Methods for surfactant removal from nanoparticle suspensions

Abstract

The present disclosure relates to the cleansing of nanoparticles in aqueous cationic surfactant solutions, including polyalkylammonium salts such as cetyltrimethylammonium bromide, as demonstrated by surfactant exchange, followed by the addition of peptizing agents to stabilize the cleansed nanoparticle solutes.

Claims

1. A method for converting CTAB-stabilized GNRs into citrate-stabilized GNRs, comprising: filtering excess surfactant from a GNR-CTAB suspension to result in a concentrated GNR-CTAB suspension; subjecting GNR-CTAB suspension to at least one centrifugation and redispersion cycle with solutions containing a mild detergent to deplete CTAB below trace levels, resulting in a CTAB-free GNR suspension; and subjecting CTAB-depleted GNR suspension to at least one centrifugation and redispersion cycle using sodium citrate solutions to produce suspensions of citrate-stabilized GNRs, devoid of detergent.

2. The method of claim 1, further comprising diluting the suspension of CTAB-stabilized GNRs to 25 mM CTAB.

3. The method of claim 1, further comprising filtering the suspension of CTAB-stabilized GNRs by stirred ultrafiltration.

4. The method of claim 1, wherein the mild detergent is sodium polystyrenesulfonate (Na-PSS).

5. The method of claim 4, the Na-PSS concentration being 0.15 wt %.

6. The method of claim 1, the diluted GNR-CTAB suspension being subject to three centrifugation and redispersion cycles with 0.15 wt % Na-PSS.

7. The method of claim 1, the CTAB-depleted GNR suspension being subjected to two centrifugation and redispersion cycles with aqueous sodium citrate.

8. The method of claim 1, the sodium citrate being at a concentration between 10 M and 10 mM.

9. The method of claim 1, the conversion of CTAB-GNRs into citrate-stabilized GNRs being 75% to 85%.

10. A method for removing cetyltrimethylammonium bromide (CTAB) from aqueous suspensions of gold nanorods (GNRs), comprising: intermediate treatment of a suspension of GNRs with sodium polystyrenesulfonate (Na-PSS) resulting in a treated GNR suspension; and subjecting the treated GNR suspension by further exchange with sodium citrate to produce citrate-stabilized GNRs (cit-GNRs).

11. A composition comprising citrate stabilized GNRs prepared by the method of claim 1.

12. A method of removing a cationic surfactant from a suspension of gold nanoparticles and preparing peptizing agent stabilized said gold nanoparticles, comprising: adding an anionic polymeric detergent to said suspension of gold nanoparticles to remove said cationic surfactant, resulting in a suspension depleted of said cationic surfactant to below detectable limit by X-ray photoelectron spectroscopy; and adding a peptizing agent to stabilize the cleansed gold nanoparticles, wherein said peptizing agent is selected from the group consisting of an anionic species, a zwitterion species, a nonionic species, and any combination thereof, and wherein said gold nanoparticles have hydrodynamic sizes between 1-200 nm.

13. A composition comprising peptizing agent stabilized GNRs, wherein the composition is essentially depleted CTAB or other cationic surfactant to below detectable limit by X-ray photoelectron spectroscopy.

14. The composition of claim 13, wherein the peptizing agent is selected from the group consisting of an anionic species, a zwitterionic species, a nonionic species, and any combination thereof.

15. The composition of claim 14, wherein the peptizing agent comprises citrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an illustration of an embodiment of the herein disclosed method for removing cetyltrimethylammonium bromide (CTAB) from aqueous suspensions of gold nanorods (GNRs) via intermediate treatment with polystyrenesulfonate (PSS), followed by further exchange with sodium citrate to produce citrate-stabilized GNRs (cit-GNRs).

(2) FIG. 2 is a process flow diagram for converting CTAB-GNRs into cit-GNRs by ultrafiltration (stage 1) with successive centrifugation/redispersion (C/R) cycles using 0.15 wt % Na-PSS (stages 2-4) and 5 mM Na.sub.3-cit (stages 5 and 6), with estimated surfactant concentrations after each stage).

(3) FIG. 3 shows normalized spectra of GNR dispersions at various stages of the cleansing process, illustrating the stability of said dispersions.

(4) FIG. 4a is a transmission electron microscope (TEM) image of GNRs initially stabilized in CTAB solutions, deposited onto a carbon-coated TEM grid.

(5) FIG. 4b is a TEM image of GNRs initially stabilized in CTAB solutions, deposited onto a carbon-coated TEM grid. FIGS. 4a and 4b are statistically identical.

(6) FIG. 5 shows optical absorbance spectra of GNRs stabilized in 100 mM, 1 mM, or 0.01 mM CTAB, before and after treatment with 5 mM sodium citrate, demonstrating the destabilizing effect of removing CTAB without the use of detergent in the cleansing process.

(7) FIG. 6a shows survey spectra by X-ray photoelectron spectroscopy (XPS) acquired from pelleted GNR samples after Stage 4 (PSS wash #3) and Stage 5 (Citrate wash #1), demonstrating the reduction of N signal (associated with CTAB) below the limit of detection.

