Magnetic networks of carbon quantum dots and Ag particles

Journal of Colloid and Interface Science 539 (2019) 203–213

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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Regular Article

Magnetic networks of carbon quantum dots and Ag particles Ling Wang, Yitong Wang, Yuanyuan Hu, Guangzhen Wang, Shuli Dong ⇑, Jingcheng Hao ⇑ Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials (Shandong University), Ministry of Education, Jinan 250100, PR China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 23 October 2018 Revised 9 December 2018 Accepted 10 December 2018 Available online 17 December 2018 Keywords: Self-assembly Carbon quantum dot Magnetic surfactant Ag DNA

a b s t r a c t Self-assembly exploits a facile non-covalent way to couple structurally different building blocks for creating soft materials with synergetic novel properties and functions. Taking advantage of magnetoproperties from magnetic surfactants as well as versatile functional ligand formed by carbon quantum dots with cysteine (cys-CQDs), the magnetic network materials were firstly constructed by using magnetic surfactants and cys-CQDs as self-assembly building blocks. Counterions of Br
, [GdCl3Br]
, [HoCl3Br]
in surfactants could control the morphology of magnetic network structures, and the concentration of magnetic surfactants manoeuvres a versatile scenario of self-assembly behavior. Self-assembly of cys-CQDs and CTAHo brought out a 10-fold increase in magnetic moment of CTAHo. The fluorescent property of carbon quantum dots firstly served as an effective indicator element to dissect the collective effect in self-assembly process. For the sake of capturing the target sequence-specific DNA molecules, in situ growth of Ag nanoparticles (AgNPs) upon the magnetic network structures was realized by synergetically electrostatic and coordinated interaction of carboxyl groups and Ag ions. The magnetic Ag self-assemblies anchored thiol-containing DNA, serving as a magnetic separation booster for the target sequence-specific DNA molecules under an applied magnetic field, which will bring light on designing magneto-functional self-assembly materials according to practical application requirements. Ó 2018 Published by Elsevier Inc.

1. Introduction Self-assembly is a coupling process of structurally different building blocks by synergetic non-covalent interactions, such as electrostatic interactions, hydrophobic interactions, hydrogen-

⇑ Corresponding authors. E-mail addresses: [email protected] (S. Dong), [email protected] (J. Hao). https://doi.org/10.1016/j.jcis.2018.12.037 0021-9797/Ó 2018 Published by Elsevier Inc.

bond interactions, in order to create versatile functional materials [1]. The excellent availability of building block units, the simplicity of synthesis and synergistic emerging properties make selfassembly work as a powerful technique for constructing functional hierarchical materials using surfactants [2]. Especially, the selfassembly of magnetic surfactants (Mag-Surfs) containing magneto-active metal complex ions, with cooperative building blocks, realized the construction of novel magnetic aggregates

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[3,4], nano-materials [5–8] and bio-complexes [9–11]. Mag-Surfs possess the advantages of facile fabrication, flexible structural design (changing its cations or anions) and high magnetosensitivity [12,13]. The amphiphilic characteristic and surfaceactivity impart Mag-Surfs with excellent self-assembly capability in exploiting non-covalent interactions and offering a facile method for preparing the magnetic functional materials [9], which will bring an innovation of magnetic materials construction and serve as a supplement for traditional sophisticated magnetoactive nanoparticles synthesis. Traditionally, self-assembly building blocks are amphiphilic molecules or polymers. Hierarchical self-assemblies by ligandmodified nanoparticles such as Au nanoparticles [5,14], provided an innovative insight into enrich the theoretical basis of aggregate research. Carbon quantum dots (CQDs) have attracted enormous attention in the field of bioimaging, sensors, biomedicine and catalysis by virtue of their superior advantages in biocompatibility, low toxicity, photobleaching resistance and tunable emission [15]. Because of various surface modified ligands, such as carboxyl, hydroxyl, nucleotide [16], amino [17] and boronic acid groups [18], individual CQD is endowed with highly effective selfassembly building block with multi-interactive sites. These functional groups will provide a controllable handle for non-covalent forces between CQDs and other cooperative molecules. Thus, self-assembly building block of individual CQD with amphiphilic molecules could control the self-assembly process and aggregates of CQDs. For example, our group has fabricated fluorescent vesicles formed by surfactants/CQDs [19], as well as responsive biological vesicles by DNA and CQDs with coordinated Ce(III) ions [20]. Besides, the coupling of CQDs and surfactant could also fabricate self-assembled nanosphere [21]. Importantly, the functional ligands on individual CQD endow CQDs assemblies with emerging synergetic application like bio-detected platform [19] and ‘‘activated” carbon dot photosensitizer [21]. Inspired by the enormous application potentials of cysteine based-CQDs (cys-CQDs), such as the effect on cellular energy metabolism process [22] and Golgi apparatus targeting ability [23], we believe the integration of cysCQDs with cooperative Mag-Surfs will construct soft functional material with magneto-responsive capability. Our group has reported in-situ Ag nanoparticles (AgNPs) synthesis utilizing glutathione-hydrogels of superlong helices as templates [24] or by reducting Ag+ through electrostatically selfassembled 50 -adenosine monophosphate [25]. Due to the excellent antibacterial functions, catalytical properties and biothiol-binding capability of AgNPs, the combination of AgNPs and hydrogel framework could be utilized to fabricate assembled Ag hybrid materials with synergetic functions and tunable mechanical properties, playing a critical role in wound healing, catalysis [26] and biomolecule

