Surfactant-Modified Ultrafine Gold Nanoparticles with Magnetic

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Surfactant-Modified Ultrafine Gold Nanoparticles with Magnetic Responsiveness for Reversible Convergence and Release of Biomacromolecules Lu Xu, Shuli Dong, Jingcheng Hao, Jiwei Cui, and Heinz Hoffmann Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04591 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

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Au@CTACe or Au@CTAGd

ACS Paragon Plus Environment

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Surfactant‐Modified Ultrafine Gold Nanoparticles with Magnetic Responsiveness for Reversible Convergence and Release of Biomacromolecules Lu Xu,† Shuli Dong,† Jingcheng Hao,*,† Jiwei Cui,*,† and Heinz Hoffmann‡ †

Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials of Ministry of Education, Shandong University, Jinan 250100, P. R. China ‡

Physikalische Chemie I, Bayreuth University, BT 95440, Germany

* Corresponding author. Tel.: +86-531-88363768, Fax: +86-531-88564750 E-mail: [email protected]

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ABSTRACT: It is difficult to synthesize magnetic gold nanoparticles (AuNPs) with ultrafine sizes (< 2 nm) based on conventional method via coating AuNPs using magnetic particles, compounds

or

ions.

Here

the

magnetic

cationic

surfactants,

C16H33N+(CH3)3[CeCl3Br]- (CTACe) and C16H33N+(CH3)3[GdCl3Br]- (CTAGd), are prepared by one-step coordination reaction, i.e., C16H33N+(CH3)3Br- (CTABr) + CeCl3 or GdCl3 → CTACe or CTAGd. A simple strategy to fabricate ultrafine (< 2 nm) magnetic gold nanoparticles (AuNPs) via surface modification with weak oxidizing paramagnetic cationic surfactants, CTACe or CTAGd, is developed. The resulting AuNPs can highly concentrate the charges of cationic surfactants on their surfaces, thereby presenting strong electrostatic interaction with negatively charged biomacromolecules, DNA and proteins. As a consequence, they can converge DNA and proteins over 90% at a lower dosage than magnetic surfactants or existing magnetic AuNPs. The surface modification with these cationic surfactants endows AuNPs with strong magnetism, which allows them to magnetize and migrate the attached biomacromolecules with a much higher efficiency. The native conformation of DNA and proteins can be protected during the migration. Besides, the captured DNA and proteins could be released after adding sufficient inorganic salts such as at cNaBr = 50 mmol·L-1. Our results could offer new guidance for a diverse range of systems including gene delivery, DNA transfection, and protein delivery and separation.

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1. INTRODUCTION Delivery of biomacromolecules such as nucleic acids and proteins are of great importance for gene therapy,1 gene silence,2 protein transport, separation and purification.3 Different types of synthetic materials have been employed to assemble nanoscale aggregates with biomacromolecules and promote their delivey.1-5 Among them, homogeneous magnetic nanoparticles have achieved great success.1,3,4 The combination of magnetic particles with biomolecules enables the resulting entities to be controlled by external magnetic force. However, the synthesis of ultrafine magnetic nanoparticles with high migration efficiency, low toxicity, and good biocompatibility is

still

challenging.

In

addition,

the

interaction

between

particles

and

biomacromolecules could often influence the native conformation and subsequent functions of biomolecules.6 Due to the tunable size, surface plasmon resonance (SPR), large specific surface area, excellent biocompatibility, and easy surface modification, gold nanoparticles (AuNPs) have been widely investigated in the fields of nanotechnology, material science and biochemistry.7-14 In particular, core-shell particles that AuNPs as the cores coated with magnetic shells (e.g., nanoparticles, compounds or ions) have aroused great interests for their potential applications in magnetic resonance imaging (MRI),9,10 photothermal therapy,4 and targeted cargo delivery.5 The properties of AuNPs highly depend on their sizes.12-14 The AuNPs with ultra-small size (< 2 nm) show excellent performance in acting as quantum dots,12 catalysis,13 and chemical or biological sensing.14 However, the ultrafine magnetic AuNPs are rarely reported, because it requires relatively harsh conditions to perfectly control the size of these core-shell nanoparticles,9-14 and most of the modified materials may need extra complicated synthesis process. 3


