Responsive Hydrogel-based Photonic Nanochains for

Subscriber access provided by READING UNIV

Communication

Responsive Hydrogel-based Photonic Nanochains for Microenvironment Sensing and Imaging in Real Time and High Resolution Wei Luo, Qian Cui, Kai Fang, Ke Chen, Huiru Ma, and Jianguo Guan Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04218 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 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

Nano Letters

Responsive Hydrogel-based Photonic Nanochains for Microenvironment Sensing and Imaging in Real Time and High Resolution Wei Luo1,3, Qian Cui1, Kai Fang1, Ke Chen2, Huiru Ma2* and Jianguo Guan1* 1. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, International School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, 430070, China. 2. School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, Wuhan, 430070, China 3. Department of Materials Science and Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois, 61801, United States

Abstract: Microenvironment sensing and imaging are of importance in micro-scale zones like microreactors, microfluidic systems and biological cells. But they are so far implemented only based on chemical colors from dyes or quantum dots, which suffered either from photobleaching, quenching or photoblinking behaviors, or from limited color gamut. In contrast, structural colors from hydrogel-based photonic crystals (PCs) may be stable and tunable in the whole visible spectrum by diffraction peak shift, facilitating the visual detection with high accuracy. However,

ACS Paragon Plus Environment

1

Nano Letters 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

Page 2 of 26

the current hydrogel-based PCs are all inappropriate for micro-scale detection due to the bulk size. Here we demonstrate the smallest hydrogel-based PCs ― responsive hydrogel-based photonic nanochains with high-resolution and real-time response by developing a general hydrogen bond-guided template polymerization method. A variety of mechanically separated stimuli-responsive hydrogel-based photonic nanochains have been obtained in a large scale including those responding to pH, solvent and temperature. Each of them has a submicrometer diameter and is composed of individual one dimensional periodic structure of magnetic particles locked by a tens-of-nm-thick peapod-like responsive hydrogel shell. Taking the pH-responsive hydrogel-based photonic nanochains for example, pH-induced hydrogel volume change notably alters the nanochain length, resulting in a significant variation of the structural color. The submicrometer size endows the nanochains with improved resolution and response time by 2~3 orders of magnitude than the previous counterparts. Our results for the first time validate the feasibility of using structural colors for microenvironment sensing and imaging, and may further promote the applications of responsive PCs, such as in displays and printing.

Keywords: photonic crystals, responsive hydrogels, photonic nanochains, microenvironment sensing and imaging, response time, color resolution

Microenvironment (local environment) sensing and imaging, which can reveal the physical or chemical states of a micro-scale zone,1 are indispensable for the study on such as diffusion, transportation, dynamic chemical reaction and intermolecular interactions in microreactors, microfluidic systems, cells, biological tissue.1,2 At present, they are most accomplished by submicrometer-sized optical sensors using fluorescent dyes, semiconductor quantum dots etc. as probes, which sense the environment usually through the variation of photoluminescent


2

Page 3 of 26 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

Nano Letters

properties (fluorescence intensity or lifetimes of luminesce fluorophores).3, 4 These ultra-small sensors have distinct advantages of the direct transportation into small zones, the remote acquirement of optical feedback and the use of numerous sensing spots for imaging.3 Thus, they have been widely exploited for the detection of pH,5, 6 temperature,7 CO2,8 O29, 10 or ions,11, 12 and cellular organelles13 in microenvironment. However, the fluorescent dyes or quantum dots commonly suffered from photobleaching, quenching or photoblinking behaviors.14 Furthermore, complicated instruments are often required to transduce the optical signals such as intensity or life-time of fluorophores into pseudo color map for the demonstration of analytes distribution.4, 15 In addition, most of the colorimetric method including absorption or fluorescent dyes can only change the color either from a colorless to colored state or from one color to another one.16 The limited colors involved represent a relative low sensitivity for visual detection of analytes. Intrinsically different from the traditional dyes and quantum dots, photonic crystals (PCs) can diffract bright structural colors stemming from the interaction between the incident light and the periodic structure, which are free from photobleaching, quenching or chemical instability, and in nature durable as long as their periodical structures persist.14, 17, 18 Among them, the responsive hydrogel-based PCs (RHPCs), which are constructed by responsive hydrogels and colloidally templated photonic crystals (e.g. opals, inverse opals or crystal colloidal arrays),are further capable of continuously shifting the diffraction peak and tuning the color in a wide range of visible spectrum due to the dramatic hydrogel volume change with external stimuli.19,

