U.S. patent application number 15/137904 was filed with the patent office on 2016-10-20 for converting nanoparticles in oil to aqueous suspensions.
The applicant listed for this patent is William Marsh Rice University. Invention is credited to Hitesh Ghanshyam BAGARIA, Gautam Chandrakanth KINI, Wen Yin Lynn KO, Michael S. WONG.
Application Number | 20160303530 15/137904 |
Document ID | / |
Family ID | 41669191 |
Filed Date | 2016-10-20 |
United States Patent
Application |
20160303530 |
Kind Code |
A1 |
WONG; Michael S. ; et
al. |
October 20, 2016 |
CONVERTING NANOPARTICLES IN OIL TO AQUEOUS SUSPENSIONS
Abstract
An improved process for converting an oil suspension of
nanoparticles (NPs) into a water suspension of NPs, wherein water,
surfactant and a non-surfactant salt is used instead of merely
water and surfactant, leading to greatly improved NP aqueous
suspensions.
Inventors: |
WONG; Michael S.; (Houston,
TX) ; BAGARIA; Hitesh Ghanshyam; (Houston, TX)
; KINI; Gautam Chandrakanth; (Houston, TX) ; KO;
Wen Yin Lynn; (Houston, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
William Marsh Rice University |
Houston |
TX |
US |
|
|
Family ID: |
41669191 |
Appl. No.: |
15/137904 |
Filed: |
April 25, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13056858 |
May 24, 2011 |
9321021 |
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PCT/US2009/052312 |
Jul 30, 2009 |
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15137904 |
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61088225 |
Aug 12, 2008 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01J 13/0008 20130101;
B01F 17/0085 20130101; Y10S 977/773 20130101; Y10S 977/786
20130101; B01J 13/0039 20130101 |
International
Class: |
B01J 13/00 20060101
B01J013/00; B01F 17/00 20060101 B01F017/00 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH STATEMENT
[0002] This invention was made with government support under Grant
No. CBET 0652073 awarded by the National Science Foundation. The
government has certain rights in the invention.
Claims
1. A method of converting nanoparticles (NPs) in oil to NPs in
aqueous suspension comprising: adding a surfactant and water and a
non-surfactant salt in any order to a mix of NPs in oil, such that
the concentration of the surfactant is above the critical micellar
concentration, and stirring at a temperature above the boiling
point of said oil and below that of water to form an aqueous
suspension of NPs wherein said suspension of NPs contain more
compact surfactant micelles than those formed in water without a
non-surfactant salt.
2. The method of claim 1, further comprising adding alcohol or
amines or both to the mix of NPs in oil.
3. The method of claim 1, wherein said surfactant is selected from
anionic, cationic, nonionic, zwitterionic, or Gemini surfactants,
and combinations thereof.
4. The method of claim 1, wherein said surfactant is sodium
1,2-bis(2-ethylhexoxycarbonyl)ethanesulfonate (AOT).
5. The method of claim 1, wherein said surfactant is selected from
cetyl trimethyl ammonium bromide (CTAB), cetylpyridinium chloride
(CPC), benzalkonium chloride (BAC), Benzethonium chloride (BZT),
amine and imidazoline salts such as primary, secondary and tertiary
amine hydrochlorides, imidazoline hydrochlorides alkyl
poly(ethylene oxide), copolymers of poly(ethylene oxide) and
poly(propylene oxide), alkyl polyglucosides, alkyl phenol
ethoxylate, polyoxyethylene esters of fatty acids, polyoxyethylene
fatty acid amides, alkyl polysaccharides, ethoxylates of alkyl
amine, castor oil, end capped synthetic alcohol, tallow amine and
alkanol amine mercaptan, dodecyl betaine, dodecyl dimethylamine
oxide, docamidopropyl betaine, coco ampho glycinate and
combinations thereof.
6. The method of claim 1, wherein said non-surfactant salt
comprises a cationic portion selected from the group consisting of
alkali, alkaline earth metal, sodium, potassium, magnesium,
calcium, and quaternary ammonium and an anionic portion selected
from the group consisting of halide, acetate, malate, fumarate,
citrate, succinate, phosphate, polyphosphates, hypochlorite,
perchlorite, carbonate, bicarbonate, bisulphite, sulphite,
sulphonate, and nitrate.
7. The method of claim 1, wherein said non-surfactant salt is
NaCl.
8. The method of claim 1, wherein said NPs in oil comprises a
mixture of two or more different NPs.
9. A method of converting nanoparticles (NPs) in oil to NPs in
aqueous suspension comprising: a. dissolving NPs into an oil having
a boiling point lower than that of water; b. preparing a surfactant
solution in a non-surfactant salt-water such that the concentration
of the surfactant is above the critical micellar concentration; and
c. adding the NP containing oil blend of step a) to the surfactant
solution and stirring the resulting dispersion at a temperature
above the boiling point of said oil and below that of water to form
an aqueous suspension of NPs d. wherein more compact surfactant
micelles are formed in said non-surfactant salt-water than those
form in water not containing a non-surfactant salt.
10. The method of claim 9, further comprising adding alcohol or
amines or both to the mix of NPs in oil.
11. The method of claim 9, wherein said stirring occurs above the
boiling point of said oil and below that of water.
12. The method of claim 9, wherein said surfactant is selected from
anionic, cationic, nonionic, zwitterionic, or Gemini surfactants,
and combinations thereof.
13. The method of claim 9, wherein said surfactant is sodium
1,2-bis(2-ethylhexoxycarbonyl)ethanesulfonate (AOT).