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In our previous work, we have reported a strategy to produce magnetic AuNPs via one-step modification.11 However, the size of the resulting particles is relatively large (> 12 nm). Due to the strong oxidizability of the modified compound, C16H33N+(CH3)3[FeCl3Br]- (CTAFe), the stoichiometric ratio of the added HAuCl4, CTAFe and NaBH4 should be carefully controlled. Excess of CTAFe or NaBH4 could lead to the reduction of [FeCl3Br]- by BH4- and the introduction of impurities. In the current study, we report a simple strategy to fabricate ultrafine (< 2 nm) magnetic AuNPs via surface modification with weak oxidizing paramagnetic cationic surfactants, CTACe or CTAGd. The two surfactants are prepared by one-step coordination

reaction

using

low-cost,

commercially

available

compounds,

C16H33N+(CH3)3Br- (CTABr) and CeCl3 or GdCl3, respectively.15,16 The prepared ultrafine magnetic AuNPs are demonstrated to be a good convergent for DNA and proteins due to the electronic interactions. There are several advantages of using these ultrafine AuNPs except for the facile fabrication, good stability and dispersibility, fast and effective binding to biomacromolecules. Firstly, the particles with sensitive magnetic responsiveness can migrate DNA and proteins with a weak external magnetic field (0.25 T), and the resulting migration efficiency of these biomacromolecules is much higher than that of using magnetic surfactants or Au@CTAFe. Secondly, low dosages (0.3 µmol·L-1) of nanoparticles is required to converge DNA or proteins, and a maximum compaction efficiency over 90% can be obtained. Thirdly, the convergence of nanoparticles and biomacromolecules is reversible via controlling the salinity of the solution. Lastly, the property of both DNA and proteins can be retained via tuning the ratio of the nanoparticles and biomolecules.

2. EXPERIMENTAL SECTION 4


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2.1. Chemicals and Materials. Cetyltrimethylammonium bromide (98%, CTABr) and sodium borohydride (96%, NaBH4) were purchased from Sinopharm Chemical Reagent Co. Ltd, China. Sodium tetrachloroaurate (III) dihydrate (NaAuCl4·2H2O), Cerium (III) chloride heptahydrate (99.9%, CeCl3·7H2O) and Gadolinium chloride hexahydrate (99%, GdCl3·6H2O) were received from Aladdin Industrial Corporation, China. All the chemicals were used without further purification. Herring testes double-strand (ds) DNA sodium salts were obtained from Sigma. Its molar weight was < 1200 bps as determined by AGE and its concentration was examined through considering the DNA base molar extinction coefficient to be 6600 mol-1·cm-1 at 260 nm in UV-vis spectra. The absorbance ratio of DNA stock solution was 1.8 to 1.9 at 260 nm and 280 nm, suggesting the absence of proteins. Myoglobin (Mb, from equine heart) was purchased from Sigma. Its concentration was calculated according to the absorbance at 409 nm by considering the molar extinction coefficient of 17100 mol-1·cm-1. Bovine serum albumin (BSA, 98%, ~66 kDa) was purchased from Sigma. 2.2. Synthesis of Magnetic Materials. Cetyltrimethylammonium trichloromonobromocerate (CTACe) and cetyltrimethylammonium trichloromonobromogadolinate (CTAGd) were prepared by mixing equal amount of CTABr with CeCl3 or GdCl3 in methanol and stirring overnight at room temperature. The solvent was then evaporated and the products were dried at reduced pressure at 378 K for 12 hr, both yielding white solids. Both Au@CTACe and Au@CTAGd nanoparticles were synthesized as follows: mixing 5 mL desired amount of CTACe or CTAGd (5 to 50 mmol·L-1) with 0.25 mmol·L-1, 5 mL NaAuCl4 in aqueous solution and stirring for 15 min. Then a fresh 5


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prepared ice-bathed NaBH4 aqueous solution (0.6 mL, 10 mmol·L-1) was added to the above solution. The resulting mixtures were vigorously stirred for 2 min and kept at 298 K. In all experiments, the concentration of DNA was controlled to be constant at 75 μmol·L-1, Mb and BSA concentrations were held at 14.6 and 20 μmol·L-1, respectively. Each complex sample was prepared by adding known amounts of biomacromolecules, nanoparticles and water to a fixed volume. Thrice-distilled water was used to prepare each sample solution. 2.3. Characterizations. The critical micelle concentrations (cmcs) of CTACe and CTAGd were determined by electrical conductivity method. A DDSJ-308A analyzer was used to perform electrical conductivity measurements. A Pyrex glass measuring cell was placed in a water bath (298 ± 0.3 K). The cmcs were determined from the break point between the higher [dκ/d(conc)] and lower [dκ/d(conc)] linear curves. The ionic dissociation constant (β) of the surfactant was estimated by the ratio of the two slopes. SQUID magnetometry was conducted with dried samples of surfactants which were placed in sealed poly-propylene tubes and mounted inside a plastic straw for measuring in a magnetometer with a superconducting quantum interference device (MPMSXL, Quantum Design, USA) and a reciprocating sample option (RSO). The data were collected at 300 K. UV-vis spectra of AuNPs and biomacromolecule-nanoparticle complex solutions were examined by a U-4100 UV/visible spectrometer, using a 10 mm path length quartz cell over a wavelength range of 220-320 nm. A 4 μL sample solution was dropped on TEM grid (copper grid, 3.02 mm, 200 mesh, and coated with Formvar film). After dried under an infrared lamp for 30 min,