20

Therefore, RHPCs can provide more discernible colors by eyes than traditional dyes-based colorimetric sensors. Moreover, as the responsive moieties which can be attached to the hydrogels are much richer than synthetic chromogens,20, 21 RHPCs are more easily employed to implement the transduction of diverse stimuli into optical signals. Hence, a great number of


3

Nano Letters 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

Page 4 of 26

RHPCs have been widely exploited in this way to visually detect ions,22-25 molecules,26-30 microbes,31, 32 pH,33-36 temperature,37-40 solvent,41-43 gas,44-46 and humidity.47-49 However, the so far developed RHPCs are all in form of films and microballs,23, 27, 30, 31, 46 which have a size of at least tens of micrometers in three dimensions, limiting their applications to bulk samples, and are unqualified

for the sensing and

imaging

of micro-scale zones

or heterogeneous

microenvironments. In addition, it often costs tens of minutes or even hours for the so far developed RHPCs to reach a balance due to the slow diffusion of analytes in bulky responsive polymers.50 This seriously impedes the real-time response and on-line sensing of RHPCs.31, 35, 36 Hence, there are great needs for the miniaturization of RHPCs. Herein, taking advantage of the smallest and independent unit cells of magnetically responsive PCs, namely dynamic one dimensional (1D) periodic structure of magnetic particles, we have proposed and demonstrated a new category of RHPCs ― responsive hydrogel-based photonic nanochains by developing a general hydrogen bond-guided template polymerization method. The as-obtained responsive hydrogel-based photonic nanochains are mechanically separated peapodlike nanochains with tens-of-nm-thick responsive hydrogel shell locking the single 1D periodic structure of magnetic particles. They have a submicrometer diameter and possess unique properties of tunable photonic bandgaps through the length alteration orginated from the hydrogel volume change with external stimuli. They are promising for microenviroment sensing and imaging in real time and high resolution. Our results for the first time approve the real-time and online monitoring or detection at a micrometer level by structural colors from hydrogelbased PCs. The hydrogen bond-guided template polymerization method developed here is proved to be universal for the synthesis of this new category of responsive hydrogel-based photonic nanochains.


4

Page 5 of 26 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

Nano Letters

Figure 1. Formation mechanism and structure characterization of pH responsive photonic chains. (a) Schematic illustration of the formation mechanism of the pH responsive photonic nanochains coated by a crosslinked poly(2-Hydroxyethyl acrylate (HEA)-co-acrylic acid (AA)) hydrogel pod shell. Monomers first concentrate around uniform superparamagnetic Fe3O4@polyvinylpyrrolidone (PVP) nanoparticles via hydrogen bonds. Then, the nanoparticles are aligned linearly by magnetic field (H) into dynamic 1D periodical structures. Subsequently, they were in-situ immobilized in crosslinked poly(HEA-co-AA) hydrogel to obtain separated pH-responsive 1D photonic nanochains by UV light-initiated radical polymerization of monomers and crosslinker. (b) Schematic illustration of hydrogen bonds formed between monomers and PVP polymer chains. Monomers have a higher concentration in the vicinity of