14. The method of claim 9, wherein said surfactant is selected from
cetyl trimethyl ammonium bromide (CTAB), cetylpyridinium chloride
(CPC), benzalkonium chloride (BAC), Benzethonium chloride (BZT),
amine and imidazoline salts such as primary, secondary and tertiary
amine hydrochlorides, imidazoline hydrochlorides alkyl
poly(ethylene oxide), copolymers of poly(ethylene oxide) and
poly(propylene oxide), alkyl polyglucosides, alkyl phenol
ethoxylate, polyoxyethylene esters of fatty acids, polyoxyethylene
fatty acid amides, alkyl polysaccharides, ethoxylates of alkyl
amine, castor oil, end capped synthetic alcohol, tallow amine and
alkanol amine mercaptan, dodecyl betaine, dodecyl dimethylamine
oxide, docamidopropyl betaine, coco ampho glycinate and
combinations thereof.
15. The method of claim 9, wherein said non-surfactant salt
comprises a cationic portion selected from the group consisting of
alkali, alkaline earth metal, sodium, potassium, magnesium,
calcium, and quaternary ammonium and an anionic portion selected
from the group consisting of halide, acetate, malate, fumarate,
citrate, succinate, phosphate, polyphosphates, hypochlorite,
perchlorite, carbonate, bicarbonate, bisulphite, sulphite,
sulphonate, and nitrate.
16. The method of claim 9, wherein said non-surfactant salt is
NaCl.
17. The method of claim 9, wherein said NPs in oil comprises a
mixture of two or more different NPs.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 13/056,858, filed Jan. 31, 2011, which was a national stage
filing of PCT/US2009/52312, filed Jul. 30, 2009, which claims
priority to U.S. Provisional Application No. 61/088,225, filed Aug.
12, 2008. Each of these applications is incorporated herein by
reference in its entirety for all purposes.
REFERENCE TO MICROFICHE APPENDIX
[0003] Not applicable.
FIELD OF THE INVENTION
[0004] The invention generally relates to an improved process for
converting an oil suspension of nanoparticles (NPs) into a water
suspension of NPs. Conventional methods of preparing NP dispersions
focus on transferring NPs from oil to water using a
surfactant--this is referred to as phase-transfer. The novelty of
the process described herein is that salt-containing water is used
instead of plain water, leading to the desired improvement in the
NP suspensions. The improvement comes in the form of more stable
aqueous NP suspensions, wherein the size and functionality of NPs
(luminescence, magnetization, electrical properties etc.) are
retained or even enhanced.
BACKGROUND OF THE INVENTION
[0005] NPs have found several applications due to their high
specific surface area and unique optical, electrical and magnetic
properties in fields such as medical diagnostics, drug delivery,
foods, energy and catalysis, combinatorial libraries,
semiconductors and electronics..sup.1-6 A key challenge in
phase-transfer of NPs involves the prevention of drastic
alterations to the functional properties of NPs while in aqueous
phase. A well documented example is the degradation in optical
properties of CdSe quantum dots (QD) in water, brought about by
water-induced fluorescence quenching. Methodologies to prevent QD
surfaces from coming in contact with water include the growth of
physical barriers such as a shell (core-shell quantum dots) or the
use of surfactant and polymeric systems to disperse and protect the
NPs from interactions with water..sup.7,8
[0006] Previous groups have employed
microemulsion/emulsion/surfactant routes to phase-transfer NPs to
water..sup.9-12 Fan et al. (2005) discuss a process to
phase-transfer NPs dissolved in chloroform by formulating an oil in
water microemulsion with chloroform as oil and cetyl
trimethylammonium bromide (CTAB) as surfactant, wherein chloroform
is evaporated by heating the microemulsion. This process gives
aqueous dispersions of NPs with high stability, but results in
drastic lowering in functional properties of NPs, specifically
luminescence..sup.10
[0007] Dubertret et al. (2002) report successful dispersion of
single quantum dots in water using twin-tailed surfactants such as
1,4-Bis(2-ethylhexyl) sodium sulfosuccinate (Aerosol OT or
AOT)..sup.11 In their proposed route, quantum dots were dissolved
in chloroform to which AOT was added, and the chloroform was
evaporated to leave behind a residue. The residue was then heated
at 80.degree. C. after which 1 mL of water was added to give a
stable clear dispersion of quantum dots in water. When the
procedure discussed above was repeated with addition of salt
(unspecified composition) in lieu of pure water, the resulting
quantum dot suspensions were reported to be unstable..sup.11 Thus,
Dubertret teaches away from using salt to improve phase
transfer.
[0008] Li et al. (2007) report phase-transfer of CdSe/ZnS
core-shell quantum dots from chloroform to water. CdSe/ZnS
core-shell quantum dots were solubilized in chloroform upon which
the mixture was transferred to a solution of Gemini 12-4-12
surfactant that resulted in the formation of oil-in-water
microemulsion. Specifically, the CdSe/ZnS core/shell quantum dots
in Li were coated with stabilizing ligands such as TOPO and these
modified CdSe/ZnS were added to a surfactant aqueous solution. The
alkanes on the stabilizing ligands interacted with the alkanes on
the surfactants to form an interdigitated bilayer structure. The
core-shell quantum dots were phase-transferred to water by
evaporating chloroform at 50.degree. C. for 30 minutes.
Photoluminescence studies revealed an increase in intensity of
core-shell quantum dots in aqueous phase over those in chloroform
and the phase transferred NP dispersion was found to be stable. The
authors also mention in the paper that the NP dispersions were
stable even after two months, in terms of photoluminescence
intensities. No photoluminescence intensities values measured after
2 months were published in the paper, however, and it is important
to note that the reported luminescence values were for a CdSe--ZnS
core-shell structure,.sup.12 not a naked QD.
[0009] Schematic 1, displayed in FIG. 20, summarizes current
methods to phase-transfer NPs, wherein NPs present in oil are added
to water that contains a dispersing/stabilizing agent (surfactant).
Surfactants have "oil-loving" (hydrophobic) and "water-loving"
(hydrophilic) groups in its chemical structure and exist as
structures called "micelles" in water. The hydrophilic ends of the
micellar structure face water, whereas the hydrophobic groups are
shielded from water and are located in the interior of the
micelle..sup.13 When oil is added to water containing surfactant
micelles, the surfactant molecules partition to the interface of
oil and water so as to disperse oil in water. These dispersions are
then heated to evaporate or "boil-off" the oil. Consequently, NPs
are contained within the hydrophobic part of the micelle and the
hydrophilic portion disperses them in water.