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TEM observations were performed with a JEOL’s JEM-2100 TEM (Japan) at an accelerating voltage of 200 kV. A BI-200SM instrument (Brookhaven) was used for the measurements of dynamic laser scattering (DLS) of DNA/surfactant or DNA-nanoparticle complex solution samples at a constant scattering angle of 90°. All sample solutions were made dust free by filtration through cellulose acetate membranes of 0.45 mm pore size. A JASCO J-810 spectropolarimeter was used to perform CD spectroscopy. Samples were located in 10 mm path length cells, and the scanning speed was controlled to 100 nm·min-1 with a measuring range over 220-320 nm. Each sample measured three times for their average value. Agarose gels were horizontally submerged in pH 7.4 TAE buffer (40 mmol·L-1 Tris, 2 mmol·L-1 Na2EDTA·2H2O, 20 mmol·L-1 glacial acetic acid) at 5 V·cm-1. DNA was visualized by ethidium bromide (0.5 μg·mL-1), and a standard DNA ladder of 5000 bps was utilized as a reference. The zeta potential of the dispersions of ultrafine AuNPs was measured with a Zeta PALS potential analyzer instrument (Brookhaven, USA) with parallel-plate platinum black electrodes spaced 5 mm apart and a 10 mm path length rectangular organic glass cell (made of polymethyl methacrylate). All samples were measured using a sinusoidal voltage of 80 V with a frequency of 3 Hz.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Physicochemical Properties of Modified Cationic Surfactants. The two paramagnetic cationic surfactants, CTACe and CTAGd, were synthesized by coordinating commercially available cationic surfactant CTABr with CeCl3 or GdCl3, respectively, in methanol. The paramagnetism of the two surfactants was proved by SQUID magnetometry that the magnetic moment overall linearly 7


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increases with increasing the strength of magnetic field (Figure 1).17 Electrical conductivity measurements (Figure S1 in the Supporting Information (SI)) indicate that the critical micelle concentrations (cmcs) of CTACe and CTAGd are 1.1 and 0.87 mmol·L-1, respectively. Compared to the cmc of CTABr (~1.0 mmol·L-1),18 the changes of anions did not greatly affect the cmcs of cationic surfactants. This is uncommon because the two larger f-block anions (i.e. [CeCl3Br]- and [GdCl3Br]-) should be less effective at screening the cation-cation repulsion and increase the cmcs.15,16 The dissociation constants (β) of CTACe and CTAGd are 0.75 and 0.24, respectively. They are much higher than that of CTABr (β = 0.11),18 indicating that the binding degrees of [CeCl3Br]- and [GdCl3Br]- to cationic micelles are much lower compared to Br-. These phenomena could be because these f-block anions are hydrophobic and can interact with the nonpolar alkyl chains of cationic surfactants,15,16 thus enhancing the whole hydrophobicity of cationic surfactants and favoring the aggregation of them. The association with the nonpolar moieties also induced the partition of these complex ions into the micellar core instead of binding to the polar groups by means of the electro-static attraction,15,16 which caused these magnetic surfactants possess higher β. 0.90

a

-1 Magnetic moment / emug

0.6 -1 Magnetic moment / emug

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.3 0.0 -0.3 -0.6 -30000

-15000 0 15000 Magnetic field / Oe

30000

b

0.45 0.00 -0.45 -0.90 -30000

-15000 0 15000 Magnetic field / Oe

30000

Figure 1. SQUID magnetometry data of cationic surfactants, CTACe (a) and CTAGd

(b) at 300 K.

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3.2. Preparation of Ultrafine Magnetic Gold Nanoparticles (AuNPs) with Paramagnetic Surfactants. The surfactant-coated ultrafine magnetic AuNPs were