enrichment [25]. Inspired by these elegant approaches, we firstly constructed the magnetic network materials with incorporation of AgNPs upon cys-CQDs/Mag-Surfs self-assembled templates. Specifically, this research provides a facile non-covalent selfassembly of individual cys-CQD and positive Mag-Surfs including (C16H33(CH3)3N+[HoCl3Br]
(CTAHo) and C16H33(CH3)3N+[GdCl3Br]
(CTAGd)) as the cross-linking reagents, to fabricate magnetic network aggregates. In this proposal, AgNPs were produced in situ without any reductant or stabilizer due to the presence of cysteine-ligands in the aggregates (Scheme 1), where cys-CQDs worked as the building blocks and the active sites to synthesize AgNPs, and Mag-Surfs served as cross-linking reagents and magnetic sources. Furthermore, we anchored thiol-modified DNA on magnetic Ag self-assemblies through AgAS bonds to explore a magnetic selector to enriching sequence-specific DNA molecules under an applied magnetic field. 2. Experimental section 2.1. Chemicals Cetyltrimethylammonium bromide (98%, CTAB), HoCl36H2O (99.9%), GdCl36H2O (99.9%), silver nitrate (AgNO3), cysteine (99%); 5,50 -Dithiobis(2-nitrobenzoic acid) (DTNB, 97%) were purchased from J&K Chemical; anhydrous citric acid (99.5%), oxalyl chloride (98%) were obtained from aladdin; tetrahydrofuran (THF, 99%) was purchased from Sinopharm Chemical Reagent Co. Ltd.. 1,6-diphenyl-1,3,5-hexatriene (DPH, 98%) was obtained from Sigma-Aldrich. DNA (thoil-containing DNA: SH-AGT CTG ACT ACA ACT, target DNA: TCA GAC TGA TGT TGA) were perchaded from Sangon Biotich, and ultrapure water with a resistivity of 18.25 MX cm prepared from a UPH-IV ultrapure water purifier (China) was used throughout the experimental process. 2.2. UV–vis spectrometry UV spectra of samples were examined by a U-4100 UV–Vis spectrometer, using 10 mm path length quartz cell. 2.3. Zeta-potential measurements The Zeta potentials of the samples were measured using a Zeta PALS potential analyzer instrument (Brookhaven, USA), equipped with parallel-plate platinum black electrodes spaced 5 mm apart and a 10 mm path length rectangular organic glass cell. All samples were measured at a sinusoidal voltage of 80 V with a frequency of 3 Hz. Each sample was measured ten times for the average value.

Scheme 1. The construction process of magnetic network clusters with AgNPs: firstly, self-assembly of cys-CQDs with Mag-Surfs including CTAHo and CTAGd; secondly, insitu synthesis of AgNPs on network templates.

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2.4. Transmission electronic microscopy (TEM)

2.9. Synthesis of Mag-Surfs

A drop of dispersion liquid was dropped on TEM grid (copper grid, 3.02 mm, 200 mesh and coated with Formvar film). After drying, TEM images were obtained with a JEM 100 cx TEM (JEOL, Japan) at an accelerating voltage of 120 kV.

The cationic surfactant CTAHo, was synthesized as reported by mixing equal molar amount of CTAB and HoCl36H2O in methanol and stirring over night at room temperature and then dehydrated in vacuo at 80 °C overnight. CTAGd was synthesized by the same method.

2.5. Conductivity measurements 2.10. Synthesis of cys-CQDs DDSJ-308A Analyzer was used to perform electrical conductivity experiments. The Pyrex glass measuring cell was placed in a water bath at 25 ± 0.1 °C. The critical micelle concentration (cmc) was determined from the break point between the higher and lower [dj/d(conc)] linear curves. The ionic dissociation constant (b) was estimated by the ratio of the slopes. Each sample was measured three times for the average value.

FTIR spectra were obtained on a VERTEX-70/70V FTIR spectrometer (Bruker Optics, Germany).

Anhydrous citric acid (25.0 g) was heated to 250 °C by an oil bath and kept pyrolyzing for 2 h to obtain black solid. After cooling to room temperature, the black solid was re-dispersed in 30 mL oxalyl chloride with vigorous stirring for 24 h under N2 atmosphere. The oxalyl chloride in mixing solution was then removed by spin steaming. After re-dispersing in THF solution, 3.0 g cysteine was added into the mixture solution with vigorous stirring for 3 days under N2 atmosphere. After removing THF, the dark product was neutralized with NaOH solution. Finally, the cys-CQDs solution was dialysed (3000 Da cutoff) and lyophilized to get the cys-CQDs powder [23].

2.7. Atomic force microscopy (AFM)

2.11. Self-assembly of cys-CQDs and surfactants

For AFM observations, samples were observed on a Tapping Mode operating with a Nanoscope IIIA at a scan frequency of 1.5 Hz and a resolution of 512  512 pixels.

Various amount of surfactants were mixed with 0.2 mgmL
1 cys-CQDs (cys-CQDs solution should be freshly prepared), the mixture solution was incubalated in 25 °C for 24 h. The final concentration of cys-CQDs is 0.1 mgmL
1.

2.6. Fourier transform infrared spectroscopy (FTIR)

2.8. Fluorescence quantum yields 2.12. Self-assembly of cys-CQDs/CTAHo/Ag assemblies The fluorescence quantum yields were measured with a spectrofluorometer (FLSP920, Edinburgh Instruments LTD) equipped with an integrating sphere, which consists of a 120 mm inside diameter spherical cavity. 3 mL of sample solution was sealed in a quartz cell (1 cm  1 cm) with a plug. The same volume of water was used as the blank sample.

10 mmolL
1 AgNO3 (1.0 mL) solution was added into 4.0 mL cys-CQDs/CTAHo complexes solution (with 0.1 mgmL
1 cysCQDs and 0.4 mmolL
1 CTAHo, cys-CQDs solution should be freshly prepared), the mixed solution was incabulated in 25 °C for 24 h (in dark condition).

b v (S-H)

v (N-H)

v (C-H) v (O-H) or v (N-H)

v (C=O) 4000

3000

v (C=C)/δ (N-H)

2000

1000 -1

Wavenumber (cm ) Fig. 1. (a) The synthesis route and structure of cys-CQDs. (b) FTIR spectrum and (c) AFM observation of cys-CQDs, insert in Fig. 1c is the height analysis of cys-CQDs.