5

Nano Letters 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

Page 6 of 26

dynamic 1D periodical structures than that in the other part of the precursor solution, insuring that the polymerization of monomers occurs mainly near those structure. (c) Schematic illustration of crosslinked poly(HEA-co-AA) hydrogel pod coated chains. (d, e) SEM, (f) TEM and (g) optical microscopy images of the as-obtained Fe3O4@PVP@poly(HEA-co-AA) pHresponsive photonic nanochains. Magnetically responsive PCs possesses tunable and reversible photonic band gap covering almost the whole visible spectra by the variation of magnetic field,51-53 but they are composed of dynamic 1D periodic structures of unconnected magnetic particles, which will collapse into randomly dispersed magnetic particles and then lose the structural colors without H. In addition, they have no responsiveness other than magnetic field. 1D peapod-like rigid nanochains which are formed by embedding the dynamic1D periodic structure of magnetic particles individually in inorganic nanoshells can retain the structural colors of magnetically responsive PCs with high resolution,54, 55 but they have invariable photonic stop bands and only respond to the direction of magnetic field. Thus, they are not suited for colorimetric sensors. In contrast, the inclusion of 1D periodic structure of magnetic particles in tens-of-nm-thick responsive hydrogels is expected to render each chain the capability of color alteration due to the hydrogel volume change with stimuli, and the diverse responsiveness due to the richness of hydrogels. In this way, the realtime and high-resolution responsiveness of structural colors to various stimuli may be achieved for microenvironment sensing and imaging. However, it is much hard to individually fix the dynamic and unlocked 1D periodic structures of magnetically responsive PCs with polymers using current approaches as they either require the large external disturbances (high temperature refluxing, violent stirring, or ultrasonic irradiation, etc.) which destroy the fragile periodicity of magnetically responsive PCs during polymerization,56-59 or often form a bulk gel due to the


6

Page 7 of 26 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

Nano Letters

uniformly distributed monomer in the solution (such as in-situ quick polymerization without harsh agitation).40,

48, 49

Hence, we firstly develop a hydrogen bond-guided template

polymerization method to cirvumvent the above problems. Taking the fabrication of pHresponsive Fe3O4@PVP@poly(HEA-co-AA) photonic nanochains as an example in Fig. 1a-c. In our experiment, monomers (HEA and AA), crosslinker ethylene glycol dimethacrylate and photo-initiator

2-Hydroxy-2-methylpropiophenone,

monodispersed

superparamagnetic

Fe3O4@PVP particles were all added to ethylene glycol forming a precursor solution. Subsequently, the polymerization was initiated by UV irradiation under magnetic field (382 Oe) and lasted for 5 mins to get the products. In our protocol, the PVP shells of the Fe3O4@PVP particles play a critical role in the formation of the 1D responsive photonic nanochains. They provide steric repulsion to counterbalance the magnetic attraction of the superparamagnetic Fe3O4@PVP particles induced by magnetic field to form 1D periodic structure of magnetically responsive PCs.52 Furthermore, they also concentrate the monomers containing hydroxyl or carboxyl groups (in this case HEA and AA) in the vicinity of the 1D periodic structure of Fe3O4@PVP particles via hydrogen bonds in the precursor solution before UV polymerization, resulting in a higher monomer concentration around those structures than that in the other part of the precursor solution. Thus, after the UV-initiated polymerization was finished, only monomers around the 1D periodic structure of magnetic particles were polymerized into mechanically separated poly(HEA-co-AA) hydrogel pod shells immobilizing the above dynamic 1D periodic structure, while the rest part of the precursor solution was still in liquid due to their relatively low monomer concentration. As a result, individual peapod-like 1D magnetic photonic nanochains are formed.