[0010] However, what is needed in the art are better methods to
phase-transfer NPs to water without disturbing key properties of
the nanoparticles.
SUMMARY OF THE INVENTION
[0011] We have found quite surprisingly that phase-transfer of NPs
in water containing salt facilitates formation of compact
surfactant micelles that helps improve NP dispersibility and retain
their functionality over several weeks. Particularly noteworthy is
the retention of functionality for the case of bare QDs and
tetrapods in aqueous phase over time periods of several weeks. This
has never been reported in previous literature, which focuses on
protective core-shell structures to shield QD surfaces from coming
in contact with water.
[0012] Schematic 2, displayed in FIG. 21, summarizes the method of
the invention. The methodology to phase-transfer NPs is the same as
described above, except that salt-containing water is used in the
invention instead of plain water. An alternate method to
phase-transfer NPs is by blending surfactants directly to oil
containing NPs upon which, the mixture is added to salt-water or
plain water and oil is evaporated. Even through this route, the
benefits of NP phase-transferred in salt-containing water is
significantly greater over phase-transfer in plain water..sup.14
Table 1 summarizes the benefits of the new process over existing NP
phase-transfer methods.
TABLE-US-00001 TABLE 1 Benefits of new phase-transfer process over
existing methods Phase-transfer in Salt- Containing Water (Using
Phase-transfer in Water newly proposed process) Retains Retains PL
Route in Process Forms PL.sup.+ in Forms in Literature Summary
System Dispersion? solution? Dispersion? solution? Fan et al. Oil +
NPs - add Oil - Yes, but all Poor PL Yes, most High PL 2005, to
surfactant Chloroform, NPs crash NPs retained 2004 solution in
plain Surfactant - out upon in solution water, stir to form CTAB,
centrifugation upon microemulsion, NP - CdSe (FIG. 1)
centrifugation heat to evaporate QDs (FIG. 1) oil and thus, form NP
dispersion in water that are stabilized by surfactants Bagaria et
Oil + NPs - add Oil - Yes, but bulk Poor PL Yes, most High PL al.
2009.sup.14 to surfactant Hexane, of NPs crash (FIGS. 2, 3) NPs
retained (FIGS. 2, 3) solution in salt- Surfactant - out upon in
solution containing water, Aerosol centrifugation upon form OT
(AOT), (FIGS. 2, 3) centrifugation microemulsion, NP - CdSe (FIGS.
2, 3) heat to evaporate QD oil and form NP dispersions.
Alternately, Oil + NPs + Surfactant - add to salt- containing water
to form microemulsion, heat to evaporate oil and thus, form NP
dispersions Dubertret Oil + NP + Oil - No, all NPs Poor PL **Not
Not et al. 2002 Surfactant, heat Chloroform, crash out applicable
applicable to evaporate oil, Surfactant - upon add water or salt
AOT, centrifugation solution to NP - CdSe (FIG. 4) disperse NP in
QD water Li et al. Oil + NP + Oil - Yes Yes To try To try 2007
Surfactant, add to Chloroform, water to form Surfactant - emulsion,
heat to Gemini evaporate oil and 12-4-12, thus form NP NP -
dispersion in CdSe/ZnS water stabilized Core-Shell by surfactant
.sup.+ PL = Photoluminescence *Dubertret et al. (2002) make the
case that when salt was added to the residue obtained by heating
the oil-NP-surfactant blend, it gave unstable dispersions. **NP
phase transfer using the process by Dubertret et al. (2002) is
compared with the phase transfer process developed by Wong and
coworkers herein (refer to Bagaria et al. 2009 for further
details)
[0013] Salts herein refer primarily to the salting-out variant
(called salting-out electrolyte or water structure makers) that are
known to have the following effects that prove useful in assisting
NP phase-transfer..sup.15 These salts readily dissociate in water
to yield anions and cations and are known to lower the critical
micellar concentration (CMC, or concentration at which surfactants
form micellar structures), resulting in the formation of compact
micelles through a charge screening mechanism..sup.16
[0014] The salt comprises a cationic portion selected from the
group consisting of alkali, alkaline earth metal, sodium,
potassium, magnesium, calcium, and quaternary ammonium and an
anionic portion selected from the group consisting of halide,
acetate, malate, fumarate, citrate, succinate, phosphate,
polyphosphates, hypochlorite, perchlorite, carbonate, bicarbonate,
bisulphite, sulphite, sulphonate, and nitrate.
[0015] As an alternate to salting-out electrolytes, salting-in
electrolytes or water structure breakers can be used to
phase-transfer NPs. These salts do not dissociate in water and are
known to increase the CMC of surfactants. Examples of such salts
include (but is not restricted to) are urea, guanidium chloride,
1-4 dioxane and the like. However, they can still facilitate
compact micelle formation from charge screening. Even here, the NP
dispersibility is better than when phase-transfer is carried out in
plain water. Hence, oil-containing NPs in the new procedure are
contained in smaller and more compact surfactant micelles than
those formed in plain water. As a consequence, NP dispersibility
upon evaporation of oil improves with superior retention in NP
properties.
[0016] The temperature at which phase-transfer is carried out is
typically high--above the boiling point of the organic solvent, but
below that of water. This allows complete evaporation of oil to
give NP dispersions in water that could be directly applicable in
biological, health-care, pharmaceutical and food related fields,
where the presence of oil is undesirable. At this high temperature,
the solubility of surfactants in water is very high (called the
Kraft temperature in literature for ionic or charged surfactants).