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3. Results and discission 3.1. Synthesis and characterization of cys-CQDs As one of building blocks in magnetic self-assemblies described in Scheme 1, the negatively charged CQDs with peripherallysubstituted cysteine ligands were synthesized through two steps, including pyrolysis of citric acid to produce carboxylic acid CQDs and amidation reaction for the attachment of cysteine anions on CQDs surface (Fig. 1a). FTIR analysis of cys-CQDs in Fig. 1b gives a stretching vibration signal of OAH or NAH at 3200–3600 cm
1. The vibrational absorption signals at 1715 cm
1 and 1627 cm
1 are ascribed to the bending vibrations of C@O and C@C, respectively. Moreover, the stretching vibration band (-SH, 2552 cm
1) of thiol groups [27] proved the successful synthesis of CQDs with cysteine groups. From X-ray photoelectron spectroscopy (XPS) measurement, the presence of carbon, nitrogen, oxygen and sulfur elements confirms the successful attachment of cysteine onto the surface of CQDs (Figs. S1–S4). The synthesized cys-CQDs are 26.9 ± 5.3 nm in diameter and 9.8 ± 3.1 nm in height, indicating their discoid morphology (Figs. 1c and S5).

3.2. Self-assembly of cys-CQDs and surfactants with different counterions Another indispensable building block for the magnetic self-assemblies is Mag-Surfs. The highly effective concentrations of metal centers impart Mag-Surfs with magneto-responsive

Zeta Potential (mV)

60

properties [9], making it possible to remotely control over the magnetic self-assemblies of cys-CQDs. Because of the highesteffective magnetic moment of lanthanide metal Ho(III) or Gd(III), Mag-Surfs containing [HoCl3Br]
and [GdCl3Br]
generally present higher magnetic susceptibility [9]. Therefore, CTAHo and CTAGd were chosen as the cooperative molecules to self-assemble with cys-CQDs for higher magnetic susceptibility. In order to obtain more insight into the effect of counterions on aggregate construction, a straightforward control test over self-assembly process of cys-CQDs with various [GdCl3Br]
, [HoCl3Br]
and Br
surfactants was performed. Upon addition of cationic surfactants, Zeta potential (n) of cys-CQDs increases from
49.7 mV to approximate 30.0 mV (Fig. 2a). FTIR spectrum in Fig. S6, the stretching vibration peak of C@O in carboxyl group of cys-CQDs appeared a redshift from 1713 cm
1 to 1693 cm
1 after self-assemble with CTAHo, which further proved that the electrostatic interaction between carboxyl groups in cys-CQDs and quaternary ammonium cations in surfactants is the leading force to drive the self-assembly behavior. Zeta potential is considered as a powerful tool to assess the colloidal stability of aggregates. Theoretically, only when n is higher than 30 mV or lower than
30 mV, colloidal particles could possess a sustainable stability in solution [28]. The significant reduction in absolute value of n results in thermodynamic instability for cys-CQDs/Mag-Surfs [20], afterwards, accompanied with the presence of precipitate within the concentration range of 0.05– 0.2 mmolL
1 for CTAHo, as well as with 0.05–0.3 mmolL
1 for CTAGd. However, the precipitate occurred for cys-CQDs/CTAB complexes in the range from 0.2 to 0.3 mmolL
1 (Fig. 2a and b). These results fully demonstrated that Mag-Surfs possess a stronger

CTAHo

b

a

30

CTAGd

Precipitate 0

CTAB CTAGd CTAHo

-30

-60 0.01

0.1

CTAB

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1

c (mM) surfactant

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c (mM) surfactant

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-1

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(mM)

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CTAHo

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(mM)

Fig. 2. (a) The Zeta potential of the mixed solution of cys-CQDs and surfactants (including CTAB, CTAGd and CTAHo). (b) The phase diagram of cys-CQDs with increasing concentration surfactants, where triangle indicates the solution phase, star configuration represents the turbidity phase and sphere indicates precipitate. Electrical conductivity measurements of CTAGd (c) and CTAHo (d), and the break point is the cmc of Mag-Surfs. The b was estimated by the ratio of the slopes. Each j was the average value calculated from the three measurements.

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capability in neutralizing the charges of cys-CQDs compared with CTAB. We speculate the unique hydrophobic property of [GdCl3Br]
and [HoCl3Br]
play a pivotal role in this higher selfassembling efficiency with cys-CQDs. The counterions [GdCl3Br]
and [HoCl3Br]
are larger than Br
, and it is supposed to be less effective to screen the repulsions between adjacent cation-cation head groups and increase the critical micelle concentration (cmc). However, compared with traditional CTAB (cmc [29] is 0.92 mmolL
1, b [30] is 0.11), Mag-Surfs CTAGd and CTAHo possess exceptional lower cmc (0.75 mmolL
1 and 0.81 mmolL
1, respectively) and higher b (0.76 and 0.77) (Fig. 2c, d and Table S1). A reasonable explanation offered by Eastoe [12,13] and our group [3,5,9,10] was that the hydrophobic counterions in Mag-Surfs could enter into the micellar core by interacting with hydrophobic moieties of alkyl chain, resulting in lower cmc and higher b. Moreover, surfactant solutions with 1,6-diphenyl-1,3,5hexatriene (DPH; 2.5 lmol/L, kex = 355 nm, kem = 428 nm, is a fluorescent apolar probe) further prove the explanation of Eastoe. DPH is solubilized by the apolar part of surfactant aggregates and consequently a significant increase in fluorescence intensity. As shown in Fig. S7, the fluorescence intensity of CTAHo (6.68 * 106) at 428 nm is obviously higher than that of CTAB (0.85 * 106), indicating the hydrophobicity of CTAHo than CTAB. The stronger fluorescence intensity of CTAHo is contributed by more apolar part in CTAHo aggregate. As such, more interactive sites of cationic head groups on Mag-Surfs interact with cys-CQDs, behaving well with a stronger self-assembled capability. The highly efficient selfassembly capability of Mag-Surfs was also proved by a sustainable DNA condense capability at high concentration of C16H33(CH3)3N+[FeCl3Br]
(CTAFe), while CTAB could de-compact DNA complexes at the same condition [30]. Similarly, magnetic Au assemblies of Au/C16H33(CH3)3N+[CeCl3Br]
(Au/CTACe) provided high selfassembly degree of DNA molecules in comparison with Au/CTAB assemblies [5]. The above discussions illustrate that the selfassembly of Mag-Surfs is a key tool to generate complex aggregates with novel topology and properties.