7

Nano Letters 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

Page 8 of 26

The SEM, TEM and optical microscopy images in Fig. 1d-g show that the products are almost flexible single nanochains, with an average diameter and length around 180 nm and 20 µm, respectively. The TEM image indicates that the nanochain has inorganic cores from Fe3O4@PVP particles wrapped by an organic layer of around 20 nm in thickness. Fig. S1a shows that the calcination process of the products from 200 °C to around 400 °C causes a weight loss of 21.68% which is attributed to the decomposition of the organic components. This result further reveals an increase in the polymer weight percentage of the nanochains compared to that of pure Fe3O4@PVP particles,52 indicating the presence of a new polymer other than PVP. The characteristic absorption band at 1721 cm-1 in Fig. S1b corresponds to the carbonyl group (C=O) in HEA and AA.60 The peak at 1155 cm-1 represents C-O-C stretching in HEA.60 The peaks at 1654, 1439 and 1280 cm-1 are the characteristic peaks of PVP.52 The dip at 568 cm-1 comes from Fe-O in Fe3O4.52 The above data prove that the 1D photonic nanochains are composed of 20-nmthick peapod-like shells of poly(HEA-co-AA) hydrogel wrapping uniform Fe3O4@PVP particles with identical inerparticle distances. To further prove the above formation mechanism of the nanochains, a number of other monomers have been substituted for HEA and AA. For example, if monomers cannot form hydrogen bonds with PVP were used such as acrylamide, no chain was observed after UV irradiation (Fig. S2a). In contrast, monomers that can form hydrogen bonds with PVP, such as methacrylic acid and 2-hydroxethyl methacrylate (HEMA) led to the formation of nanochains (Fig. S2b and c). Moreover, when the precursor solution containing only AA monomers is neutralized to a pH near 7.0, no chain was obtained due to the deterioration of hydrogen bonds between AA and PVP (Fig. S2d). All these phenomena clearly support that the hydrogen bonds between the monomer and PVP are essential for the fabrication of the nanochains. Therefore, the


8

Page 9 of 26 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

Nano Letters

PVP molecules anchored on the 1D periodic structure of magnetic particles of magnetically responsive PCs function as templates for the acceleration of polymer pod shell growth and formation of peapod-like photonic nanochains. These results are also in good accordance with the earlier reports that PVP could largely increase the reaction rates of monomers which could form hydrogen bonds around them via template polymerization.61-64

Figure 2. Optical properties of pH responsive hydrogel-based photonic chains. (a-c) Schematic illustration of the color generation, pH induced color changing process and dark field optical microscopy images of the as-obtained Fe3O4@PVP@poly(HEA-co-AA) photonic nanochains of the same batch in different pH buffer solutions (0.1 M buffer, 0.15 M NaCl). (d)


9

Nano Letters 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

Page 10 of 26

The corresponding reflection spectra of the photonic nanochains under different pH. (e) Dependence of λmax to magnetic strength of photonic chains. This plot demonstrates that not magnetic field strength but only pH variation can alter the color of the chains. (f) Optical microscopy images of the same chain at different pH. H = 100 Oe unless labelled in the figures. λmax is the wavelength corresponding to the maximum of diffraction peak. Figure 2a-c depict the generation and variation of structural color of the same batch of Fe3O4@PVP@poly(HEA-co-AA) photonic chains in different pH buffer solutions under magnetic field. As illustrated in the schemes of color changing process (Fig. 2a-c), the photonic nanochains under magnetic field diffract distinct colors following Bragg’s law.51-53 It also implies that the diffracted color is angle-independent just like their previous 1D bulk PC counterparts,65 and red shifts with decreasing the angle between the incident light and the magnetic field direction, which equals to (90°-θ). When the pH nearby the photonic nanochains increase, the carboxyl groups in the poly(HEA-co-AA) hydrogel pod shells deprotonate into carboxylates. This, in turn, makes the hydrogel pod shells swell due to the higher solubility of the latter than the former.35 As a result, the interparticle distances, namely, lattice constants of the photonic nanochains become larger leading to the red shift of the diffracted wavelength. This can be confirmed by the corresponding dark field optical microscopy images, which indicate that the color changes from blue to red as the pH increases. In these optical microscopy images, each bright dot observed represents a separated photonic chain orientated perpendicular to the imaging planes. As each dot is independent of each other, it can be regarded as a single pixel. This suggests their potential for high-resolution sensing and imaging. Typically, the high ionic strength, which exists widely in most of application scenarios, such as body fluids, may screen the electrostatic forces between ionic groups and suppress the volume