Hence, their ability to form compact micelles in order to
accommodate introduced oil is lower. Salts of the salting-out
variety overcome this problem, wherein at appropriate
concentrations they reverse the solubility trends to make
surfactants available for stabilizing NPs during high temperature
phase-transfer. This is facilitated by the salt disrupting the
hydrogen bonding network of water with surfactants..sup.16 In
addition to reversing solubility trends of ionic surfactants, salts
also enhance compact micelle formation through a charge screening
mechanism. Addition of salt, by making surfactants available for
NPs, also increases the fraction of particles that get transferred
from oil to water.
[0017] Three things should be noted. First, phase-transfer of NPs
may be carried out at room temperature, but is preferably carried
out at temperatures above the boiling point of oils and below that
of water to be directly applicable in the pharmaceutical and
food-related sectors. Second, alcohols or amines are sometimes
required to be added in conjunction with salts to create a stable
NP dispersion. Alcohols and amines are known to enhance the
compactness of surfactant films by promoting hydrophobic
interactions between surfactant monomers and through dissolution of
liquid crystalline micelles of surfactants. Finally, in addition to
the procedure described above, it is noteworthy that surfactants
could be initially mixed with the oil-NP blend instead of being
present in water or salt-containing water. There are slight
differences in the NP phase-transfer trends, but the benefits of NP
phase-transfer in salt-containing water is still significantly
greater over phase-transfer in plain water.
[0018] Addition of salt sometimes results in the formation of
liquid crystalline surfactant aggregates that may make the
dispersion appear turbid. It may be desirable to remove these
aggregates from the NP dispersion for certain applications. In such
cases, the removal of aggregates may be easily performed either by
centrifugation of the NP dispersion or by passing the
phase-transferred NPs through an appropriate filtration device
leading to clear NP dispersions. Alternatively, the liquid
crystalline phases may be dissolved by addition of alcohols or
amines.
[0019] Phase-transfer by this newly proposed method results in
dispersions that have following features: (1) High stability of NPs
from aggregation for several weeks/months, and (2) Retention in
functional properties of NPs for several weeks/months. Stability
from aggregation and retention of NP functionality is significantly
better by this route when compared to dispersions obtained by
conventional methods that phase-transfer NPs to water.
[0020] The steps are generally described as follows:
[0021] 1. Dissolve NPs into oils having boiling points lower than
that of water, such as hexane or chloroform.
[0022] 2. Prepare a surfactant solution in water/salt-containing
water such that the concentration of the surfactant is above the
CMC. Add the NP containing oil blend (as prepared in step 1) to the
surfactant solution and stir the resulting dispersion (called
Oil-in-Water (O/W) emulsion) for 1 hour using a magnetic stirrer.
Heat the emulsion in a water bath for 20-30 minutes, at a
temperature above the boiling point of the oil. Cool the resulting
solution so as to form the NP dispersion in water.
[0023] 3. An alternate procedure would be to add surfactant to the
NP containing oil blend to form a NP-oil-surfactant mix. Add water
to this mixture to form Water-in-Oil (W/O) microemulsion. Add a
dosed amount of microemulsion to water/salt-containing water such
that the surfactant concentration is above the CMC, and stir the
solution for 1 hour so as to form an emulsion. Heat the emulsion in
a water bath for 20-30 minutes, at a temperature above the boiling
point of the oil if it is desired to remove the oil. Cool the
resulting solution so as to form the NP dispersion in water.
[0024] 4. Another alternative would be to add surfactant to the NP
containing oil blend to form a NP-oil surfactant mix. Add a dosed
amount of this NP-oil surfactant mix to water/salt-containing water
such that the surfactant concentration is above the CMC, and stir
the solution for 1 hour so as to form an emulsion. Heat the
emulsion in a water bath for 20-30 minutes at a temperature above
the boiling point of the oil. Cool the resulting solution so as to
form the NP dispersion in water.
[0025] Other possible variations are described in the following
sections.
[0026] The present inventive methods differ from those in the cited
art such as Li (2007) or Li et al, Sensors and Actuators B:
Chemical (2008) because a non-surfactant salt and surfactant is
used.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1: UV-Vis spectra (left) of CdSe NPs phase transferred
in water and 3 g/L NaCl solution with CTAB as the surfactant, and
photographs (right) of these samples. Both samples were centrifuged
at 9000 rpm for one hour after centrifugation.
[0028] FIG. 2: UV-Vis absorbance spectra (top) and PL spectra
(bottom) of spherical CdSe QDs transferred from hexane to water
containing different levels of salt concentration (Salt=NaCl at 3
g/L, Surfactant=AOT).
[0029] FIG. 3: % PL of spherical CdSe quantum dots as a function of
time, when transferred from hexane to water containing different
levels of salt concentration (Salt=NaCl, Surfactant=AOT).
[0030] FIG. 4: Phase-transferred CdSe QDs using the procedure as
described by Dubertret et al. (2002). A blend of
chloroform-AOT-CdSe was heated to boil off chloroform. The
resulting residue were added to (i) water and NaCl solutions (ii) 1
g/L (iii) 2.5 g/L (iv) 3 g/L and (v) 5 g/L and centrifuged at 9000
rpm for 1 hour. It is evident that the QDs crashed out for all
cases indicating unstable dispersions.
[0031] FIG. 5: UV-Vis absorbance spectra (top) and PL spectra
(bottom) of CdSe tetrapods transferred from hexane to water
containing different levels of salt concentration (Salt=NaCl at 2.5
g/L, Surfactant=AOT).
[0032] FIG. 6: % PL of CdSe tetrapods as a function of time, when
transferred from hexane to water containing different levels of
salt concentration (Salt=NaCl, Surfactant=AOT).
[0033] FIG. 7: UV-Vis absorbance spectra (top) and PL intensity
spectra (bottom) of concentric spherical CdSe/ZnS core-shell
quantum dots, transferred from hexane to water containing different
levels of salt concentration (Salt=NaCl at 3 g/L,
Surfactant=AOT).
[0034] FIG. 8: % PL of CdSe/ZnS core-shell particles as a function
of time, when transferred from hexane to water containing different
levels of salt concentration (Salt=NaCl, Surfactant=AOT).