Generally, diverse fluorescence changing of CQDs could be realized by their self-assembly behavior. By virtue of covalent selfassembly of glucose and boronic acid on CQDs, glucose could quench CQDs fluorescence [18]. There had a fluorescencedecrease phenomenon for CQDs by generating CQDs/DNA vesicles owing to the aggregation-induced fluorescence quench phenomenon [20]. On the contrary, vesicles constructed by CQDs and anionic surfactants brought a fluorescence enhancement, in

9

d

12

e

9

9

6

Intensity

Intensity

3.3. Mechanism investigation of magnetic networks by cys-CQDs and CTAHo

6 3

Intensity

12

The difference of hydrophilic-hydrophobic property between magnetic counterions [9] and traditional Br
not only induces different self-assembly capability of cys-CQDs, but also results in discrepant aggregation structure for cys-CQDs complexes. As shown the TEM images in Fig. 3b and c, as well as the diameter measurements in Fig. 3e and f, apart from aggregates with approximate 200 nm, cys-CQDs/Mag-Surfs complexes also present the connected network structures in the size of several microns; nevertheless, cys-CQDs/CTAB complexes display as globular structures with approximately 200 nm in size (Fig. 3a and d). Generally, the colloidal particles or aggregates would produce a light scattering phenomenon only when their sizes are smaller than wavelengths of visible light (400–700 nm). The insert photos in Fig. 3a is a good interpretation, where, the opaline solution of cys-CQDs/CTAB is ascribed to light scattering of newly formed aggregate, similar to the light scattering phenomenon caused by the vesicles of CQDs/ anionic surfactant complexes [19]; but for cys-CQDs/Mag-Surfs network structure, their large diameter would induce a completely different light reflection phenomenon instead of light scattering, behaving a macroscopically transparent solution in inserts of Fig. 3b and c. The hydrophobic properties of Mag-Surfs should be account for the connected network structure formation. Obviously, by regulating the kind of counterions in surfactants, various morphologies of cys-CQDs could be realized by self-assembly process.

3

0

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Size (nm)

1000

10000

f

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1

10

100

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1000

10000

1

10

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1000

10000

Size (nm)

Fig. 3. Effect of counterions in surfactants on cys-CQDs self-assembly. TEM images of cys-CQDs self-assembled with 0.4 mmolL
1 CTAB (a), CTAGd (b) and CTAHo (c), in which the inserts are the macroscopically photos of cys-CQDs aggregates. Size analysis of cys-CQDs is presented as interacted with 0.4 mmolL
1 CTAB (d), CTAGd (e) and CTAHo (f).

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which the binding of surfactants could tune the surface states of CQDs and prevent from fluorescence quenching of CQDs [19]. Inferred from the results of the literature, fluorescence property is an ingenious and sensitive indicator element to reflect the aggregate state of CQDs, in favor of the mechanism investigation on the self-assembled process of cys-CQDs/CTAHo. As shown in Fig. S8, the fluorescence emission spectra of cys-CQDs solution present a wavelength-dependent excitation behavior, which is generally interpreted to be related to emissive traps or electronic conjugate structures [31]. The maximum emission wavelength is approximately 465 nm with a 390 nm excitation, which can be selected for the following investigation (Fig. S8). The measured fluorescence quantum yield at 465 nm emission was calculated to be 2.27%. Fluorescence ratio at 465 nm emission of cys-CQDs/CTAHo aggregates and individual cys-CQDs is defined as F/F0, contributing to describe the fluorescence variation of cys-CQDs aggregates with a function of CTAHo concentration. As shown in Fig. 4a, the F/F0 of cysCQDs/CTAHo aggregate solution has three variation tendency: firstly, the F/F0 of cys-CQDs/CTAHo aggregate solution exhibits a continuous decrease within 0.02–0.2 mmolL
1 of CTAHo (Fig. S9a); Secondly, an unexpected increase of F/F0 occurred on 0.3 mmolL
1 of CTAHo, followed by an obvious decrease of F/F0 in the range from 0.4 to 0.7 mmolL
1 of CTAHo (Fig. S9b); Thirdly,

cys-CQDs/CTAHo aggregate solution presented an abrupt enhancement of F/F0 upon 0.8 mmolL
1 of CTAHo (Fig. S9c). Combining with diameter measurement and TEM result analysis, we thought the three variation tendency of F/F0 is precipitate phase, aggregate formation phase and aggregate re-solubilization phase of cysCQDs/CTAHo complexes. Within precipitate phase, F/F0 of cys-CQDs/CTAHo solution was continuously decreasing within 0.02–0.2 mmolL
1 of CTAHo (Fig. 4a). In this region, the absolute values of n is lower than 30 mV (Fig. 2a), inducing a thermodynamic unstable state [28] for cys-CQDs/CTAHo complex solution accompanied with precipitate generation. Thus, the decrease of F/F0 is ascribed to the reduction of cys-CQDs in the solution. The aggregation formation phase ranged from 0.3 to 0.7 mmolL
1 CTAHo, and cys-CQDs/CTAHo complex solution was a thermodynamic stable transparent solution (n > 30.3 mV) instead of precipitate in this phase (Fig. 2b). The critical concentration for aggregate formation is 0.3 mmolL
1 of CTAHo, presenting a pronounced increase of F/F0 is observed in Fig. 4a. As the concentration of CTAHo being within 0.3–0.7 mmolL
1, connected network structure with a diameter of several microns could occur by the complexes of cys-CQDs coupled with CTAHo (as shown in Fig. 4c, the concentration of CTAHo is 0.4 mmolL
1). They are in