prepared by reducing NaAuCl4 (0.25 mmol·L-1) with NaBH4 (0.6 mmol·L-1) in the presence of the paramagnetic surfactants (5 to 50 mmol·L-1). When the concentration of CTACe or CTAGd is in the range of 20-30 mmol·L-1 or 20-40 mmol·L-1, respectively, brownish-yellow dispersion of gold colloids can be obtained, indicating the formation of 1 to 2 nm AuNPs (Figure S2 in the SI).19 UV-vis spectra of these colloidal AuNPs solutions do not present typical SPR absorbance peaks of AuNPs at around 520 nm (Figure 2a and 2b), indicating that the size of the resulting magnetic AuNPs is less than 2 nm, and thus demonstrating quantum confinement effect.20 High-resolution transmission electron microscopy (HR-TEM) images confirm the formation of Au@CTACe and Au@CTAGd nanoparticles (Figure 2c and 2d, Figure S3 in the SI). The average sizes of Au@CTACe and Au@CTAGd nanoparticles are 1.7 ± 0.5 nm and 1.6 ± 0.5 nm, respectively. The zeta potential (ς) of Au@CTACe nanoparticles modified with 20 or 30 mmol·L-1 of CTACe are +53.4 ± 2.7 mV and +77.9 ± 4.0 mV, respectively. Au@CTAGd nanoparticles modified with 20 or 40 mmol·L-1 CTAGd have similar ς values compared to Au@CTAGd nanoparticles, which are +56.8 ± 4.0 and +71.7 ± 3.6 mV, respectively. The ς measurements prove the successful modification of AuNPs with CTACe or CTAGd. ς data can be applied to evaluate the thermodynamic stability of colloid particles.

High ς values, typically above +30 mV or below -30 mV, indicate the high stability of particles, while the intermediate ς values often suggest a thermodynamic instability and the tendency of aggregation.21,22 As a consequence, the two kinds of ultrafine magnetic AuNPs should have good thermodynamic stability and dispersibility that can inhibit the appearance of aggregation. 9


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0.6

a

cCTACe / mmolL

0.7

-1

0.6

5.0 10 20 25 30 40 50

0.5 0.4 0.3

0.3 0.2

0.1

0.1

500

600 700 Wavelength / nm

5.0 10 20 30 40 50

0.4

0.2

0.0 400

cCTAGd / mmolL-1

b

0.5 Abs / a.u.

0.7

Abs / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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800

0.0 400

500


800

Figure 2. UV spectra of the resulting AuNP dispersions modified with of CTACe (a)

or CTAGd (b). HR-TEM images of the magnetic AuNPs modified with 30 mmol·L-1 CTACe (c) or 30 mmol·L-1 CTAGd (d). Scale bars are10 nm. T = 298 K. 3.3. Highly Efficient Convergence of DNA Induced by Au@CTACe. A precise

control over the native conformation of DNA and proteins is usually required during the transportation.6 In the case of gene delivery, the applied tools should not only be able to effectively relieve the charge repulsion between the negatively charged DNA and the phospholipid bilayers, but also induce the transition DNA from original stretched coil state to compacted global ones to ensure an effective entry of DNA into cells through the endocytosis.23 Although the compaction of DNA is necessary, the secondary structure of DNA must be protected during the delivery to avoid denaturation.23 According to previous reports,24 multivalent cationic species with at least three charges are required to compact DNA via relieving the charge repulsion between the adjacent phosphate groups on DNA backbones to make them approach each other and 10


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leads to the size reduction of DNA. Cationic surfactants can condense DNA once they self-assemble into micelles with multi-charges in the vicinity of DNA backbones.23 Due to the severe electrostatic interaction, the cmcs of cationic surfactants in the presence of DNA are usually much lower than those in the absence of DNA. It is defined as the critical association concentrations (cacs).23 If the compacted DNA is in a sufficiently high concentration, with the charge repulsion between the adjacent DNA molecules significantly reduced by the binding of surface micelles, partial DNA/surfactant complexes will aggregate with each other and fall from the solutions.23 Therefore, the phase separation can serve as an indicator of the presence of DNA compaction.17 According to the phase diagram shown in Figure 3a, the critical concentration of surfactant CTACe in condensing 75 μmol·L-1 DNA is 50 μmol·L-1. Ultrafine Au@CTACe nanoparticles (the stock solution was prepared with 0.25 mmol·L-1 NaAuCl4 and 30 mmol·L-1 CTACe) instead of surfactant CTACe can reduce the dosage for DNA condensation by more than two orders of magnitude that only 0.4 μmol·L-1 is needed (Figure 3a). The dosage for DNA compaction is also much lower than that of our previously reported Au@CTAFe nanoparticles (2 μmol·L-1).5 Changes of the average hydrodynamic diameters (2Rh) of DNA upon the addition of Au@CTACe or CTACe could provide detail information on the compaction process. As shown in Figure 3b and 3c, the compaction happens at 0.4 μmol·L-1 Au@CTACe and 50 μmol·L-1 CTACe, respectively. A maximum size reduction of 91.8% DNA is achieved as the dosage of AuNPs arrives at 10 μmol·L-1, in contrast to CTACe, only 81.4% size reduction of DNA can be ensured at 300 μmol·L-1 CTACe, demonstrating that using Au@CTACe instead of CTACe can also significantly improve the DNA condensation efficiency. The compaction efficiency of DNA induced by the Au@CTACe is also much higher than that of our previously 11