10

Page 11 of 26 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

Nano Letters

expansion of ionic hydrogel. As a result, pH-responsive hydrogel-based PCs usually exhibit a limited tunable color range and a low sensitivity.33 In contrast, Fig. 2d demonstrates that in high ionic strength aqueous solutions (0.1 M buffer, 0.15 M NaCl), our photonic nanochains shift the diffraction peak up to 210 nm between pH 3.6 and 7.2, suggesting that they, even if in small sizes, have a better sensitivity than the earlier reported counterparts under the same ionic strength.35, 36 This can be explained by the high loading of ionic groups and the relatively large initial interparticle distance rendering by the non-close packed nature of 1D periodical structure. It is worth noting that the as-obtained photonic nanochains need magnetic field to align and diffract structural colors, but the magnetic strength does not affect their colors. As shown in Fig. 2e, with the increasing of magnetic field strength from 60 Oe to 700 Oe, the wavelength corresponding to the maximum of the diffraction peak (λmax) remain unchanged, only the variation of pH alters the position of λmax. This is probably assigned to the high modulus of the crosslinked hydrogel between particles in the nanochains. The magnetic attraction exerted on the particles are insufficient to compress the hydrogel. Thus, the effect of magnetic field here is only to align the nanochains. This advantage that λmax is independent on magnetic field strength endows our photonic nanochains with good measurement accuracy and precision. Fig. 2f demonstrates the corresponding chain lengths (L) of the same nanochain at pH of 7.2 and 3.8. The ratio L pH 7.2/L pH 3.8 = 23.3 µm / 16.7 µm = 1.4. This value is quite close to λmax, pH 7.2/λmax, pH 3.8

= (680 nm)/(480 nm) = 1.42, demonstrating a linear proportion of λmax to L (or interparticle

spacing d) and suggesting that the structural color of the 1D photonic nanochains changes with pH via d.


11

Nano Letters 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

Page 12 of 26

Figure 3. Response time measurement of pH responsive photonic chains. (a) Schematic illustration of the Y-shaped microfluidic device. Finite element analysis simulation of the (b) flow rate and (c) pH value distribution for a plane xy in the central microfluidic channel. (d) Dark field optical microscopy image of the steady microfluid. The channel profile is outlined by dashed line segments. Numerous chains flow across rectangular area in (b-d) at a constant rate which lead to a calculated response time from red to blue: distance (100 µm) / speed (2.5 mm/s) = 40 ms. H = 400 Oe.

The as-prepared photonic nanochains are also expected to have fast response time to chemical environment change due to their tens nanometer thick polymer shell. In order to confirm it, a Yshaped microfluidic channel (Fig. 3a) was fabricated. During the experiment, a neutral aqueous solution containing the photonic nanochains and a HCl aqueous solution were injected respectively from the upper and lower inlet to make a constant flow of 6 µl/min in the microchannel after the Y-junction, while a magnetic field of 400 Oe was applied perpendicularly to the channel. In this case, the Reynolds number is 0.342, much smaller than that corresponding to turbulent flows. This implies a laminar flow in the microchannel, as further confirmed by the simulated flow rate distribution (Fig. 3b). Thus, there occurs only diffusion of protons along y-


12

Page 13 of 26 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

Nano Letters

axis without disturbance across the interface of the two fluids. Based on this, we have employed finite element analysis to simulate the pH distribution, which remains static over time. The simulated color transition stripe in Fig. 3c was set between pH 5.0 to 3.6 to match experiment color change of photonic nanochains, during which the nanochains have the most abrupt color change from red to blue as shown in Fig. 2d. Fig. 3d is the dark field optical microscopy image demonstrating the transition of photonic nanochains from red to blue due to the protons diffusion. In the area labelled by the squares in Figure 3b-d, the flow speed is approximately 2.5 mm/s. The distance in Figure 3d for photonic nanochains to horizontally flow across the color stripe is 100 µm. Thus, the response time of photonic nanochains is less than 100 (µm) / 2.5 (mm/s) = 40 ms. This approves that the as-developed pH responsive photonic nanochains have a response time shorter by at least 2~3 magnitudes than their previous bulk counterparts in the form of film or microballs,34-36, 66 promising the applications in fast pH microsensing. Cycling stability is important for optical devices. Since the as-obtained photonic nanochains have a hydrogel as thin as tens of nanometers, which greatly shortens the diffusion length of analytes, they exhibit no obvious hysteresis during the cycling process (Fig. S3). In contrast, porous RPCs like inverse opals with interconnected voids can achieve a diffusion-limited response rate only in the early stage of swelling, but exclusively perform hysteresis when the hydrogel swells and occupies much of their voids.66


