Instantaneous Magnetically Assembled and Hydrophilic Photonic

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Instantaneous Magnetically Assembled and Hydrophilic Photonic Crystals with Controlled Diffraction Colors Shuge Tang, Caiqin Wang, Nan Liu, Ya Li, Ping Han, and Quanjun Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05967 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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The Journal of Physical Chemistry

Instantaneous Magnetically Assembled and Hydrophilic Photonic Crystals with Controlled Diffraction Colors

Shuge Tang,†,

‡, §

Caiqin Wang,‡,

§, ∥

Nan Liu,*,

†, ‡, §, ∥

Ya Li,

‡, §, ∥

Ping Han,

†

Quanjun Lu†

†

College of Public Health, Zhengzhou University, Zhengzhou 540001, P. R. China

‡

School of Public Health, Guangzhou Medical University, Guangzhou 511436, P. R.

China §

Institute of Environmental & Operational Medicine, Academy of Military Medicine,

Chinese Academy of Military Sciences, Tianjin 300050, P. R. China ∥

School of Public Health, Lanzhou University, Lanzhou 73000, P. R. China

*Corresponding author at: E-mail addresses: [email protected] (Nan Liu) Supporting Information

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ABSTRACT: Time-consuming assembling process of the traditional photonic crystals (PCs) and non-water-dispersibility of the reported magnetic responsive PCs (MRPCs) have greatly limited the application especially in the biotechnological fields. Herein, the hydrophilic and size-controllable Fe3O4@ poly (4-styrenesulfonic acid-co-maleic acid (PSSMA) @SiO2 MRPCs were fabricated by orderly assembling of the core-shell colloidal nanocrystal clusters via a two-step facile synthesis approach. Due to the rich carboxyl and hydroxyl groups of PSSMA, the obtained MRPCs have excellent properties of hydrophilicity, high surface charge which presented magnetically tunable photonic structural colors and rapidly reflection signal in aqueous solution under external magnetic field within 1 s. The diffraction color of the MRPCs in the entire visible range could be tuned by adjusting the magnetic strength or the nanoparticle size, which bring a clearly change of the structure color from brilliant red to modena by naked eyes. Thus, the magnetically sensitive MRPCs with low-cost, tunable size and fast optical signal response indicate a promising application in optical system, biosensors and biomedical imaging.


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1. INTRODUCTION Magnetic induction is an ultra-fast method to assemble photonic crystals (PCs). Over the past decade, with the characteristics of easier preparation, higher saturation intensity, facilitated magnetization and colorful responsiveness under external magnetic field (EMF), magnetic responsive PCs (MRPCs) have gained considerable attentions due to its rapid and reversible response to EMF. It exhibits promising prospects in the fields of anti-counterfeiting,1-2 information encryption technologies, 3-5

magnetometer 6 and chemsensor,7-9 etc. The structure of MRPCs is always composed of core and shells. Commonly, the

typical core is Fe3O4, and shells are commonly the high polymers, such as polyacrylic acid (PAA),10-11

polyvinylpyrrolidone (PVP),12

oleic acid (OA),2

poly

(4-styrenesulfonic acid-co-maleic acid) (PSSMA)13 and carbon (C)14-15 coating on the core surface as the stabilizer which providing the repulsion force during the building-up process under EMF. What’s more, another kind of silica (SiO2) layer is coated on the surface of above-mentioned Fe3O4 CNCs to prevent the detachment of polyelectrolytes. Recent studies suggested that the magnetic colloidal nanocrystal clusters (MCNCs) could be assembled into ordered chain-like photonic structures along the direction of the EMF, because of the balance of magnetic dipole-dipole interaction force and steric-repulsion (or electrostatic force) among the magnetite particles in ferrofluid.16-18 Until now, several methods have been developed for the synthesis of MRPCs. But the drawbacks such as the complicated and severe synthesis