1.5

b

a

F/F0

1.2

cmc = 0.81 mM

Precipitate

1.0 mM Aggregate resolubilization

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Fig. 4. Mechnism investigation of cys-CQDs/CTAHo self-assembly. (a) The fluorescence changes of cys-CQDs/CTAHo complexes with an increasing concentration of CTAHo. (b) The diameter analysis of cys-CQDs interacted with CTAHo at 0.4, 0.8 and 1.0 mmolL
1. TEM images of cys-CQDs/CTAHo complexes including 0.4 mmolL
1 (c), 0.8 mmolL
1 (d) and 1.0 mmolL
1 CTAHo (e), respectively. (f) The schematic diagram illustrating the self-assembled process for cys-CQDs and CTAHo at 0.4, 0.8 and 1.0 mmolL
1, indicating the aggregate formation, aggregate redissolution at initial and terminal stage.

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agreement with the aggregate peak from diameter analysis in Fig. 4b. Two factors are supposed to be accountable to the formation of the aggregation phase: lower than their cmc of CTAHo (0.81 mmolL
1 obtained from conductivity measurement), individual CTAHo molecule would electrostatically be absorbed onto the negatively charged cys-CQDs; Meanwhile, the hydrophobic interaction could spontaneously drive adjacent alkyl chains of CTAHo to interact with each other and serve as a bridge to connect cys-CQDs/CTAHo units into huge network clusters (as illustrated by Fig. 4f). Within aggregate formation phase, a slight decrease of F/F0 was observed with gradual increasing concentration of CTAHo. We speculate that the formation of cys-CQDs aggregation facilitates the nonradiative energy transfer, producing aggregationinduced fluorescence quench phenomenon to a certain degree. This phenomenon was also observed CQDCe/DNA vesicles in which DNA could control the self-assembly and dis-assembly process and further regulate CQDs fluorescence [20]. The re-solubilization of aggregates appeared once the concentration of CTAHo was 0.8 mmolL
1, and F/F0 of cys-CQDs/CTAHo solution turned to increase again. One can find in Fig. 4b that at 0.8 mmolL
1 CTAHo, the aggregate formation peak disappeared, and instead a new peak about 30–80 nm generated. This phenomenon was in agreement with TEM result in Fig. 4d. There had an obvious size decrease of cys-CQDs/CTAHo network structures. More importantly, when the concentration of CTAHo reached to 1.0 mmolL
1, the size of cys-CQDs/CTAHo network structures continued to reduce to about 30 nm (Fig. 4b and e), and F/F0 of cysCQDs/CTAHo solution enhanced (Fig. 4a). We believe the aggregate state of CTAHo is the critical element for the size variation of cysCQDs/CTAHo network structures. From conductivity measurement in Fig. 2d, the cmc of CTAHo was 0.81 mmolL
1. Upon exceeding the cmc, CTAHo would spontaneously self-assemble into micelles by hydrophobic interaction, meanwhile, these micelles of CTAHo could electrostatic interaction with cys-CQDs. There exists a competition between micelles and molecules of CTAHo in the process of electrostatically interacting with cys-CQDs. The advantage of more positive charges makes CTAHo micelles preferential bind to the surface of cys-CQDs, which increased the charged repulsion effect of cys-CQDs complexes, resulting in a size reduction of cys-CQDs/CTAHo network structures in Fig. 4b and f. Therefore, we can speculate that in the initial stage of aggregation resolubilization, the cys-CQDs/CTAHo network structures were composed of units cys-CQDs/CTAHo micelles, adjacent units are connected with hydrophobic tails of individual CTAHo molecule (0.8 mmolL
1 CTAHo in Fig. 4f).

150

The terminal stage of aggregation re-solubilization started at 1.0 mmolL
1 CTAHo, the size of cys-CQDs/CTAHo network structures was about 30 nm (Fig. 4b and e). Combining with the fact that a single cys-CQD is 26.9 nm from AFM analysis, we deduced that these aggregates were made of single cys-CQD with peripherallyabsorbed CTAHo micelles. This dis-assembly of connected cysCQDs/CTAHo network structures into single smaller cys-CQDs/ CTAHo aggregate plays a critical role in the enhancement of F/F0 (Fig. 4a and f). The fluorescent properties of cys-CQDs aggregates are affected by self-assembly process, which is completely different from the quenching mechanisms for metal ions like Cu2+ and Hg2+, by inner filter effect [17] and effective electron or energy transfer process [32], respectively. Take advantage of fluorescent property of CQDs as an effective indicator element for studying the microstructure of CQDs aggregates, could provide detailed interpretation for self-assembly and bring an innovative enlightenment to aggregate research. Therefore, the self-assembly process of cys-CQDs and CTAHo undergoes three phase variation including precipitate, aggregate formation and aggregate re-solubilization phase. In aggregation formation phase, individual CTAHo molecule would electrostatically interacted with cys-CQDs, accompanied with the hydrophobic interaction between adjacent alkyl chains of CTAHo, which serve as a bridge to connect cys-CQDs/CTAHo units into huge network structures (Fig. 4f). For aggregation re-solubilization region, the large connected cys-CQDs/CTAHo network structures deassembled into smaller cys-CQDs/CTAHo aggregate: at the initial stage, the cys-CQDs/CTAHo network structures were composed of units cys-CQDs/micelles of CTAHo, adjacent units are connected with hydrophobic tails of individual CTAHo molecule; but for terminal stage, cys-CQDs/CTAHo aggregates are composed of single cys-CQD with peripherally-absorbed CTAHo micelles (Fig. 4f). 3.4. Redox and magnetic behaviors for cys-CQDs assemblies Considering the functional groups of cys-CQDs including amine, carboxyl and sulfhydryl, self-assembly with Mag-Surfs would bring out some distinctive collective properties and functions. According to DTNB-thiols assay method [33], the sulfhydryl on cys-CQDs is 1.6325 lmolmg
1 (principles and step of calculation is shown in Figs. S10 and S11). According to Fig. 5a and b, as integrating with surfactants containing Br
, [GdCl3Br]
or [HoCl3Br]
for 2 h, cysCQDs assemblies still possess 85% thiol groups. However, after 24 h incubation, the stretching vibration peak of -SH at 2552 cm
1 [27] on cys-CQDs/CTAHo assemblies disappeared,