[0035] FIG. 9: UV-Vis absorbance spectra of C.sub.60/Bucky balls
transferred from hexane to water containing different levels of
salt concentration (Salt=NaCl, Surfactant=AOT).
[0036] FIG. 10: UV-Vis absorption spectra of Gold (Au) NPs
transferred from hexane to water containing different levels of
salt concentration (Salt=NaCl, Surfactant=AOT).
[0037] FIG. 11: Iron-oxide NPs (i) in hexane, (ii)
phase-transferred to plain water and (iii) phase-transferred into 3
g/L NaCl.
[0038] FIG. 12: PL spectra of spherical CdSe QDs, transferred from
chloroform to water containing different levels of salt
concentration (Salt=NaCl at 3 g/L, Surfactant=AOT).
[0039] FIG. 13: UV and PL spectra of CdSe transferred from hexane
to water containing sodium sulfate (Na.sub.2SO.sub.4).
(Surfactant=AOT).
[0040] FIG. 14: UV-Vis spectra of CdSe NPs transferred from hexane
to water containing CaCl.sub.2 (surfactant=AOT). CdSe spherical QDs
phase transferred into CaCl.sub.2 solution at (i) 0.05 g/L and (ii)
0.1 g/L. Photographs taken under regular white light.
[0041] FIG. 15: UV-Vis spectra and photograph (under white light)
of CdSe NPs transferred from hexane to water containing 1.0 g/L of
sodium citrate (surfactant=AOT).
[0042] FIG. 16: UV-Vis spectra of CdSe NPs phase transferred in
water and 3 g/L NaCl solution with DDAB as the surfactant Both
samples were centrifuged at 9000 rpm for one hour after
centrifugation.
[0043] FIG. 17: UV-Vis spectra of CdSe NPs transferred from hexane
to water containing NaCl at 3 g/L using a surfactant blend of AOT
(anionic) and Polystep C-M4S (non-ionic) surfactant.
[0044] FIG. 18: UV-Vis spectra of CdSe NPs in 3 g/L NaCl before and
after centrifugation showing the removal of scattering components
in phase-transferred solutions.
[0045] FIG. 19: Stability of 3 nm CdSe NPs in KCl solution after
transfer from hexane to water or 3 g/L NaCl solution using
AOT-Polystep C-M4S surfactant blend.
[0046] FIG. 20: Schematic 1 displays the conventional method of
phase-transferring NPs from oil to plain water.
[0047] FIG. 21: Schematic 2 displays the inventive method of
phase-transferring NPs from oil to salt water.
DETAILED DESCRIPTION OF THE INVENTION
[0048] The invention is a novel way of converting oil preparations
of NPs to NPs dispersed in aqueous phase, using an additional
ingredient--salt. Phase-transfer by the newly proposed method
results in dispersions that have following features: (1) High
stability of NPs from aggregation for several weeks/months, and (2)
Retention in functional properties of NPs for several weeks/months.
Stability from aggregation and retention of NP functionality is
significantly better by this route when compared to dispersions
obtained by conventional methods that phase-transfer NPs to water.
Further, addition of salt also increases the fraction of particles
that get transferred from oil to water.
[0049] A preferred salt for use in the invention is NaCl, but many
other salts can be used, provided only that the salt function to
lower the critical micellar concentration and to form compact
micelles by charge screening of surfactants and does not otherwise
alter the functionality of the NP, oil or surfactant. In preferred
embodiments, the salt comprises a cationic portion selected from
the group consisting of alkali, alkaline earth metal, sodium,
potassium, magnesium, calcium, and quaternary ammonium and an
anionic portion selected from the group consisting of halide,
acetate, malate, fumarate, citrate, succinate, phosphate,
polyphosphates, hypochlorite, perchlorite, carbonate, bicarbonate,
bisulphite, sulphite, sulphonate, and nitrate. Salts that can be
used include, but are not limited to, NaCl, KCl, CaCl.sub.2,
MgCl.sub.2, Na.sub.2SO.sub.4, K.sub.2SO.sub.4, CaSO.sub.4,
Na.sub.3PO.sub.4, K.sub.3PO.sub.4, Ca.sub.3(PO.sub.4).sub.2,
Na.sub.3Citrate (sodium citrate), NaI, Na.sub.2HPO.sub.4 (disodium
hydrogen phosphate), sodium acetate, EDTA and quarternary ammonium
salts.
[0050] Preferred oils for use in the invention should be of low
molecular weight so that the boiling point of the oil is lower than
that of water. This constraint may be overcome, however, by a
simple liquid extraction process of NPs from higher molecular
weight oils, through the use of low molecular weight organic
solvents with higher selectivity towards NPs. This requires an
additional step in the phase-transfer process, however, and is less
preferred. Alternately, for the case where the NPs are insensitive
to high temperatures, the high molecular weight organic solvents
can be boiled off and the NPs may be precipitated and/or dried
before being redissolved into lower molecular weight solvents.
[0051] Additional variations in the invention include other types
of anionic surfactants, such as, but not limited to, sodium dodecyl
benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), sodium
laureth sulfate (SLES) or anionic lipids like phosphatidic acid,
phosphotidyl glycerol, phosphotidylinositol, etc., over and above
herein demonstrated anionic aerosol-OT (AOT). Other categories of
surfactants might also be employed in the invention, such as
cationic (positively charged), nonionic (neutral), zwitterionic
(positive and negative charge) and Gemini surfactants (2 regular
surfactants linked together by spacer molecules). Examples of
cationic surfactants include cetyl trimethyl ammonium bromide
(CTAB), cetylpyridinium chloride (CPC), benzalkonium chloride
(BAC), benzethonium chloride (BZT), amine and imidazoline salts
such as primary, secondary and tertiary amine hydrochlorides and
imidazoline hydrochlorides. Examples of nonionic surfactants
include alkyl poly(ethylene oxide), copolymers of poly(ethylene
oxide) and poly(propylene oxide), alkyl polyglucosides, alkyl
phenol ethoxylate, polyoxyethylene esters of fatty acids,
polyoxyethylene fatty acid amides, alkyl polysaccharides,
ethoxylates of alkyl amine, castor oil, end capped synthetic
alcohol, tallow amine and alkanol amine mercaptan. Examples of
zwitterionic surfactants include dodecyl betaine, dodecyl
dimethylamine oxide, docamidopropyl betaine and coco ampho
glycinate.