conditions and lower yield (only 100 mg production was obtained in one synthetic experiment) hinder its wide application. One of the traditional methods, such as emulsion polymerization method, i.e., PS was coated on the surface of co-precipitated iron oxide and the response of the magnetic colloid under EMF usually require several minutes even 60 min for its low concentration of magnetite.19 It was also reported a high-temperature hydrolysis method to prepare Fe3O4 CNCs with different sizes from 30 to 180 nm by increasing the amount of sodium hydroxide (NaOH).20 Beside the two methods, a “one-pot” hydrothermal synthesis is emerging for relatively simple operation and stable repeatability.16, 21-23 The facile procedure only includes reactants mixing into a homogeneous solution, transferring into Teflon-lined stainless-steel autoclave and maintaining at the reaction temperature which makes it possible to repetition and batch production. Guan firstly reported a fabrication of MRPCs with a PVP polymer shell using long range repulsive force.24 The color tunable ranges in organic solvent was up to 280 nm. However, the as-obtained MCNCs haven’t been capable of assembling to PCs in aqueous solution under EMF. More recently, Chen reported a hydrothermal syntheses method of the magnetocaloric stimuli-responsive Fe3O4@C, but the reaction time was as long as 72 h.25 All the drawbacks above constrained the application of MRPCs in biotechnological and bio-medicine field. In the present work, to the best of our knowledge, the instantaneous magnetically assembled and hydrophilic MRPCs we reported characterized as water solubility, monodisperse superparamagnetic property and size-controlling by using of PSSMA as the surfactant via “one-pot” hydrothermal synthesis. Here are several advantages of our synthesis strategy. Firstly, compared with the previously reported inverse opal PCs26 and the opal closest-packing PCs27-28 in our group, the fabrication process of


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the MRPCs is easy-to-operate and cost-efficient; and the reaction time is further shortened to 9 h and the assembling is instantaneous (less than 1s), which greatly improve the efficiency of self-assembled PCs in laboratories; moreover, the size of Fe3O4 CNCs shows a homogeneous morphology and can be simply adjusted by the addition amount of water, which lays a groundwork for the PCs diffraction color shift under EMF. Finally, PSSMA offers a high charge on the surface of the Fe3O4 core for its long-range steric repulsion and high dispersibility in water, which leading to strong and rapid PCs response of Fe3O4@PSSMA@SiO2 by controlling the applied EMF intensity or the size of MRPCs in the entire visible spectrum. There are significant applications of the Fe3O4 CNCs MRPCs in drug delivery,29-30 bio-enrichment31-32 and fluorescent amplification,33 etc. The photonic bandgap and color of MRPCs could be changed by external environment such as temperature, humidity, pressure, salt ion concentration and small molecule compound, and those changes could produce visible color changes. In the future, the MRPCs will be combined with recognition molecules such as aptamer, antibody, etc. to assemble detection sensors for naked-eye chemical pollutant detection.

2. EXPERIMENTAL 2.1 Chemicals and Materials. Anhydrous ferric chloride (FeCl3≥97.0% and FeC12≈ 2.0%), tetraethylorthosilcate (TEOS, 98%) and PSSMA (Mw≈20 000, SS: MA=1:1) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Ethylene glycol (EG) is analytical grade and was pre-dried by enough molecular sieve (type 3), other common reagents and solvents were of analytical grade and directly used without further purification. Water was deionized and ultrafiltered using a Milli-Q apparatus



(Millipore, Bedford, MA), which had a minimum resistivity of 18 MΩ·cm. 2.2 Apparatus. Zeta potentials and the size distribution of the Fe3O4 CNCs were collected by using a zeta potential analyzer (3000 HS, Malvern, UK). The structures and properties of the Fe3O4 CNCs were characterized using X-ray diffraction (XRD, D/MAX-250, Rigaku, Japan). The morphology of the particles was characterized by a transmission electron microscopy (TEM, H 7650, Hitach, Japan) at a voltage of 100 kV and scanning electron microscopy (SEM, 200 FEG FEI, Quanta, USA）in high vacuum mode at 10 kV accelerating voltage. The magnetic properties of the samples were investigated by physical property measurement system (PPMS-9, Quantum Design, USA) with applied EMF between -10k and 10k Oe at 300 k. X-ray photoelectron spectroscopy (XPS) spectrum was captured by ESCALAB 250xi (Thermo-Fisher, USA). Fourier transform-infrared spectroscopy (FT-IR) spectra were measured on a FT-IR spectrometer (Nexus 870, Nicolet, USA), with KBr pellets as the sample matrix. The dark-field images were captured by an Eclipse Ci-S/Ci-L (Nikon, Japan). The digital photographs and video were captured by digital camera D7100 (Nikon, Thailand). The reflection spectrums of the PCs were recorded by a spectrometer (Maya 2000, Ocean Optics, USA). The intensity of the EMF was determined by Gaussmeter (HT 20, Shanghai Hengtong, China). Cake-like NdFeB (N 50) magnet with a center field strength of 2500Gs was employed used behind the vial as EMF for the color tracking of the prepared MRPCs. A laboratory self-made electromagnet prepared by our laboratory was employed as the EMF (Figure S1) in the measurement of reflection spectra. 2.3 Synthesis of MCNCs Core. Fe3O4@PSSMA CNCs were synthesized by a “one-pot” hydrothermal method. In a typical procedure, 12 mmol FeCl3, 110 mmol