(8) FIG. 6b shows high-resolution (regional) XPS spectra of the C 1s region, indicating citrate as the predominant surface species on the GNR samples at Stage 5 and 6.

(9) FIG. 7 shows attenuated total reflectance-infrared (ATR-IR) spectra of the cleansing process, indicating bulk ligand exchange.

(10) FIG. 8 shows surface-enhanced Raman scattering (SERS) spectra of the cleansing process, again indicating bulk ligand exchange. Loss of signal intensity at 1140 cm.sup.1 indicates the displacement of PSS from the GNR surface by citrate ions. The Raman band between 1420 and 1480 cm.sup.1 represents common CH bending modes present in all surfactants.

DETAILED DESCRIPTION

(11) For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended.

(12) Referring to FIG. 1, in one embodiment, described herein is an efficient method for removing cetyltrimethylammonium bromide (CTAB) from aqueous suspensions of gold nanorods (GNRs) via intermediate treatment with polystyrenesulfonate (PSS), followed by further exchange with sodium citrate to produce citrate-stabilized GNRs (cit-GNRs). Citrate-stabilized nanoparticles are well known to support a variety of surface modification protocols in aqueous media, with minimal interference by the displaced electrolyte. The surface exchange process was monitored in stages by x-ray photoelectron spectroscopy (XPS), an invaluable tool for quantitative elemental analysis of surface adsorbates, as well as by attenuated total reflectance infrared (ATR-IR) spectroscopy and surface-enhanced Raman scattering (SERS). Analyses indicate that CTAB removal by PSS treatment is highly efficient, as is the subsequent displacement of PSS by citrate. The cit-GNRs are fully dispersible in low-salt solutions, and are easily functionalized by further ligand exchange.

EXAMPLE

(13) A six-stage protocol was developed to convert CTAB-GNRs (6826 nm, .sub.LPR=713 nm) into cit-GNRs, using standard ultrafiltration and centrifugation equipment. CTAB-GNRs were prepared on a gram scale by the method described by Khanal and Zubarev, then diluted with water to prevent premature flocculation of CTAB-GNRs during the initial purification stages. In a typical process, a suspension of GNRs (optical density (O.D.)=3.2) stabilized in 25 mM CTAB was concentrated to a few mL by stirred ultrafiltration to remove excess surfactant, then diluted with deionized water (Stage 1).

(14) Referring to FIG. 2, a process flow diagram is presented for converting CTAB-GNRs into cit-GNRs by ultrafiltration (stage 1) with three centrifugation/redispersion (C/R) cycles using 0.15 wt % Na-PSS (M.sub.w=70 kDa; stages 2-4), and 2 C/R cycles using 5 mM Na.sub.3-cit (stages 5 and 6), with estimated surfactant concentrations after each stage. The first three C/R cycles depleted CTAB to trace levels: the residual CTAB in the PSS-stabilized GNR suspension after Stage 4 is <1 M based on simple dilution factors, however the free CTAB concentration is likely even lower due to its favorable adsorption to PSS. The PSS-GNRs were then subjected to two additional C/R cycles using 5 mM sodium citrate (Stages 5 and 6) for exchange with PSS, yielding stable dispersions of cit-GNRs. A complete procedure with exact volumes and concentrations is provided in Appendix-A.

(15) The conversion of CTAB-GNRs into cit-GNRs was achieved with an overall efficiency of 75-85%, based on initial and final optical density (O.D.) values of the GNR dispersions at 713 nm. The main source of loss is due to incomplete recovery of GNRs during the first two C/R cycles. Absorption spectroscopy and TEM analysis of the citrate-stabilized GNRs indicated essentially no changes in optical absorption (FIG. 3, which are normalized spectra of GNR dispersions after Stages 1, 3, 5 and 6) or size distribution (FIGS. 4a and 4b, which are respectively TEM images of CTAB-stabilized GNRs (68.226.2 nm; N=102) and citrate-stabilized GNRs (68.525.7 nm; N=134)). On the other hand, the exchange of PSS to citrate was readily discernible by electrokinetic measurements: the zeta potential of GNRs in dilute Na-PSS solution (0.25 mg/mL, pH 5) was 55.68.3 mV, while that of cit-GNRs in dilute phosphate buffered solution (PBS) adjusted to pH 9.5 (I8.2 mM) was less negative (mean 26.913.2 mV; mode 21.17.1 mV). The cit-GNR suspensions are stable for months at low ionic strength (I<0.1 M) but aggregate if dispersed in 1 M NaCl or standard PBS buffer. It is thus concluded that the cit-GNRs behave similarly as other citrate-stabilized Au nanoparticles.

(16) The incubation of GNRs with Na-PSS (minimum 1 hour, prior to each C/R step) is essential for producing well-dispersed cit-GNRs. It is established that PSS adsorbs weakly onto GNR surfaces in the absence of CTAB, resulting in metastable dispersions whose relative stability depends on Na-PSS concentration, suggestive of depletion effects. Attempts to exchange CTAB directly with citrate invariably led to rapid aggregation, regardless of surfactant concentrations. For instance, FIG. 5 shows absorbance spectra of GNR dispersions stabilized in 100 mM, 1 mM, or 0.01 mM CTAB, before and after treatment with 5 mM sodium citrate; all GNR dispersions were strongly aggregated after several hours' exposure to citrate, in the absence of PSS treatment.