b

a

Thiol %

120

v (S-H) 2552

90

60 CTAB CTAGd CTAHo

30

cys-CQDs cys-CQDs/CTAHo

0 0.0

0.1

0.4

c (mM) surfactant

0.5

0.6

4000

3000

2000

1000

-1

Wavenumber (cm )

Fig. 5. (a) The thiol percentage of cys-CQDs after self-assembling with CTAB, CTAGd and CTAHo for 2 h. (b) FTIR analysis of cys-CQDs as well as cys-CQDs interacted with CTAHo for 24 h.

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-1

Magnetic moment (emu g )

0.6

0.3

0.0

0 0.4 0.8

-0.3

-0.6 -10000

-5000

0

5000

10000

Magnetic Field (Oe) Fig. 6. SQUID magnetometry of cys-CQDs/CTAHo complexes with 0, 0.4 and 0.8 mmolL
1 CTAHo.

Ag 3d scan

f

indicating a slow oxidization process of thiol caused by CTAHo. In our previous work, it has been proved that the oxidizability of magnetic surfactants CTACe and CTAFe could induce a spontaneous size self-growth phenomenon of Au nanoparticles in the process of emulsion self-assembly [5]. Moreover, [FeCl3Br]
has a stronger oxidizability than [CeCl3Br]
, and could produce magnetic Au nanoparticle with larger size than that of CTACe [5–7]. We believe magnetic CTAHo and CTAGd also have oxidizability, and it accounts for the disappearance of sulfhydryl. It is undeniable that the oxidizability of thiol groups caused by CTAHo is slow and mild, in comparison, H2O2 could degrade sulfhydryl rapidly within 30 min (Fig. S12). The use of lanthanide metal such as Ho(III) and Gd(III) is important for Mag-Surfs due to the highest-effective magnetic moments of metal centers [9], which makes Mag-Surfs exhibit paramagnetic behavior with no saturation magnetization in Figs. 6 and S13. Unexpectedly, after coupling with CTAHo, cys-CQDs present an significantly increased magnetic moment from original 0.054 emug
1 into 0.433 emug
1 (0.4 mmolL
1 CTAHo) and 0.580 emug
1 (0.8 mmolL
1 CTAHo, more than 10 times) (Fig. 6). This result fully

g

~6 eV

1727

621 without Ag with Ag

1696

380

376

372

Binding Energy (eV)

368

364

2000

1500

1000

500

-1

Wavenumber (cm )

Fig. 7. (a) AgNPs synthesized in situ on the magnetic network cluster templates (produced from 10.0 mmolL
1 Ag+). (b) AgNPs self-assembled with network clusters produced from 2.5 mmolL
1 (b), 5.0 mmolL
1 (c) and 10.0 mmolL
1 Ag+ (d). (e) The formation of AuNPs based on magnetic network clusters. (f) XPS of a dried sample of magnetic Ag assemblies. (g) FTIR spectrum of magnetic network clusters with or without coordinated AgNPs.

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illustrated that cooperative self-assembly of cys-CQDs and CTAHo provides a facile noncovalent method for the synergetic enhancement of magnetic susceptibility value for both building blocks. A video of cys-CQDs/CTAHo complexes ‘‘walked” under the attraction of external magnet (1 T) proved a sensitive and rapid responsibility for magneto-assemblies at room temperature (Video S1). The fantastic paramagnetism enhancement could be accounted for by the ‘‘classical models of colloid aggregates” [9]. According to the classical models, electric double layer of colloidal particles is divided into two regions, the so-called Stern and diffuse layer [12]. Herein, although the magnetic [HoCl3Br]
ions in diffuse layer will be released by the charge competition of cys-CQDs, those [HoCl3Br]
ions in the Stern layer remain unchanged, which could bind and magnetize cys-CQDs network clusters. Eastoe et al. demonstrated the hydrophobic counterions could interact with the alkyl chains of Mag-Surfs by hydrophobic interactions and further partition into the micellar core [12,13], indicating the magnetic [HoCl3Br]
in diffuse layer also contribute to the magnetization of network clusters. In addition, a certain coupling effect in the anionic metallic centers could make the network clusters retain long-range ordering instead of being disorderly dispersed in solution [4]. Importantly, a higher holmic content by integrating more [HoCl3Br]
ions in network clusters, has a larger magnetic unit density which brings out a higher magnetic moment of 0.8 mmolL
1 CTAHo than that of 0.4 mmolL
1 CTAHo (Fig. 6). 3.5. Magnetic network clusters as templates to synthesis Ag nanoparticles The presence of cysteine groups [34,35] in magnetic cys-CQDs/ CTAHo network clusters make it feasible to in situ growth of AgNPs without any reducing agents, producing magnetic network materi-