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NaAc, 3g PSSMA and a series of water volumes were firstly added into the pre-dried EG (120 mL). The solution was vigorously stirred for 30 min; the homogeneous yellow solution was prepared. Afterwards, 45 mmol solid NaOH were added into the solution, and the color of the reaction solution was changed from light brown to black immediately. Finally, the as-obtained solution was transferred into a 200 mL Teflon-lined stainless-steel autoclave. Then the autoclave was sealed and kept at 190 °C in oven for 9 h. After it was naturally cooled down to room temperature, the yielded black magnetic particles were collected by a magnet and washed with a mixture of ethanol and water for several times, finally dispersed in water with the volume of 45 mL. 2.4 Synthesis of MCNCs Core-shell Nanostructure. The modified StÖber method was applied for SiO2 coating;22 initially, 6 mL Fe3O4@PSSMA was mixed with 80 mL ethanol and 4 mL ammonium hydroxide, the mixture was sonicated for 5 min before transferring into a 250 mL three-neck flask under 600 rpm stirring and 50 °C water bath. After vigorous stirring for 10 min, 200 µL TEOS was added into the above mixture for every 20 min, the thickness of SiO2 shell was determined by the addition of TEOS. After continuous stirring for 1 h, the Fe3O4@PSSMA@SiO2 CNCs were isolated by a magnet and rinsed with ethanol and water for several times, and then dried in a vacuum oven at 60 °C overnight. After that,10 mg Fe3O4@PSSMA@SiO2 CNCs were added into 2mL aqueous solution to form a homogeneous solution, and then, MRPCs with different colors were acquired under EMF.

3 RESULTS AND DISCUSSION

3.1

Verification

of

formation

of MRPCs

under

EMF

The

prepared

Fe3O4@PSSMA@SiO2 could form MRPCs structure under EMF in aqueous solution.



Figure 1 a showed that a rainbow-like color effect can be successfully achieved within 1 s when the Fe3O4@PSSMA@SiO2 aqueous solution placed in front of the NdFeB magnet with circular disk-like shape. The reason for the appearance of the rainbow-like color was that the edge in the permanent magnet behaved divergent directions. It could be observed that the rainbow-like diffraction colors were red to blue (blue-shift) from center to the edge of the magnet. Simultaneously, the EMF intensity distribution simulated by COMSOL Multiphysics 5.3 (Figure 1 b) was gradually increased from the center to the edge of the magnet and was in accordance with the responsive diffraction colors. We also found that the bright diffraction color rapidly vanished back to brown (Inset of Figure 1 a) rapidly after the EMF removal.

Figure 1. (a) Photo of the rainbow-like colors effect displayed with a NdFeB magnet with circular disk shape. Inset: A bottle of Fe3O4@PSSMA@SiO2 CNCs without magnet placed (5 mg/mL in aqueous solution and the size is 220 nm). (b) The EMF strength distribution simulated by COMSOL Multiphysics 5.3.

3.2 Customization of the controllable MRPCs size

The difference between the building blocks of MRPCs and general MCNCs is that MRPCs will not be attracted by EMF, but rapidly assemble to bright PCs structure which were visible by naked eyes. The reason is that MRPCs are in possession of uniform particle size and high surface charge. In the process of the formation of