(17) The ligand exchange process from Stage 3 (PSS wash #2) to Stage 6 (citrate wash #2) was evaluated by XPS, based on the binding energies of various atomic species (Table 1 and FIGS. 6a and 6b). FIG. 6a shows survey XPS spectra acquired from pelleted GNR samples after Stage 4 (PSS wash #3) and Stage 5 (Citrate wash #1), indicating the disappearance of the N 1s signal. FIG. 6b shows high-resolution (regional) XPS spectra of the C 1s region, indicating relative changes in carbon subtype after treatment with citrate. The nitrogen (N 1s) content in the GNR samples was reduced to the limit of detection by Stage 5, thereby establishing the essentially complete removal of CTAB. The XPS signals for carbon (C 1s), oxygen (O 1s), sodium (Na 1s), and sulfur (S 2p) were also greatly reduced upon washing with 5 mM citrate (Stage 5), indicating the gross displacement of Na-PSS from the GNR surfaces (FIG. 6a). The S 2p signal was almost completely gone by the second citrate wash (Stage 6), and below trace levels in the supernatant. High-resolution analysis of the C is region further revealed that citrate exchange was accompanied by a change in the population of carbon subtypes, with a reduction in hydrocarbon species (CH/CC; 284.8 eV) and an increase in the density of carboxyl groups (C(O)O; 288.2 eV), corresponding with the adsorption of citrate (FIG. 6b).

(18) TABLE-US-00001 TABLE 1 XPS analysis of GNR samples at different purification stages Stage bkgrnd.sup.b Stage bkgrnd.sup.b Stage 5.sup.a bkgrnd.sup.b Stage bkgrnd.sup.b Element 3.sup.a (PSS (PSS 4.sup.a (PSS (PSS (Cit. (Cit. 6.sup.a (Cit. (Cit. (B.E., eV)c #2) #2) #3) #3) #1) #1) #2) #2) C 1s (283) 210 93.8 205 67.6 8.00 9.27 6.44 1.04 N 1s (401) 1.51 1.28 0.03.sup.d 0.03.sup.d O 1s (530) 80.3 39.3 86.7 32.4 4.14 11.7 4.54 3.02 Na 1s 21.8 13.3 26.3 13.2 1.28 4.47 1.79 0.60 S 2p (167) 24.1 11.4 26.6 9.19 0.47 0.47 0.09 <0.03.sup.d Ag 3d 0.25 0.26 0.28 0.26 Si 2s (101) 1 1 1 1 Au 4f (82) 1 1 1 1 .sup.aSignals from pelleted GNR samples, normalized to Au 4f peak area. .sup.bSignals from supernatant of GNR samples, normalized to Si 2s peak area. .sup.cBinding energies from survey XPS spectra. .sup.dAt or below limit of detection.

(19) The ligand exchange process was characterized by ATR-IR spectroscopy and SERS, to identify the molecular species associated with the GNR samples (FIGS. 7 and 8, respectively). IR analysis of pelleted GNR samples from Stages 1, 3, and 4 confirmed the presence (and depletion) of CTAB and PSS, with the former producing strong bands associated with sp.sup.3 CH stretching (2800-2900 cm.sup.1), CH.sub.3 deformation (1450-1480 cm.sup.1), and CN stretching (900-950 cm.sup.1), and the latter producing peaks associated with the SO.sub.3.sup. group (1020-1200 cm.sup.1). PSS displacement by citrate anions produced additional spectral changes, dominated by CO.sub.2.sup. stretching (1575 cm.sup.1) and CH.sub.2 deformation (1390 cm.sup.1). SERS analysis also confirmed that PSS was adsorbed onto GNRs by the enhanced SO.sub.3.sup. Raman signal, but was displaced after two washes with Na.sub.3-cit. Overall, these vibrational analyses further supported the conclusions drawn from the XPS data.

(20) The herein disclosed method describes a general approach for depleting CTAB and other cationic surfactants from aqueous nanoparticle suspensions, allowing their clean exchange with various surface ligands. Specifically, an efficient method is disclosed for removing cationic surfactants such as cetyltrimethylammonium bromide (CTAB) from aqueous suspensions of nanoparticles using sodium polystyrenesulfonate (Na-PSS), followed by the addition of peptizing agents to stabilize the cleansed nanoparticle solutes, including but not limited to anionic species such as citrate and oxalate, zwitterionic species such as sulfobetaines and carboxybetaines, or nonionic species such as Tween-20. In the case of citrate-stabilized GNRs, the materials are compatible with conjugation chemistries commonly applied to most metal colloids, and are useful for evaluating changes in particle size as a function of ligand structure.

(21) Those skilled in the art will recognize that numerous modifications can be made to the specific implementations described above. The implementations should not be limited to the particular limitations described. Other implementations may be possible.

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