13

Nano Letters 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

Page 14 of 26

Figure 4. pH responsive hydrogel-based photonic nanochains for the microenviroment sensing and imaging. (a) Schematic illustration of the experiment to show the dynamic process of acid-base aqueous solution (pH = 1) and buffer solution (pH=7.2). Visulization of the color changing process was recorded by CCD camera. (b) Time dependent darkfield optical microscopy images of the pH responsive photonic nanochain solution after one drop of acid was added. (c) Magnified darkfield optical microscopy image of rainbow stripe. (d) The size distribution of the color spots in (c), obtained by measuring 100 spots. H = 100 Oe in (b,c). As the Fe3O4@PVP@poly(HEA-co-AA) photonic nanochains have much reduced size and fast response time, they can be used as water dispersible probes for real-time, high-resolution sensing and imaging of chemical reaction through direct observation of naked eye. In order to prove it, a dynamic acid-base titration processes was implemented in Fig. 4a. Firstly, 50 µl of buffer solution containing the photonic nanochains with a pH of 7.2 was placed above a glass


14

Page 15 of 26 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

Nano Letters

substrate which mounted on an inverted microscope. Subsequently, a tiny droplet of diluted hydrochloric acid (HCl) aqueous solution with pH = 1 was added from above. As shown in Fig. 4b, before adding the acid droplet, all the photonic nanochains under magnetic field diffracted red color homogenously in the whole solution. After the droplet falls, the mixed position immediately turns into deep blue due to the low pH value and is distinctly different from the residual red area (Video S1). Then, high concentration of protons starts to diffuse and creates a clear rainbow stripe in the boundary zone which represents pH gradient. As time goes on, the rainbow stripe becomes wider, suggesting the protons in the high and low regions need time to reach an equilibrium. In an enlarged area of the rainbow stripe in Fig. 4c, a high density of bright spots with different colors are observed. 100 dots were chosen randomly to show a size distribution of them in Fig. 4d. The lateral resolution of our photonic nanochains is estimated to be around 2 µm, which is at least 2~3 orders of magnitude smaller than that of their previous bulk counterparts in the form of film or microballs.21,

67, 68

The fact that the average size

observed from our dark field optical microscopy is larger than the diameter of each chain observed in the SEM and TEM images, which is in the range of nanometers, can be explained by the limited resolution of the microscope. The as-developed 1D photonic nanochains sensors are unlike traditional optical chemical sensors such as fluorescent dyes or nanoparticles, which commonly rely on the variation of fluorescence intensity or life-time to detect and need complex instruments to transduce optical signals into pseudo color.4 They are also different from traditional colorimetric sensor which normally change color between a colorless to colored state or from one color to another one.16 Distinctively, the 1D photonic nanochains employ λmax or structural color, which is continuous in the whole visible spectra, for sensing. The possibility of using all the colors in visible spectrum


15

Nano Letters 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

Page 16 of 26

facilitates the direct visual observation in micron-sized small area with more accuracy and without any additional instruments.

Figure 5. Composition modulation of responsive hydrogel-based photonic nanochains. (a) TEM images and (b) the corresponding tunable color range at pH between 7.2 and 3.6 of the Fe3O4@PVP@poly(HEA-co-AA) photonic nanochains with different stiffness. (c) Schematic illustration for the fabrication of solvent responsive Fe3O4@PVP@poly(HEMA-co-AA) photonic nanochains and (d) their corresponding reflection spectra depending on the ethanol volume fraction in aqueous solution. (e) Schematic illustration for the fabrication of temperature responsive Fe3O4@PVP@PNIPAM photonic nanochains and (f) their corresponding reflection spectra depending on temperature. H = 100 Oe in (b, d, f). The hydrogen bond-guided template polymerization developed here is robust and versatile for the preparation of 1D photonic nanochains with controllable structures and responsiveness.