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reported Au@CTAFe nanoparticles which can offer a maximum 84.8% size reduction of DNA at 50 μmol·L-1.5 Agarose gel electrophoresis (AGE) provides an extra demonstration of DNA condensation promoted by the Au@CTACe (Figure S4 in the SI). The insertion of monovalent positively charged dye ethidium bromide (EB) into the double helix allows DNA to show clear fluorescent bands in an agarose gel,5 the binding of multivalent cationic Au@CTACe on a DNA backbone prevents the double helix combining with EB.5

a

CTACe

Au@CTACe

-1 cDNA = 0.075 mmolL

1E-4

1E-3

0.01

0.1

-1

log c / mmolL 400

b

400

c

300

300

2R / nm h

2R / nm h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200

200

100 100 0.0

0.2

0.4

c

CTACe

0.6

/ mmol  L

-1

0.8

1.0

0

1E-3

c

Au@CTACe

0.01

/ mmolL

-1

0.1

Figure 3. (a) Phase behavior of 75 μmol·L-1 DNA mixed with different

concentrations of Au@CTACe or CTACe, respectively. The filled green or red symbols refer to the region in which precipitates are discovered. Dynamic light scattering (DLS) results show the changes of the 2Rh of 75 μmol·L-1 DNA upon the addition of various amounts of CTACe (b) and Au@CTACe (c), respectively. T = 298 K. 12


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As a result, the fluorescent signal of DNA becomes indistinct and even disappears.5 Corresponding phenomena, which predicate the compaction of DNA, could be traced with the amount of the magnetic AuNPs reaching 0.4 μmol·L-1. Condensed DNA particles with smaller size are typically easier to cross cell membranes through the endocytosis.11,23 The ultrafine magnetic Au@CTACe can compact DNA with both a very low dosage and a high efficiency, it could be regarded as one kind of potential powerful DNA delivery tool. The good performance may benefit from two aspects. First, the negatively charged AuNPs serve as vectors which highly adsorb the positive charge of CTACe, thereby neutralizing the charge of the phosphate groups on DNA backbones with a high efficiency. Second, the ultrafine size (< 2 nm) of Au@CTACe particles results in less steric hindrance when interacting with adjacent phosphate groups. This helps the adjacent phosphate groups approach each other much closer when the particles adsorb on DNA backbones than using the large-sized Au@CTAFe and CTACe micelles as compaction agents. 3.4. Highly Efficient Migration of DNA Controlled by Au@CTACe. In a low

strength magnetic field (0.25 T), Au@CTACe (the stock solution was prepared with 0.25 mmol·L-1 NaAuCl4 and 30 mmol·L-1 CTACe) at a very low concentration of 0.3 μmol·L-1 can induce a highly effective migration of 75 μmol·L-1 DNA towards the magnet. This behavior enhances locally the concentration of DNA-nanoparticle complexes in the vicinity of the magnetic field, making the complexes exceed their solubility limit to produce precipitates at the point of the highest field density (Figure 4a). According to the phase behavior (Figure 3a) and DLS results (Figure 3c), the magnetic AuNPs at this concentration cannot form aggregates with DNA. However, after the sample is exposed to be an applied low strength magnetic field (0.25 T), a stepwise migration of DNA can be detected with UV-vis spectra (Figure 5a). The maximum absorbance band of DNA at 260 nm (OD260) decreased vs. time, which 13


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equates to a reduction in the amount of DNA in the upper solution (the solution above the magnet).25 Most of the DNA molecules are transported to adjacent to the magnet surface and result in large amount of precipitates after 6 h, because it is hard to detect the DNA absorbance in the supernatant (Figure 5a). The increase in the baseline indicates the aggregation of DNA and Au@CTACe. When replacing the delivery tool with 30 μmol·L-1 surfactant CTACe, the amount is equal to that modified on the nanoparticle surface, much lower migration efficiency can be obtained with 98% DNA molecules remained in the upper solution over 6 h (Figure 5a). In the absence of external magnetic fields, the absorbance band of DNA remains constant over 96 h (Figure 5b), meaning that no aggregation of Au@CTACe and DNA happens. Although being exposed to a magnetic field (0.25 T), Au@CTABr prepared with the same method using equal amount CTABr instead of CTACe at the same concentration cannot induce the migration of DNA, by virtue of that no variation in the OD260 (Figure 5b) occurred after 4 days. This behavior confirms the importance of the modified magnetic CTACe in promoting the migration of DNA when using AuNPs.