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Fe3O4@PSSMA, anhydrous FeCl3, NaAc and EG were acted as iron source, the supplier for an alkalescent reaction atmosphere and the solvent for higher viscosity. PSMMA was absorbed onto the surface of the clusters to form a steric-hindrance layer. Owing to the rich carboxyl and hydroxyl groups of PSSMA coated on the surface of Fe3O4, the MCNCs behaved excellent hydrophilicity and well-monodispersed property, and the surface charge was up to -64 mV. The sizes of MRPCs for display photonic colors under EMF were bespoke by the addition amount of PSSMA or TEOS in both two steps. The diameter of the core of Fe3O4 CNCs was depended on the addition volumes of water (Fig. 2 a-d) while keeping all the other parameters constant. The phenomenon could be interpreted by the following two reasons. Firstly, OH- was ionized by water molecules serves as the reaction center in the synthesis process of the Fe3O4 nanocrystals. In the reaction system, with the invariant amount of iron source (FeCl3), more volume of water resulted in more OH-. This means that the additional water was negatively correlated with the sizes of Fe3O4 CNCs. Secondly, the ionized OH- in the reaction system might enhance the surfactant’s electrostatic repulsion on the surface of Fe3O4, which making the primary nanocrystals aggregation into minor particles.



Figure 2. Size distribution and TEM images of MRPCs by using different amounts of water, i.e., 600, 450, 300 and 225 µL for a-d, respectively. Insets are the TEM images of Fe3O4@PSSMA (a-d). Fe3O4@PSSMA@SiO2 CNCs synthesized by using different amounts of TEOS, i.e., 200, 400, 600 and 800 µL, respectively (e-h); (i) The relationship between the thickness of the SiO2 shells of MRPCs and addition volume of TEOS; The hysteresis curves (j) of the above Fe3O4@PSSMA and Fe3O4@PSSMA@SiO2 with different thicknesses of silicon shell (e-h); (All scale bars are 100 nm in the insets).

In the second preparation step for SiO2 coating by using the ø=110 nm Fe3O4@PSSMA CNCs as the core, different sizes of Fe3O4@PSSMA@SiO2 CNCs could be obtained by a modified StÖber method. It could be observed that the PSSMA coated Fe3O4 (insets of Figure 2 a-d) and Fe3O4@PSSMA@SiO2 CNCs (Figure 2 b-e) were presented almost reasonable uniformity in size and rough surface. As a result, the diameter of SiO2 cladding layer of the synthesized MRPCs was growing with the increasing addition volume of TEOS illustrated in Figure 2 (e-h). The shell is dense and fully covers the metallic core, which is in favor of maintaining the chemical stability. Catalyzed by ammonia, TEOS was decomposed onto the Fe3O4 surface (ø=110 nm, Figure 2 a) with the aid of PSSMA. It also suggested that the diameters


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and the shells of the prepared MRPCs were positively correlated with the addition volumes of TEOS (Figure 2 i). The SiO2 shell was essential in this system because it could protect the Fe3O4@PSSMA core and the increased diameters of the build-up MRPCs could be tuned in tailor-made size in a broad visible spectrum. It also clearly revealed that the CNCs of Fe3O4@PSSMA and Fe3O4@PSSMA@SiO2 proved the typical superparamagnetism and have no remanence and coercivity in Figure 2 (j). All those properties were attributed to the composition of small primary nanocrystals. The saturation magnetization was declined with the increasing thickness of the SiO2 layer which attributing to the non-magnetic shell. 3.3 Optimization of experimental conditions In order to obtain optimum PCs

properties, the reaction conditions, such as the amount of PSSMA (3 g) and NaOH (45 mmol) were optimized for the best surface potential of the CNCs and crystallite size. A crucial prerequisite to the response under EMF of MRPCs is the sufficient negative charges on the surface of the MCNCs. After PSSMA coating from 1.2 to 3 g, the surface charge of Fe3O4@PSSMA was changed from -17 to -64 mV which beyond the cut-off point (-30 mV) for electrostatic stabilization of the colloidal suspensions (Figure 3 a and S 2), so the MCNCs enabled to respond sensitively and reversibly under EMF and exhibited bright diffraction colors. In virtue of the constant weight of iron source, PSSMA on the surface of Fe3O4 CNCs might be saturated and the zeta potential would not be raised yet. A high zeta potential value could keep stable even after 4 months storage in water (Figure S 3) in the follow-up experiment. The strongest peak of 311 facet was opted for analyzing the mean sizes of the Fe3O4 primary crystals in the Fe3O4@PSSMA CNCs, according to Debye-Scherrer equation, the crystallite size was calculated as 12.8, 18.7, 25.8 and 31.7 nm, respectively.34-35 It suggested that the width of the half peak was positively correlation with the amount



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of NaOH in the XRD pattern in Figure 3 (b) which exhibiting similar diffraction spectrum. Apparently, more amount of NaOH facilitated the alkalinity of the reaction system and accelerates the deoxidization of Fe3+ to Fe2+ which resulted in a larger size of crystallite.36

Figure 3. (a) Zeta potentials of Fe3O4@PSSMA CNCs with different addition amounts of PSSMA. (b) XRD patterns of the prepared Fe3O4@PSSMA CNCs by addition of different amounts of NaOH (75, 60, 45 and 30 mmol).