[0052] Alcohols and amines that can be used to stabilize the
dispersions include, but are not limited to, methanol, ethanol,
isopropanol, butanol, monoethanol amine, diethanol amine,
triethanol amine, ethylenediamine.
[0053] Other embodiments of the invention include phase-transfer
NPs using bicontinuous/O/W microemulsions, O/W nanoemulsions/W/O
nanoemulsions, in addition to herein demonstrated phase-transfer
using W/O microemulsions. Also, W/O emulsions,
water-in-oil-in-water (W/O/W) multiple emulsions or
oil-in-water-in-oil (O/W/O) multiple emulsions can advantageously
be employed.
[0054] In one embodiment, the NPs are solubilized in oil,
surfactant added to the NP-oil blend and the resulting NP-oil
surfactant blend is phase-transferred at high temperature in
salt-containing water.
[0055] In another embodiment, the NPs are incorporated in
microemulsions (W/O/bicontinuous/O/W) or in emulsions (W/O or O/W)
or in multiple emulsions (W/O/W or O/W/O), the oil boiled off and
the resulting NP surfactant mix phase-transferred in
salt-containing water.
[0056] In another embodiment, the water may contain different types
of surfactants or their mixtures, containing different types of
salts such as sodium citrate, sodium iodide, sodium hydrogen
phosphate, sodium acetate, sodium bromide etc. or their mixtures
and under conditions of different pH and phase-transfer
temperatures. Other salts included but not restricted to are weakly
dissociating salts that act as salting-in electrolytes or water
structure breakers, such as urea, guanidium chloride and 1-4
dioxane.
[0057] In another embodiment, the method involves phase-transfer of
mixtures of different NPs from oil to water to produce dispersions
with dual/multiple functionalities. For example, Au and Pd
nanoparticles can be mixed in hexane and AOT and phase-transferred
to salt containing water to result in assemblies of Au--Pd
nanoparticles that can catalyze aqueous phase reactions.
Alternately, the catalytic properties of Au or Pd could be combined
with the optical properties of CdSe tetrapods or quantum dots to
give novel NP assemblies with optically responsive catalytic
properties.
[0058] In another embodiment, the method allows control over
self-assembly of NP aggregates comprising of a single type of NP or
mixtures of different NPs as a function of salt and/or alcohols. As
an example, controlled crystallization of quantum dots could be
obtained by phase-transferring them using oppositely charged
surfactants (cationic and anionic) in salt water.
[0059] In another embodiment, the method allows control over
self-assembly of NP aggregates comprising of a single type of NP or
mixtures of different NPs and as a function of evaporation rates of
oils that solubilize and host the NPs. The self-assembled
aggregates obtained through procedures described above would
potentially have novel optical, electrical, catalytic and magnetic
properties.
[0060] In another embodiment, polymers are incorporated in
phase-transfer solutions so as to add stability and functionality
to NP dispersions and as avenues to create new structures such as
functional/smart capsules, micro/nanowires etc. As an example,
controlled crystallization of quantum dots or formation of NP
assemblies/nanowires could be obtained by phase-transferring them
using combinations of oppositely charged surfactants and polymers
in salt water.
[0061] In another embodiment, the method allows the formation of NP
dispersions stable across a wide range of ionic strength, pH and
temperatures using mixed surfactants of ionic and non-ionic types,
for high ionic-strength biological applications and as possible
hydrocarbon/oil/gas sensing agents in hydrocarbon reservoirs.
Example 1
[0062] This example demonstrates benefits obtained by
phase-transferring different types of NPs into salt-containing
water over conventional processes that phase-transfer NPs to plain
water. In all experiments, the oil used was hexane, surfactant used
was AOT at 1.5 g/L and salt used was NaCl at 3 g/L, unless
specified otherwise.
[0063] Table 2 summarizes the results of phase-transfer experiments
carried out in plain water, and in salt containing water, featuring
NPs of different compositions and shapes. Phase-transfer
experiments were carried out as per the descriptions provided in
Table 1 (Bagaria, et al., 2009). In all experiments for this
section, the oil used was hexane, surfactant used was Aerosol OT
(AOT) and salt used was sodium chloride (NaCl).
[0064] Phase-transfer was carried out at 85.degree. C. All
resulting NP dispersions were characterized by their "dispersion"
and "functionality" attributes. Dispersion refers to the settling
of NPs in time and was recorded visually or after centrifugation at
9000 rpm for 1 hour by UV measurements (where applicable).
Functionality refers to measurable physical properties that are
specific to a type of NP, such as photoluminescence etc.
[0065] Inorganic semiconductor NPs (CdSe) have a characteristic
property called fluorescence, wherein nanoparticles upon exposure
to UV light, emits light in the visible region (appears
orange/red/green etc.). Photoluminescence (PL) is the quantitative
measure of fluorescence recorded by a fluorometer that was used to
differentiate between NP dispersions of CdSe, when
phase-transferred to plain water and water containing salt. CdSe is
available in different shapes such as spheres, tetrapods and
concentric spheres (CdSe/ZnS core-shell), whose dispersibility in
plain water, and in water containing salt were also characterized
by PL measurements (FIGS. 2,3 and FIGS. 5-8). Retention in PL of
CdSe dispersions prepared in plain water and in water containing
salt was investigated as a function of time for QDs (FIG. 3),
tetrapods (FIG. 6) and core-shell particles (FIG. 8). All time
studies of PL are reported as a fraction of original PL value of
respective CdSe NP types in hexane, so as to have a reference point
for comparison.