Figure 4. Efficient migration of DNA (a), myoglobin (Mb) (b), and bovine serum

albumin (BSA) (c) induced by the Au@CTACe (a) and Au@CTAGd (b and c) particles in the presence of an NdFeB magnet (0.25 T) at 298 K. The high DNA migration efficiency of the Au@CTACe may be attributed to that AuNPs highly concentrate the magnetic cationic surfactants on their surfaces and greatly enhance the electrostatic interaction between CTACe and DNA. Meanwhile, 14


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the large amounts of surface coated CTACe endow AuNPs with strong magnetism, making them be able to effectively magnetize and promote the migration of DNA through the electrostatic interaction. The capability of the synthesized magnetic AuNPs in inducing DNA to migrate towards external magnetic fields could help functional gene to be delivered to certain cells or tissues in clinical situations, which may assist to obviously improve the gene transfection efficiency. The targeted migration behavior of DNA can be better understood with circular dichroism (CD) spectra (Figure 5c). Due to the base stacking and the helicity, herring testes DNA expresses a positive band at 275 nm as well as a negative band at 242 nm in a CD spectrum.26 After introducing a 0.25 T NdFeB magnet, apart from the decrease in the intensity of the two main bands resulted from the reduction in the DNA concentration in the upper solution, obvious red shift in the two band positions can be observed as the migration time reaches 1 h (Figure 5c). This phenomenon is indicative of the appearance of the DNA compaction from stretched coil state to small-sized global one.26 DLS data further reveal the changes of the DNA compaction efficiency vs. the migration time (Figure 5d). It is found that a maximum size reduction of 81% DNA can be gained with the migration time of reaching 2 h. Moreover, the negative band which is an indicator of the persistence of the secondary structure of DNA exists throughout the transport (Figure 5c), proving that DNA maintains its native β-form during the migration process.26 It seems that the Au@CTACe nanoparticles can serve as one kind of versatile DNA targeted delivery tool, which can not only compact DNA to enable the entry of it into cells through the endocytosis, but also protect its native conformation to ensure that the functionality of gene can be well maintained after crossing cell membranes. After removing the magnet, the intensity and the position of the two opposite bands remain unchanged over time (Figure 5e), meaning that no further migration and compaction occur. It 15


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suggests that the Au@CTACe can also act as one kind of magnetic nano-switch to regulate the “open” and “close” of the DNA compaction with the help of a low strength magnetic field. 1.2

a

0h 1h 3h 6h CTACe 6 h

Abs / a.u.

0.9 0.6 0.3 0.0

1.2

225


DNA without magnet 96 h Au@CTABr 96 h

b

10 CD / mdeg

Abs / a.u.

0.9 0.6

300

c

0h 1h 3h 6h

5 0

0.3 -5

0.0

225


225

300 5

d

400

4


300

e

3

300

CD / mdeg

2R / nm h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200

2 1 0

-1

100 0

1

2

3 4 Time / h

5

6

225


300

Figure 5. UV-vis spectra of 75 μmol·L-1 DNA mixing with 0.3 μmol·L-1

Au@CTACe in the presence (a) or absence (b) of an NdFeB magnet (0.25 T). (c) CD spectra of the complex solution of 75 μmol·L-1 DNA and 0.3 μmol·L-1 Au@CTACe in a 0.25 T magnetic field. (d) DLS data revealing the variations of the 2Rh of 75 μmol·L-1 DNA/0.3 μmol·L-1 Au@CTACe complexes vs. the migration time in an 16


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applied magnetic field (0.25 T). (e) CD spectra of the sample that have migrated for 2 h (solid line) and the same sample after withdrawing the magnet for another 24 h (dotted line). T = 298 K.

3.5. Highly Efficient Precipitation and Migration of Proteins Induced by Au@CTAGd. The ability in magnetic controlling of efficient targeted migration of

biomacromolecules is expanded to proteins. Myoglobin (Mb) and bovine serum albumin (BSA) are selected to confirm the binding and migration capability of the ultrafine magnetic AuNPs. The phase behavior of various concentrations of the Au@CTAGd (the stock solution was prepared with 0.25 mmol·L-1 NaAuCl4 and 20 mmol·L-1 CTAGd) mixing with 14.6 μmol·L-1 Mb and 20 μmol·L-1 BSA, respectively, is displayed in Figure 6a. As shown, 2 μmol·L-1 and 0.6 μmol·L-1 Au@CTAGd are required to aggregate with Mb and BSA and to separate them from aqueous solution, respectively. In comparison, as shown in Figure 6b, at least 50 μmol·L-1 and 20 μmol·L-1 CTAGd are required to induce the precipitation of Mb and BSA, respectively. In our previous work,5 the precipitation requires 10 and 4 μmol·L-1 Au@CTAFe for Mb and BSA, respectively. It seems that apart from obviously reducing the dosage for DNA compaction, using ultrafine AuNPs to highly concentrate magnetic cationic surfactants is able to strongly decrease the critical concentration for protein separation. The higher efficiency of Au@CTAGd in precipitating proteins than Au@CTAFe is a consequence of larger amount of CTAGd modified on the surface of ultrafine AuNPs. According to our previous data,11 the higher oxidizability of CTAFe makes the modified amount of it on the surface of AuNPs should be carefully controlled to avoid the reduction of CTAFe by excess NaBH4. Compared with CTAFe, the weak oxidizability of CTAGd enables a larger amount of it to be modified on the particle surface and endowing the resulting AuNPs 17