16

Page 17 of 26 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

Nano Letters

Changing the crosslinking degree in the poly(HEA-co-AA) hydrogel can easily modulate the flexibility of the nanochains. Fig. 5a demonstrate that when the crosslinking degree is 2%, interparticle distances in the chain are very close and the polymer shell appear very thin due to the large volume contraction of hydrogel in dry state. In contrast, when the crosslinking degree goes higher, interparticle distances and the polymer shell in the chain become larger and thicker respectively owing to the less flexibility of hydrogel to shrink. Fig. S4 also indicates that the nanochains with a crosslinking degree of 2% are like soft strings, while those with a crosslinking degree of 6% are much more rigid like rods. Fig. 5b shows that the crosslinking degree of the hydrogel has a great impact on the tunable range of diffracted wavelength, which is also originated from the flexibility of photonic chains. This 1D nanochain structure assembled by colloidal particles with a continuously controllable rigidity from flexible to rigid may benefit the study of colloidal analogues of molecular structures or magnetic microswimmer.69-71 Furthermore, this hydrogen bond-guided template polymerization method can also be extended to fabricate other responsive photonic nanochains by using monomers with different functional groups. For example, when HEA was substituted with HEMA, solvent-responsive Fe3O4@PVP@poly(HEMA-co-AA) photonic chains can be obtained (Fig. 5c, d). For those functional monomers which do not form hydrogen bonds with PVP directly, such as Nisopropylacrylamide (NIPAM), layer-by-layer hydrogen bonds absorption are adopted (Fig. 5e). Poly acrylic acid macromolecules first absorb on the PVP anchored on the Fe3O4@PVP particles via the hydrogen bonds between carboxyl groups and pyrrolidinone rings. Subsequently, NIPAM monomers form hydrogen bonds with carboxyl groups on poly acrylic acid chains. After polymerization, temperature-responsive Fe3O4@PVP@PNIPAM photonic chains are acquired (Fig. 5f). The above solvent and temperature responsive photonic nanochains suggest the


17

Nano Letters 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

Page 18 of 26

versatility and universality of our method to obtain 1D photonic nanochains with different responsiveness. In summary, we have proposed and fabricated a new category of RHPCs―responsive hydrogel-based photonic nanochains, which are mechanically decoupled peapod-like nanochains with tens-of-nm-thick responsive hydrogel shells wrapping single 1D periodical structures of magnetic particles, by developing a hydrogen bond-guided template polymerization method. In our protocol, the hydrogen bonds formed between monomers (such as acrylic acid) and PVP anchored to the surfaces of uniform superparamagnetic Fe3O4 nanoparticles concentrate the monomers on the adjoining area of 1D periodical structures of magnetically responsive PCs, which are formed under magnetic field, leading to responsive hydrogel-based photonic nanochains after UV light-initiated copolymerization. Taking pH responsive peapod-like Fe3O4@PVP@poly(HEA-co-AA) photonic nanochains for example, they exhibit significantly tunable length and structural colors by modulating pH with a resolution of about 2 µm and response rate less than 40 ms, which are both 2~3 orders of magnitude smaller than the ones from previous RHPCs due to the submicrometer size. Furthermore, we have for the first time demonstrated the real-time, high-resolution and online monitoring of acid-base neutralization via visual color changes of the photonic nanochains, suggesting their promising application in microenvironment sensing and imaging. This is intrinsically different from the previous optical sensors based on chemical colors stemming from microprobes of fluorescent dyes or quantum dots etc. The as-developed hydrogen bond-guided template polymerization method is general, facile and versatile, and can be used to fabricate various responsive photonic nanochains (such as pH, solvent and temperature) in large scale and controllable way by simply changing the corresponding functional monomers and other parameters. Our results may also facilitate the