als with abundant AgNPs. After adding 10 mmolL
1 AgNO3 into cys-CQDs/CTAHo (0.4 mmolL
1) solution, the produced AgNPs grew entirely on the cys-CQDs/CTAHo network clusters rather than in bulk solution (Fig. 7a). TEM images showed that the locally grown AgNPs on network clusters have an average size of 18.9 nm (Fig. S14). Furthermore, as XPS result shown in Fig. 7f, the signals at 368 and 374 eV with an interval of 6.0 eV, correspond to the binding energies of Ag 3d5/2 and Ag 3d3/2 [25,36], respectively, confirming the existence of AgNPs on magnetic network clusters. The concentrations of Ag+ possess a manipulative ability to control the quantity of AgNPs on cys-CQDs/CTAHo network templates. Only when AgNO3 reached 10 mmolL
1, there could exist extremely abundant AgNPs locating along the magnetic network clusters (Fig. 7d), otherwise, only sporadic AgNPs could be observed on the surface of network templates (Fig. 7b and c). Three interaction sites including COO
, NH+3 and S
of cysteine could provide potential coordination interaction with AgNPs by producing AgAO, AgAN and AgAS linkages, which promote AgNPs synthesis using cysteine as reducing and capping agent [34–37]. FTIR results in Fig. 7g show that carboxyl groups in cysteine were the main binding sites with AgNPs: the formation of AgAO linkages weakened the CAO bond of cysteine and their corresponding COO
wagging mode [35] at 621 cm
1 disappeared after AgNPs formation; correspondingly, C@O bond of carboxylate groups becomes stronger and the C@O stretching vibration occurs a blue shift from 1696 cm
1 to 1727 cm
1 (Fig. 7g). The oxidization effect of thiol caused by CTAHo as well as the amidation reaction between ACOOH and ANH2 might account for the absence of AgAN and AgAO linkages. Besides coordination effect, the electrostatic interaction of COO
and Ag+ plays a critical role in AgNPs generation along with cys-CQDs/CTAHo network clusters, inhibiting Ag formation in bulk solution. As a contrast, Au nanoparticles (AuNPs) grew

120

2.0

b

1.0

0.5

c 82.6%

90

Captured DNA

1.5

Abs. (a. u.)

97.6%

target-DNA 1.9 μg/mL 3.7 μg/mL 7.3 μg/mL 11.0 μg/mL

75.7%

60

35.9% 30

0.0 220

240

260

280

Wavelength (nm)

300

32 0

0 1.85

3.70

5.55

7.40

9.25

11.10

DNA (μg/mL)

Fig. 8. (a) The construction process of magnetic Ag assemblies-anchor to enrich sequence-specific DNA molecules (TCA GAC TGA TGT TGA) via DNA hybridization. (b) The solution of target-DNA (TCA GAC TGA TGT TGA) as well as the supernatant of magnetic Ag assemblies with various amount of targeted DNA (Ag assemblies-hybrid). (c) The capture efficiency of magnetic CQDs-Ag self-assemblies as a function of target DNA.

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along the edge rather than upon cys-CQDs/CTAHo magnetic network clusters owing to the negatively charged repulsion between + COO
and AuCl
4 ions (Fig. 7e). In conclusion, Ag ions initially
interacted with negatively charged COO in cysteine by electrostatic attraction and then were reduced in situ by carboxyl residues of cysteines, formatting magnetic network clusters combination with AgNPs. 3.6. Magnetic CQDs-Ag self-assemblies for sequence-specific capture of DNA The separation of target DNA with specific nucleic acid sequence is a bottleneck in molecular diagnostics. It has been reported that Ag-S linkage makes it feasible for hydrogels based Ag to enrich and separate thiol-containing biomolecules like cysteine, glutathione and myoglobin [25]. Inspired by this, the above AgNPs assemblies are supposed to do something well for DNA identification. Anchoring thiol-modified DNA (SH-AGT CTG ACT ACA ACT) onto magnetic cys-CQDs/CTAHo-Ag assemblies by virtue of Ag-S bonds was defined as Ag assemblies-anchor. As collected in Fig. 8a, the elaborate Ag assemblies-anchor served as a magnetic selector to facilitate capturing sequence-specific DNA molecules (TCA GAC TGA TGT TGA) via complementary base pairing principle. An extra magnetic field at the bottom of the mixed solution, magnetic Ag-DNA complexes could be attracted migrating towards the magnet, thus, the absorbance in UV spectrum of the supernatant could provide a quantitative analysis of DNA capture efficiency. SEM element mapping images (Fig. S15) clearly showed the presence of P element from DNA thiol-containing molecules and Ag element of cys-CQDs/CTAHo-Ag assemblies, proving the successful anchor of SH-AGT CTG ACT ACA ACT onto magnetic Ag assemblies. The maximum absorbance of target DNA is 260.5 nm in UV–vis spectra, after complementary base pairing with Ag assembliesanchor, a red shift of UV absorbance from 260.5 to 262.5 nm could be discovered in Fig. 8b. After adding 1.9, 3.7 and 7.3 lgmL
1 target DNA, magnetic Ag assemblies-anchor exhibited high efficiency of 97.6%, 82.6% and 75.7% in capturing target TCA GAC TGA TGT TGA (Fig. 8b and c). However, magnetic Ag assemblies-anchor had a limited sequence-specific capture of DNA at high concentration of target DNA. When adding 11.0 lgmL
1 target DNA, the capture efficiency is reduced to 35.9% (Fig. 8c). The steric effect caused by DNA molecules and the non-specificity absorption of DNA molecules by cys-CQDs/CTAHo network clusters might account for the low DNA capture efficiency at a concentration of 11.0 lgmL
1 target DNA. The anchor of thiol-containing DNA and the capture of target DNA had no much effect on the morphology of cys-CQDs/ CTAHo-Ag assemblies (Figs. S16 and S17). 4. Conclusions In this work, fluorescence carbon quantum dots with cysteine as ligands (cys-CQDs) were exploited as building block to ionic self-assembly with Mag-Surfs, generating magnetic network structures in a facile non-covalent way. Two pivotal factors affect the aggregation way of the network clusters: (i) counterions of Br
, [GdCl3Br]
and [HoCl3Br]
in surfactants could manipulate the hierarchical morphology of cys-CQDs assemblies including globular aggregates or network clusters; (ii) the concentration of CTAHo controlled cys-CQDs assemblies occurring a phase transformation of precipitate, aggregate formation or aggregate redissolution. Self-assembly of cys-CQDs and CTAHo would bring out some distinctive collective properties and functions, such as efficient regulating the magnetic moment of CTAHo, meanwhile, fluorescent property of cys-CQDs could serve as an effective indicator element to dissect the collective effect in self-assembly process.