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a higher surface charge density as well as stronger electrostatic interactions with proteins. Complexes of 1.0 μmol·L-1 Au@CTAGd and 14.6 μmol·L-1 Mb verified the magnetically induce migration of proteins. In UV-vis spectra, Mb exhibits a strong Soret band at 409 nm, as well as Qα and Qβ bands at 641 and 502 nm, respectively. These three characteristic bands of Mb suggest metmyoglobin with a diamagnetic heme group as well as a six-coordinate geometry with a strongly bound water molecule.27 In the absence of a magnetic field or using equal amount of Au@CTABr (prepared with 20 mmol·L-1 CTABr in the same method for obtaining Au@CTAGd) with a 0.25 T NdFeB magnet, the UV-vis absorbance of protein-nanoparticle complexes do not change after 144 h (Figure 6c), indicating that no aggregation or migration happens. When being exposed to be a 0.25 T low strength magnetic field for 6 days, the absorbance of Mb in the upper solution decreases by about 73.3% (Figure 6d), with a lot of Mb-Au@CTAGd mixtures being transported to the magnet surface and resulting in brown precipitates (Figure 4b). The increase of baseline predicates the formation of aggregates. In former reports, either using magnetic surfactants or magnetic Au@CTAFe particles with much higher dosage (i.e., 3030 μmol·L-1 C12H25N+(CH3)3[HoCl3Br]- (DTAHo) or 5 μmol·L-1 Au@CTAFe) much lower migration efficiency is obtained with at least 40% (DTAHo system) and 34% (Au@CTAFe system) Mb remained in the supernatant.4,11 This behavior proves our proof-of-concept that ultrafine magnetic AuNPs modified with large amount of paramagnetic cationic surfactants can also act as vectors to increase the migration efficiency of proteins. The strong binding of oppositely charged particles, compounds or heavy metal ions to the charged and hydrophobic side chains can easily denature Mb.16,28 The 18


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denaturation usually brings difficulties for protein delivery, separation and purification. The addition of micromolar Au@CTAGd does not lead to any shifts in the band positions of Mb (Figure 6d), evidencing that the binding of Au@CTAGd does not bring any changes in the tertiary structure in the vicinity of the heme prosthetic group and no disruption in the native conformation occurrs during the migration process. Similar results could be discovered after adding anionic surfactants or Au@CTAFe to Mb without denaturation.5,29 However, further increase in the amount of the surface modified CTAGd would lead to evident red shift in the three typical band positions of Mb (Figure S5 in the SI), suggesting that Au@CTAGd prepared with too many CTAGd (> 30 mmol·L-1) should denature Mb. This is why the Au@CTAGd particles prepared with 20 mmol·L-1 surfactants are chosen to migrate proteins. Au@CTAGd can also control over effective targeted migration of BSA (20 μmol∙L-1) in an applied magnetic field (0.25 T) with a very low concentration (0.4 μmol∙L-1). Due to the tryptophan residues exposed to aqueous solutions, BSA presents a fluorophore absorbance band at 278 nm in UV-vis spectra.30 The increase in the baseline in UV-vis spectra in Figure 6e is the indicative of the formation of BSA/Au@CTAGd aggregates. The complexes are drawn towards the surface of the magnet to decrease the overall protein concentration in the upper solution (Figure 6e). Nearly all the BSA are transported to adjacent to the magnet surface over 120 h as no clear protein band can be traced in the upper solution. UV-vis measurements confirm no disruption in the native conformation of BSA during the transport. Denaturation of BSA should result in clear shifts in the residue band positions in principle.31 Apart from the decrease in the intensity due to the decreases in the concentration of BSA, no position changes in the band at 278 nm were observed (Figure 6e), demonstrating that 19


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the coating of nanoparticles showed negligible effect on the conformation of BSA. An evident lower migration efficiency is gained after choosing surfactant CTAGd (8 μmol∙L-1, the amount that coats on the particle surfaces) as delivery agent. Over five days, there is still approximate 85% BSA to be left in the upper solution (Figure 6e). Similar lower efficiency can also be found in our former work,5 86% is remained in the upper solution at 3 μmol∙L-1 Au@CTAFe.

a

BSA

b

BSA

Mb

Mb

0

1

10

-1

c / mol L Au@CTACe 2.5

0

10

20

40 c CTAGd

60 80 -1 molL

100

Mb without magnet Au@CTABr

c

Abs / a.u.