TABLE-US-00002 TABLE 2 Phase-transfer Results of different NP
Types+ Phase-transfer in Salt- Phase-transfer in Water Containing
Water Forms Retains PL* Forms Retains PL in NP Type NP Shape
Dispersion? in solution? Dispersion? solution? Inorganic Spheres
Yes, but a No, poor PL Yes, NPs Yes Semiconductor (FIGS. 2, 3)
significant retained in NPs (CdSe) fraction of NPs solution upon
crash out centrifugation upon centrifugation Tetrapods Yes, but a
No, poor PL Yes, most Yes (FIGS. 5, 6) significant NPs retained
fraction of NPs in solution crash out upon upon centrifugation
centrifugation Core-Shell Yes, but a No, poor PL Yes, most Yes
CdSe--ZnS significant NPs retained Spheres fraction of NPs in
solution (FIGS. 7, 8) crash out upon upon centrifugation
centrifugation Organic NPs Spheres (FIG. No, significant Not
applicable Yes, most Not applicable (C.sub.60/Bucky 9) fraction of
NPs NPs phase- Balls) stick to transferred stirrer/glass and
retained vial in solution upon centrifugation Inorganic Spheres
(FIG. Yes, but Not applicable Yes, most Not applicable Metal NPs
10) significant NPs retained (Au) fraction of NPs in solution crash
out upon upon centrifugation centrifugation Inorganic Spheres (FIG.
Yes, but a Not applicable Yes, most Not applicable Metal Oxide 11)
significant NPs retained NPs (Iron fraction of NPs in solution
Oxide) crash out upon upon centrifugation centrifugation Inorganic
Nanorods No, significant Not applicable Yes, most Not applicable
Oxide NP (Data not fraction of NPs NPs phase- (Tungsten shown)
stick to transferred Oxide) stirrer/glass and retained vial in
solution upon centrifugation .sup.+All NPs were phase transferred
from hexane using the surfactant AOT at 1.5 g/L. The salt used in
phase-transfer studies was NaCl at 3 g/L unless mentioned otherwise
*PL--Photoluminescence
[0066] Organic NP (C.sub.60) transferred to plain water and to
water containing salt was characterized by UV spectroscopy (FIG.
9). Visual inspection showed that phase-transfer of C.sub.60 to
water that did not contain salt was a near transparent solution
while that transferred to water containing salt had a light
grayish/pink color. It is noteworthy that in the former case, a lot
of C.sub.60 particles were stuck to the magnetic stirrer rod and
the glass walls of reaction vials after phase-transfer, while when
transferred with salt, this was not observed. Absorbance value is
an indicator to the quantity of material present suggesting
superior phase-transfer of C.sub.60 nanoparticles in water
containing salt.
[0067] Inorganic gold NPs (Au) transferred to water plain water and
to water containing salt was characterized by UV spectroscopy (FIG.
10). Visual inspection showed that phase-transfer of Au to plain
water was a near transparent solution while that transferred to
water containing salt had a distinct pink color (data not shown,
see Bagaria et al. 2009). As in the case of C.sub.60
phase-transfer, many Au particles stuck to the magnetic stirrer rod
and the glass walls when phase transferred in plain water, while
with salt this was not observed. Absorbance peaks is an indicator
to the quantity of material present, suggesting superior
phase-transfer of Au nanoparticles in water containing salt.
[0068] Visual inspection of phase-transferred inorganic metal-oxide
NPs (iron-oxide spheres) also showed that the fraction of NPs
transferred from oil to water was higher in salt-containing water
(FIG. 11) as evidenced by the darker colored NP dispersion.
Example 2
[0069] Table 3 summarizes the results of phase-transfer experiments
carried out in plain water with no salt, and in salt-containing
water, featuring oils that are non-polar in nature such as hexane
and polar oils such as chloroform. Phase-transfer experiments were
carried out as per the descriptions provided in Table 1 (Baragia et
al., 2009). In all experiments for this section, the surfactant
used was AOT, salt used was NaCl at 3 g/L and the NPs
phase-transferred were spherical CdSe QDs. Phase-transfer was
carried out at 85.degree. C. PL measurements were used to
characterize the dispersions (FIG. 12). The PL of CdSe QDs phase
transferred from chloroform to salt-containing water was higher
than that of CdSe QDs phase transferred to plain water. Thus, the
benefits of dispersing NPs in salt-water is independent of the oil
that initially stores the NPs.
TABLE-US-00003 TABLE 3 Phase-transfer Results of CdSe QDs from
Different Oils.sup.+ Phase-transfer in Salt Solution Phase-transfer
in Water (NaCl) CdSe QD Retains PL* in CdSe QD Retains PL* in Oil
Disperses? solution? Disperses? solution? Hexane (FIG. Yes, but No
Yes, most NPs Yes 2) significant fraction retained in of NPs crash
out solution upon upon centrifugation centrifugation Chloroform
Yes, but No Yes, most NPs Yes (FIG. 12) significant traction
retained in of NPs crash out solution upon upon centrifugation
centrifugation .sup.+The surfactant used was AOT at 1.5 g/L, NPs
used was CdSe QDs and salt used was NaCl at 3 g/L. *PL:
Photoluminescence
Example 3
[0070] Table 4 summarizes the results of phase-transfer experiments
carried out in water with salts other than sodium chloride--namely
sodium sulfate (Na.sub.2SO.sub.4, FIG. 13), calcium chloride
(CaCl.sub.2, FIG. 14) and sodium citrate (Na.sub.3Citrate, FIG.
15). Phase-transfer experiments were carried out as per the
description provided in Schematic 2, shown in FIG. 21. In all
experiments for this section, the oil used was hexane, the
surfactant used was AOT and the NPs phase-transferred were
spherical CdSe QDs. Sodium sulfate solutions were prepared at
concentrations of 0.19, 0.38 and 0.72 g/L. Levels of CaCl.sub.2
were fixed at 0.05 and 0.1 g/L, whereas the study with
Na.sub.3Citrate was done at 1 g/L. Phase-transfer was carried out
at 85.degree. C. UV and/or PL measurements were used to
characterize the dispersions. A control run for the experiment is
described as a footnote of Table 4.