The experiments above demonstrated the effects of synthetic conditions on the properties of Fe3O4 CNCs. Under the following conditions with 3 g PSSMA, 45 mmol NaOH and given amount of H2O and SiO2, the MRPCs with controlled diffraction colors can be fabricated.

Table 1. Comparison of four synthesis methods of MRPCs Type of shell

Carbon

Polyvinylpyrrolidone

Okadaic acid

PSSMA

Zeta potential (mV)

-43

-17

-35

-64

Sodium dodecyl sulfate solution

Water

120

200 9 This work

Dimethyl formamide, Dispersing media

Ethanol

ethanol and dichloromethane

Diffraction wavelength range under EMF (nm)

252

280

Synthesis time (h)

72

10

18

Reference

37-38

12, 24

2, 39


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Compared with the general methods of packing PCs, monodisperse nanomaterial such as SiO2, MMA or PS was used to construct inverse opal PCs or the opal closest-packing PCs, the assembling time was up to 3 days28 . The self-assembling cycle of MRPCs was reduced to 1s, which improved the self-assemble efficiency at a large extent. Typical methods for the synthesis of MRPCs are listed in Table 1. Compared with the previous works listed in Table 1, the surface zeta potential is significantly improved, the reaction time (9 h) is much shorted than that of those works (10-72 h); the diffraction and the wavelength range in aqueous solution under EMF is moderate. The most interesting aspects of our prepared MRPCs are that the fabrication method is facile, the prepared Fe3O4@PSSMA@SiO2 CNCs can be stably monodispersed in water with a relatively long period without precipitation and turbidity and rapidly assemble to PCs under EMF. Our prepared MRPCs can achieve a color tunable range (200 nm) as wide as almost in the whole visible spectrum in a moderate aqueous solution which is much more suitable for bio-molecular action. It indicates that the optical response time of the as-constructed MRPCs is less than 1 s, as fast as that of the PAA-capped Fe3O4 CNC-particle-based40 and steric repulsion-based24 MRPCs.



Figure 4. (a) FTIR spectra and (b) XRD of Fe3O4@PSSMA (Black line) and Fe3O4@PSSMA@SiO2 CNCs (Red line).

As magnetic nanoparticles, the Fe3O4@PSSMA and Fe3O4@PSSMA@SiO2 shown typical properties. The monodispersed Fe3O4@PSSMA MRPCs were investigated by TEM (insets of Figure 2 a-d) and SEM (Figure S 4). As demonstrated in Figure 4 (a), the strong stretching vibration peak at 587 cm-1 indicated the existence of Fe3O4 but not Fe2O3 (575 cm-1) in the sample, and the peaks in 1655 and 3420 cm-1 arising from the C-O stretching peak confirmed the hydrophilic carboxyl on the surface of Fe3O4 which coming from PSSMA coating on it.41 Another red line demonstrated the appearance of 1100 cm-1 and weaker vibration of Fe-O which presented the successful coating of SiO2. XRD measurement was employed to characterize the structure of the obtained Fe3O4@PSSMA and Fe3O4@PSSMA@SiO2. There were eight characteristic diffraction peaks (labeled in Figure 4 b) of the as-constructed Fe3O4@PSSMA, which was in agreement with that of the standard Fe3O4 both in the reflection peak position and the relative intensities.42-43 Due to the natural amorphous property of SiO2, there was no lattice fringe but a broader diffraction peak at 15-30° in the XRD pattern of Fe3O4@PSSMA@SiO2. 3.4 Responsiveness of the prepared MRPCs under EMF What the MRPCs

differed from the ordinary magnetic particles was the ability of rapid self-assemble to PCs under EMF. It exhibited the above-mentioned chain-like structure under the vertical EMF in a laboratory self-prepared dark-field microscope and exhibits densely light spots in the field of vision in Figure 5 (a). Otherwise, the MCNCs behaved a “meteor”-like alignment along the direction of EMF lines if the EMF direction declined with an angle within 90 º (Figure 5 b). To reveal the behavior of the magnetic particles under the EMF, a copper grid was prepared by immersing it inclined into the magnetic assembled MCNCs aqueous solution. It displayed an ordered chain-like


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nanostructure (Figure 5 c) and the MCNCs were assembled along with the EMF direction.