2.0 1.5 1.0 0.5 0.0

2.5

300

d

400

500

0.8

e

Abs / a.u.

0.6

1.5

700

0h 24 h 72 h 120 h CTAGd 120 h

0.4

1.0

0.2

0.5 0.0

600

Wavelength / nm 0h 24 h 96 h 144 h

2.0 Abs / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

300

400 500 600 Wavelength / nm

700

0.0

250

275 300 325 Wavelength / nm

350

Figure 6. Phase behavior of 14.6 μmol·L-1 Mb or 20 μmol·L-1 BSA mixing with

different concentrations of Au@CTAGd particles (a) or CTAGd surfactants (b). The 20


Langmuir

filled symbols refer to the region in which precipitates can be found. UV-vis spectra of 14.6 μmol·L-1 Mb/1 μmol·L-1 Au@CTAGd mixtures in the absence (c) or presence (d) of an external magnetic field (0.25 T). (e) UV-vis spectra of mixtures containing 20 μmol·L-1 BSA and 0.4 μmol·L-1 Au@CTAGd in the presence of external magnetic force (0.25T). T = 298 K. 3.6. Release of Biomacromolecules from Magnetic AuNPs. After being

transported to the vicinity of the magnet surface, adding sufficient (50 mmol·L-1) NaBr

can

effectively

screen

the

electrostatic

interaction

between

these

biomacromolecules and magnetic AuNPs, leading to the release of DNA and proteins. From CD (Figure 7a) or UV-vis spectra (Figure 7b and 7c), the absorbance of all the biomolecule-nanoparticle complexes recover. Further experimental results, as shown in Figure S6 in the SI, show that 50 mmol·L-1 is the minimum amount of NaBr required for completely releasing these biomolecules from AuNPs as the absorbance can only completely recover once the amount of NaBr reaches 50 mmol·L-1. It can be compared to DNA, Mb and BSA after the addition of NaBr. This method demonstrates a simple strategy for reversible convergence and release of DNA and proteins based on magnetic AuNPs, which provides a promising avenue for the delivery of biomacromolecules for biomedical applications.

10

CD / mdeg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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DNA NaBr 6h

a

5 0 -5 220

240

260

280

300

Wavelength / nm 21


320

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2.5

Mb 144 h NaBr

b

c

1.5 1.0 0.5 0.0

300

400

500

600

Wavelength / nm

BSA 120 h NaBr

0.6

Abs / a.u.

2.0

Abs / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

0.4 0.2 0.0 240

700

270

300

Wavelength / nm

330

Figure 7. Release of DNA (a), Mb (b) or BSA (c) upon addition of 50 mmol·L-1 NaBr

with pure DNA, Mb and BSA as a comparison. T = 298 K.

4. CONCLUSIONS We demonstrate a simple strategy to synthesize ultrafine magnetic AuNPs (< 2 nm) via one-step surface modification. The successful preparation of this kind of

responsive nanoparticles could made possible by using weak oxidizing magnetic cationic surfactants CTACe or CTAGd as modified compounds. Because AuNPs act as vectors that highly concentrate the positive charge of cationic surfactants on their surfaces

to

strongly

enhance

the

electrostatic

interaction

between

the

biomacromolecules and surfactants, they can result in a very low dosage for DNA and protein convergence. The CTACe or CTAGd endows AuNPs with strong magnetism, and thereby the biomacromolecule-nanoparticle complexes exhibit much higher sensitivity and faster responsiveness even exposure to low strength magnetic fields (0.25 T) compared to magnetic surfactants and magnetic AuNPs with larger size. The native conformation of DNA and proteins can be maintained during the migration process. The captured DNA and proteins could be released after adding sufficient inorganic salts. We envision that our method for preparing functional AuNPs to control the migration of biomacromolecules with high efficiency can be effectively 22


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and non-invasively applied to various systems in nanotechnology, material science and biomedicine.

AUTHOR INFORMATION Corresponding Author

* E-mail: [email protected]; Tel.: +86-531-88366074, Fax: +86-531-88564750. Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is financially supported by the NSFC (Grant No. 21420102006). SUPPORTING INFORMATION AVAILABLE

Determination of cmcs by electrical conductivity, photograph of a brownish-yellow solution sample, HR-TEM images of the magnetic AuNPs modified with 20 mmol·L-1 CTACe (a) or 40 mmol·L-1 CTAGd (b), AGE data, UV-vis and CD spectra. This information is available free of charge via the Internet at http://pubs.acs.org.

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TOC Au@CTACe or Au@CTAGd

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Surfactant-Modified Ultrafine Gold Nanoparticles with Magnetic

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