phase-inverted PAA membranes. Quantum dots, silver NPs and TiO2 NPs ranging from 20 nm to 150 nm were filtered separately using the phase-inverted PAA membranes. The general filtration efficiency was found to be above 80% with the highest single efficiency reaching 99.97%. Performance of the PAA membrane was compared with commercial filters. Results showed that qualitative filter papers barely captured any silver NPs. Nylon filter membranes gave an overall filtration efficiency of 44.52%. The overall average filtration efficiency of aluminum oxide membrane was 78.28%. Unlike commercial membranes, embodiments of the PAA membranes exhibited superior performance. Phase-inverted PAA membranes were found to exhibit superior durability and higher efficiency. In silver NPs filtration, PAA membranes had overall filtration efficiency of 98.5%, while in filtration of quantum dots the overall average efficiency of 87.46% was reported. PAA coated filter papers exhibited comparable filtration efficiency as PAA membrane without filter paper as the substrate.
Implementation III also illustrates the capability of embodiments of phase-inverted PAA membranes for the separation of engineered NPs. Several NPs mixtures were tested, consisting of metal-based NPs, such as a mixture gold NPs with various sizes, and a mixture of TiO2, silver, gold NPs. Other mixtures were consisted of organic and inorganic NPs such as gold NPs and polystyrene nano-beads. No matter which mixture, the separation mechanism using phase-inverted PAA membranes were in accordance with the pore sizes of these membranes employed. These membranes have only size-selectivity but no elemental selectivity was found. However, chemical modification to the PAA membrane might improve the selectivity of separation. These modifications may include, for example, functional groups on PAA. Modified PAA could either be more hydrophilic or be more hydrophobic, and hence improve its selectivity to organic NPs or inorganic NPs. These membranes can slightly aggregate NPs on membrane surface, which can block some pores on the surface and hence affected the efficiency of separation. The aggregation effect may influence the calibration of NPs after each step of separation.
Implementation IV shows that PAA membranes are not just filters; they could simultaneously serve as sensors for silver NPs detection and quantization. The first part of Implementation IV describes optical method for detection of silver NPs with a detection limit of 1 μg/ml. However, this method is based on redox chemical reaction and may only apply to NPs which have no capping layer on their surface. For those capped NPs, they are protected by the capping layer from this redox chemical reaction.
Implementation IV also discusses electrochemical method for quantitative detection of silver NPs. Utilizing PAA membrane as a concentration and sensor substrate, we reported a detection limit of 100 ppb with a filtration volume of 15 ml. These methods may be selective to particular types of particles (e.g., silver NPs). This implementation also describes utilization of EDTA to mask the interfering ions which improved the selectivity of this detection method. However, in some examples, because the PAA membrane was attached to gold electrodes, the sensitivity was decreased by contact potential. Compared with AAS verification which has a detection limit of 100 ppb, electrochemical detection of silver NPs using PAA membrane has a similar result for the concentrations of silver NPs samples with a higher detection limit of 200 ppb for 10 ml sample. Some embodiments may benefit from improvements in fabrication of PAA membrane electrode. One way to improve the fabrication can be a direct fabrication of PAA membrane coated onto gold mesh instead of simple attachment to gold electrode after fabrication of the stand-alone membrane. In this way, the contact potential between the PAA membrane and gold could be decreased. Another method that can be considered is to improve the conductivity of PAA by adding other highly conductive materials such as gold NPs and carbon nano-tubes.
Method for electrochemical detection of silver NPs without PAA membrane was presented in the last part of Implementation IV. Although this method can detect silver NPs in aqueous solution as well, it can eliminate the effect of silver ions. The function of NaCl in buffer solution was investigated. The addition of NaCl to buffer solution improved the sensitivity of detection. Method of utilizing PAA membrane is more selective and sensitive than that without PAA membrane.
Summarily, the discussion above identifies various embodiments of PAA-based membranes (e.g., phase-inverted PAA membranes) using a range of synthetic approaches. These embodiments can apply to novel applications, for example, PAA membranes have shows Raman activity as nano-filters and as sensors for engineered nano-materials.
As used herein, an element or function recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural said elements or functions, unless such exclusion is explicitly recited. Furthermore, references to “one embodiment” of the claimed invention should not be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.
This written description uses examples to disclose embodiments of the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.
This application claims the benefit of priority to U.S. Provisional Application No. 61/851,596, filed on Mar. 8, 2013 and entitled “MEMBRANE NANOFILTERS.” The content of this application is incorporated by references herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61851596 | Mar 2013 | US |