Figure 5. (a) Dark-field optical image of CNCs assembled with a vertical and (b) along a horizontal EMF, (c) TEM image of typical nanochains induced by EMF.

The interaction forces between adjacent particles were the significant factors in the build-up system of MRPCs including the long-range steric-repulsion force and the dipole-dipole interaction between magnetic particles. The former one was provided by the polymer-PSSMA capped on the Fe3O4. Another force can be explained by F=3µ(1-3cos2θ)/d4, in which µ is the magnetic moments, θ is the angle of the adjacent

particles and the direction of the EMF, d represents the distance between the center of the two neighbored particles.44 Thus, the dipole-dipole interaction between the two MCNCs was attractive along the direction of the field and repulsive perpendicular to the direction of the field. In the relative stable system of MRPCs, the formation of the chain-like structure was the result of the balance of the dipole-dipole attraction and the steric-repulsion force. Furthermore, these forces above-mentioned were the indispensable factors to hinder the aggregation between the chains. According to Bragg law,45 λ=2 nd sin θ, where λ is the peak wavelength of incident light, i.e. the band-gap; n the effective refractive index of the solution, d the lattice spacing representing the center-to-center distance between two adjacent CNCs, and θ the glancing angle between the incident light and diffraction crystal plane. In this method, the water’s refractive index, n is 1.33; when the incident angle of the



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EMF is vertical to the sample, θ is 90°. Thus, the diffraction color of the dynamic photonic chains could be tuned through controlling d46

through changing the

strength of EMF or the size of the MCNCs.42, 47 The prepared MRPCs with controlled sizes and magnetic properties were synthesized by a facile one-pot hydrothermal method and a two-step bonding procedure. The properties of uniform size, superparamagnetism and long-range steric repulsion were provided by the surfactant making it possible to respond under EMF rapidly and reversibly in aqueous solution. As explained in Figure 6 a, the vial filled with 2 mL 5 mg/mL as-prepared Fe3O4@PSSMA@SiO2 (ø=160 nm) colloidal aqueous solution was placed in front of an NdFeB permanent magnet (with a magnetic intensity of 2500 Gs in the center). Once the NdFeB magnet was moved to the vial from far to near slowly, the color of colloidal solution also gradually varied from red to modena (Figure 6 a) and the corresponding reflection spectra peaks blue-shift almost 200 nm (Figure 6 b), which could be obviously observed by naked eyes (Video S 1). It was as similar as that under a laboratory self-made electromagnet (Figure S 1). When the EMF intensity was increased by controlling the magnet-sample distance (Figure S 6), the distance between adjacent particles (d) was closer with it, and the λ was also decreased (Figure 6 b).


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Figure 6. (a) Photographs of rainbow-like colors of the Fe3O4@PSSMA@SiO2 CNCs aqueous solution (ø=160 nm, 2 mL, 5 mg/mL) driven by EMF; The above schematic illustrations are the diffraction color change of the assembled CNCs as reduce the EMF intensity from left to right. (b) Reflection spectra of the same sample in varied strength of EMF.

Moreover, the diffraction color of the as-obtained MRPCs in aqueous solution could also be turned by the size of MCNCs under EMF. The laboratory self-made NdFeB magnet array (Figure 7 a) was set up with the inter-attraction of the several bar-shape NdFeB magnets (center-magnet strength is 2500 Gs). The MRPCs displayed color stripes as the magnet below. Figure 7 c (1-4) and d revealed the red shift of colloidal colors and corresponding spectrum peak under a lab-made magnet with the increased size from 150 to 230 nm.



Figure 7. (a) the lab-made magnet; (b) Comparison of four Fe3O4@PSSMA@SiO2 CNCs sizes (1-4); (c) Diffraction color change of 1-4 under the same magnet; (d) The corresponding reflection spectra of sample 1-4.