TABLE-US-00004 TABLE 4 Phase-transfer Results of CdSe Using
Different Salts.sup.+ Salt Retains Salt Concentration Disperses
NPs? PL* of NPs? Sodium sulfate 0.19 g/L Yes, most NPs Yes
(Na.sub.2SO.sub.4) retained in solution (FIG. 13) upon
centrifugation 0.38 g/L Yes, most NPs Yes retained in solution upon
centrifugation 0.72 g/L Yes, most NPs Yes retained in solution upon
centrifugation Calcium chloride 0.05 g/L Yes, most NPs Not measured
(CaCl.sub.2) retained in solution (FIG. 14) upon centrifugation 0.1
g/L Yes, but most NPs Not measured crash out upon centrifugation
Sodium citrate 1 g/L Yes, most NPs Not measured (Na.sub.3Citrate)
retained in solution (FIG. 15) upon centrifugation .sup.+All NPs
were phase-transferred from hexane to salt-containing water, using
AOT at 1.5 g/L.
Example 4
[0071] Table 5 summarizes the results of phase transfer experiments
carried out through the use of different surfactant types, namely
CTAB (cationic), DDAB (cationic) and a mixture of AOT (anionic) and
Polystep C-M4S (C.sub.9-Phenyl-EO.sub.4-SO.sub.3--Na.sup.+:
non-ionic) surfactants. For all the experiments, the oil used was
hexane and the salt used was NaCl. Upon phase-transfer, the samples
were centrifuged at 9000 rpm for 1 hour and the supernatant NPs
were retained. As shown by UV-Vis spectra and/or photographs in
FIG. 1 (CTAB), FIG. 16 (DDAB) and FIG. 17 (AOT and Polystep C-M4S),
the phase-transfer of CdSe NPs using the above surfactants was
found to be superior in the presence of salt over plain water.
TABLE-US-00005 TABLE 5 Phase Transfer of NPs using different
surfactants.sup.+ Phase-transfer in Water Phase Transfer in salt-
Forms containing water dispersion in Forms dispersion Surfactant
solution? Retains PL* ? in solution? Retains PL* ? CTAB (1.5 g/L)
Yes, but most No Yes, most NPs are Yes (FIG. 1) NPs crash out
retained in solution upon upon centrifugation centrifugation DDAB
(1.5 g/L) Yes, but most Not measured Yes, most NPs are Not measured
(FIG. 16) NPs crash out retained in solution upon upon
centrifugation centrifugation AOT + Polystep C- Yes, but most Not
measured Yes, most NPs are Not measured M4S (Total 1.5 g/L) NPs
crash out retained in solution (FIG. 17) upon upon centrifugation
centrifugation .sup.+All phase transfer experiments were carried
out from hexane to plain or salt-containing water using AOT as
surfactant at 1.5 g/L. CdSe QDs were the NPs phase transferred and
NaCl was the salt used. Unless mentioned otherwise, the
concentration of NaCl was 3 g/L as the surfactant and with CdSe QDs
as NPs. *PL = photoluminescence
Example 5
[0072] After NPs are phase transferred with the surfactant AOT, the
dispersion may appear turbid due to liquid crystalline surfactant
aggregates. The presence of these aggregates results in an overall
high level of absorbance due to scattering, as shown in the UV-Vis
spectra of CdSe NPs in FIG. 2. These aggregates can be removed by
centrifugation as evidenced by the near-zero absorbance at higher
wavelengths in the curve indicated as `after centrifugation` (FIG.
18). The photographs of CdSe NPs clearly show the removal of the
turbidity from the dispersion upon centrifugation leading to clear
CdSe dispersions (refer to Bagaria et al. 2009 for color
images).
Example 6
[0073] The stability of NPs to high ionic strength solutions is
higher, when NPs are phase transferred using surfactants in
salt-water over plain-water. FIG. 19 depicts the percentage of NPs
retained in high ionic strength solutions of KCl, after solutions
of CdSe QDs that were phase transferred in plain water and salt
(NaCl) containing water, were added to varying levels of
ionic-strength of KCl. A blend of AOT and Polystep C-M4S
surfactants was used to phase transfer NPs from hexane to plain or
salt-containing water. Upon phase transfer, the NPs were
centrifuged at 9000 rpm for 1 hour to remove non-dispersed NPs and
liquid crystalline aggregates of AOT. Prior to stability studies in
high ionic strength solutions of KCl, the concentration of NP
solutions phase-transferred in salt-water was adjusted to match NP
concentrations in plain water by diluting with brine and measuring
UV-absorbance. For stability studies in ionic strength solutions,
KCl solutions were prepared at varying levels of concentration. NPs
in salt-containing water and plain water were added (dilution 1 in
5) such that the final KCl concentration ranged between 3 to 10
g/L. The solutions were centrifuged at 9000 rpm for 1 hour after
which UV measurements were made on the supernatant solutions. The
percentage NP retained is the ratio of concentration of NPs
(measured from absorbance values at the first exciton-542 nm)
contained in a specified ionic strength solution of KCl after
centrifugation to the concentration of NPs contained in the lowest
ionic strength solution (3 g/L KCl) after centrifugation, as a
reference point. As a note, this reference point was selected since
there was no apparent difference in NP concentrations as measured
by UV, before and after the centrifugation step. From FIG. 19, it
is apparent that NPs stabilized by compact micelles in
salt-containing solutions had a higher tolerance to ionic strength
when compared with NPs stabilized by loosely packed micelles in
plain water.
[0074] In conclusion, the efficacy of the new phase-transfer
process is herein demonstrated to be useful across many different
NP, oil, salt and surfactant types and this methodology is broadly
applicable.
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* * * * *