Integration of Mag-Surfs into assembled complexes via selfassembly makes it possible to enrich and separate DNA molecules by electrostatic interaction under an applied magnet [6,7,10]. However, how to capture sequence-specific DNA is still a challenge. In this work, the presence of cysteine ligands makes it feasible to construct the magnetic network materials with incorporation of AgNPs upon cys-CQDs/Mag-Surfs network templates, followed an anchoring of thiol-modified DNA on AgNPs surface through Ag-S bonds. The whole self-assemblies served as a magnetic selector to facilitate enriching sequence-specific DNA molecules by DNA base pairing model. This novel approach is of general interest to researchers designing magneto-functional self-assembly materials. Our future work is focus on designing the magnetic assemblies with recovery and recycling capability. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 21420102006 & 21773144), the Natural Science Foundation of Shandong Province (ZR2018ZA0547) and the Post-Doctoral Innovation Talent Support Scheme (Grant No. BX20180181). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2018.12.037. References [1] C.F.J. Faul, M. Antonietti, Adv. Mater. 15 (2003) 673–683. [2] C.F. Faul, Acc. Chem. Res. 47 (2014) 3428–3438. [3] L. Xu, W. Zhao, J. Hao, Y. Zhao, D. Wang, H. Xu, J. Lu, Langmuir 32 (2016) 10226–10234. [4] W. Zhao, S. Dong, J. Hao, Langmuir 31 (2015) 11243–11248. [5] L. Wang, Y. Wang, S. Dong, Y. Deng, J. Hao, ACS Appl. Mater. Interfaces 10 (2018) 5348–5357. [6] L. Xu, L. Feng, S. Dong, J. Hao, Chem. Commun. 51 (2015) 9257–9260. [7] L. Xu, S. Dong, J. Hao, J. Cui, H. Hoffmann, Langmuir 33 (2017) 3047–3055. [8] T.M. McCoy, P. Brown, J. Eastoe, R.F. Tabor, ACS Appl. Mater. Interfaces 7 (2015) 2124–2133. [9] L. Wang, S. Dong, J. Hao, Curr. Opin. Colloid In. 35 (2018) 81–90. [10] L. Wang, Y. Wang, J. Hao, S. Dong, Biomacromolecules 18 (2017) 1029–1038. [11] L. Xu, Y. Wang, G. Wei, L. Feng, S. Dong, J. Hao, Biomacromolecules 16 (2015) 4004–4012. [12] P. Brown, A. Bushmelev, C.P. Butts, J. Cheng, J. Eastoe, I. Grillo, R. Heenan, A.M. Schmidt, Angew. Chem. Int. Ed. 51 (2012) 2464–2466. [13] P. Brown, A. Bushmelev, C.P. Butts, J.C. Eloi, I. Grillo, P.J. Baker, A.M. Schmidt, J. Eastoe, Langmuir 29 (2013) 3246–3251. [14] T. Bollhorst, K. Rezwan, M. Maas, Chem. Soc. Rev. 46 (2017) 2091–2126. [15] S.Y. Lim, W. Shen, Z. Gao, Chem. Soc. Rev. 44 (2015) 362–381. [16] S. Singh, A. Mishra, R. Kumari, K.K. Sinha, M.K. Singh, P. Das, Carbon 114 (2017) 169–176. [17] Y. Dong, R. Wang, G. Li, C. Chen, Y. Chi, G. Chen, Anal. Chem. 84 (2012) 6220– 6224. [18] P. Shen, Y. Xia, Anal. Chem. 86 (2014) 5323–5329. [19] X. Sun, Q. Zhang, K. Yin, S. Zhou, H. Li, Chem. Commun. 52 (2016) 12024– 12027. [20] L. Wang, Y. Wang, X. Sun, G. Zhang, S. Dong, J. Hao, Chem. Eur. J. 23 (2017) 10413–10422. [21] Q. Jia, J. Ge, W. Liu, L. Guo, X. Zheng, S. Chen, M. Chen, S. Liu, L. Zhang, M. Wang, H. Zhang, P. Wang, Adv. Healthcare Mater. 6 (2017) 1601419. [22] F. Li, Y. Li, X. Yang, X. Han, Y. Jiao, T. Wei, D. Yang, H. Xu, G. Nie, Angew. Chem. Int. Ed. 57 (2018) 2377–2382. [23] R. Li, P. Gao, H. Zhang, L. Zheng, C. Li, J. Wang, Y. Li, F. Liu, N. Li, C. Huang, Chem. Sci. 8 (2017) 6829–6835. [24] G. Li, Y. Wang, L. Wang, A. Song, J. Hao, Langmuir 32 (2016) 12100–12109. [25] Y. Hu, D. Xie, Y. Wu, N. Lin, A. Song, J. Hao, Chem. Eur. J. 23 (2017) 15721– 15728. [26] Y. Hu, W. Xu, G. Li, L. Xu, A. Song, J. Hao, Langmuir 31 (2015) 8599–8605. [27] N. Mahanta, S. Valiyaveettil, Nanoscale 3 (2011) 4625–4631. [28] L. Xu, L. Feng, R. Dong, J. Hao, S. Dong, Biomacromolecules 14 (2013) 2781– 2789. [29] T. Asakawa, H. Kitano, A. Ohta, S. Miyagishi, J. Colloid Interf. Sci. 242 (2001) 284–287. [30] L. Xu, L. Feng, J. Hao, S. Dong, Colloids Surf. B 134 (2015) 105–112.

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