4 CONCLUSIONS In summary, we have reported an instantaneous magnetically assembled and hydrophilic PCs with controlled diffraction colors by a “one-pot” method with characteristics of uniform morphology and abundant surface charge. The synthesized method was facile with high yield and the size of MCNCs was easy to be controlled by adjusting the volume of water. The prepared MCNCs of Fe3O4@PSSMA and Fe3O4@PSSMA@SiO2 could be highly dispersed in aqueous solution and achieve diffraction colors change in whole visible spectrum with the alteration of magnetic strength or the nanoparticle size. These unique characteristics of our prepared MRPCs as instantaneous response to EMF and well stable in aqueous solution will greatly broader the biological and medical applications of PCs, such as biosensors, bio-imaging and color display areas, etc.

■ ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: ……. Photo of magnetic device, zeta potential, SEM image, XPS spectrum, XRF results and stability of the sample, magnetic field distribution, and colors variation along the increase of magnetic field strength (PDF).


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■ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. Tel/fax: +86-20-37103611. ORCID

Nan Liu: 0000-0002-8895-3169 Author Contributions

The manuscript was written and discussed through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS We acknowledge the support from the National 863 Young Scientist Program of China (Grant No. 2015AA020940), the National Natural Science Foundation of China (Grant Nos. 81273078 and 81472941) and the Natural Science Foundation of Tianjin City (Grant No. 16JCZDJC39500).

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TOC Graphic



Figure 1. (a) Photo of the rainbow-like colors effect displayed with a NdFeB magnet with circular disk shape. Inset: A bottle of Fe3O4@PSSMA@SiO2 CNCs without magnet placed (5 mg/mL in aqueous solution and the size is 220 nm). (b) The EMF strength distribution simulated by COMSOL Multiphysics 5.3. 61x31mm (300 x 300 DPI)


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Figure 2. Size distribution and TEM result of Fe3O4@PSSMA by using different amounts of water: 600 µL for (a); 450 µL for (b); 300 µL for (c); 225 µL for (d). Insets are the TEM images of Fe3O4@PSSMA synthesized; TEM images of Fe3O4@PSSMA (a) and Fe3O4@PSSMA@SiO2 CNCs synthesized by using different amounts of TEOS: (e) 200 µL; (f) 400 µL; (g) 600 µL; (h) 800 µL; (i) The hysteresis curves of the above Fe3O4@PSSMA (a) and Fe3O4@PSSMA@SiO2 with different thicknesses of silicon shell (e-h); (j) The thickness of the shells of Fe3O4@PSSMA@SiO2 of different addition volume of TEOS (e-h). The correlate of the diameter and the shell of the prepared Fe3O4@PSSMA@SiO2 with the addition volume of TEOS. (All scale bars are 100 nm in the insets.) 55x39mm (300 x 300 DPI)



Figure 3. (a) Zeta potentials of Fe3O4@PSSMA CNCs with different addition amounts of PSSMA. (b) XRD patterns of the prepared Fe3O4@PSSMA CNCs by addition of different amounts of NaOH (75, 60, 45 and 30 mmol). 48x18mm (300 x 300 DPI)


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Figure 4. (a) FTIR spectra and (b) XRD of Fe3O4@PSSMA (Black line) and Fe3O4@PSSMA@SiO2 CNCs (Red line). 48x19mm (300 x 300 DPI)



Figure 5. (a) Dark-field optical image of CNCs assembled with a vertical and (b) along a horizontal EMF, (c) TEM image of typical nanochains induced by EMF. 32x10mm (300 x 300 DPI)


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Figure 6. (a) Photographs of rainbow-like colors of the Fe3O4@PSSMA@SiO2 CNCs aqueous solution (ø=160 nm, 2 mL, 5 mg/mL) driven by EMF; The above schematic illustrations are the diffraction color change of the assembled CNCs as reduce the EMF intensity from left to right. (b) Reflection spectra of the same sample in varied strength of EMF. 90x115mm (300 x 300 DPI)



Figure 7. (a) the lab-made magnet; (b) Comparison of four Fe3O4@PSSMA@SiO2 CNCs sizes (1-4); (c) Diffraction color change of 1-4 under the same magnet; (d) The corresponding reflection spectra of sample 1-4. 36x13mm (300 x 300 DPI)


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TOC 28x9mm (300 x 300 DPI)


Instantaneous Magnetically Assembled and Hydrophilic Photonic

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