18

Page 19 of 26 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

Nano Letters

application of photonic crystals in area which need fast response and high resolution, such as real-time continuous monitoring, printing and displays. EXPERIMENTAL SECTION Materials: Acrylic acid (AA), 2-Hydroxyethyl acrylate (HEA), 2-hydroxethyl methacrylate (HEMA), acrylamide, ethylene glycol dimethacrylate (EGDMA), 2-Hydroxy-2-methylpropiophenone (HMPP), ethylene glycol (EG), sodium acetate trihydrate, acetic acid solution (0.1 M) were purchased form Sigma-Aldrich and were used as received. 2-(N-Morpholino) ethanesulfonic acid (MES) and phosphate buffered saline (PBS) packs were purchased from Thermo Scientific Inc. Preparation of Fe3O4@PVP@poly(HEA-co-AA)(or Fe3O4@PVP@poly(HEMA-co-AA)) photonic nanochains: Fe3O4@PVP particles with a diameter around 150 nm were synthesized according to the previously reported method.52 After rinsing with ethanol 2~3 times, they were dispersed in ethanol with a concentration of 15 mg/cm.3 Then, 5 g Fe3O4@PVP particles ethanol solutions were centrifuged and the supernatant solutions were decanted. The remaining Fe3O4@PVP particles were mixed with 1.39 g HEA (or 1.56g HEMA), 1.29 g AA monomers, 0.12 g cross-linker EGDMA, 0.1 g photo-initiator HMPP, 30 g ethylene glycol (EG) and 10 g distilled water. This mixture was vortexed for 15 min to form a homogeneous brown precursor solution. After that, 5 g of precursor solution was transformed into a glass beaker (outer diameter 50 mm, height 72 mm) and placed above a neodymium magnet (diameter 10.1 cm, height 1.2 cm) at different distances for 1 min before the UV light (Black-Ray B-100AP) was turned on for 5~10 min. At last, the product was rinsed with ethanol and water, then dispersed in pH 7.2 PBS buffer solution (0.1 M buffer, 0.15 M NaCl). Fabrication and operation of Y-shaped microfluidic channel: The mask of the Y-shaped channel was fist prepared by standard photolithography process. Then, the pattern of Y-shaped channel was transferred from the photomask to SU8-2075 photoresist on a 4 inch silicon wafer. Subsequently, PDMS was poured on to the silicon wafer mold. After 30 min in 85 oC oven, the PDMS was detached from the silicon wafer and finally sealed with a glass slide from the bottom. Each inlet channel is 300 µm (width) and 100 µm (depth). After the Y-junction, the dimension is 600 µm (width) and 100 µm (depth), as shown in Figure 3. The inlet ends of the Y-shaped channel are connected to two 1 ml syringes with solution via tubes. The syringes are driven by a dual channel syringe pump operating at 3 µl/min. Finite Element Analysis: Numerical simulations in Figure 3 were carried out using a commercial Finite Element Method (FEM) based software package COMSOL Multiphysics TM. The model utilizes the Laminar Flow (SPF) and Transport of Diluted Species (TDS) to simulate flow rate and pH distribution at steady state in the Y-shaped microchannel. The study is stationary in 2D dimension. The diffusivity of H+ was set as 8×10-5 cm2/s in water. The flow rate was set as 3 µl/min for each inlet channel. Characterization: Transmission electron microscopy (TEM) images were captured on JEM-2100F (JEOL, Japan) with an accelerating voltage of 200 kV. Field emission scanning electron microscopy (FE-SEM) images were performed on a Hitachi S-4800 scanning electron microscope. The FT-IR spectrum was obtained using a 60-SXB


19

Nano Letters 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

Page 20 of 26

FTIR spectrometer in the range of 400‐4000 cm-1 with a resolution of 4 cm-1. Thermal analysis of Fe3O4 nanoparticles was conducted on a NETZSCH-STA449C/G under air at a heating rate of 10 oC·min-1. The reflective spectra were obtained using an Ocean Optics USB 2000+ spectrometer in the range of 300-1000 cm-1 with a resolution of 0.30 nm. Microscopy images were recorded through an optical microscope (Leica DMI 3000M, Germany).

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge in another document. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and *E-mail: [email protected] Author Contributions W.L. H.M., and J.G. conceived the idea. W.L., Q.C., and K.F. participated in the fabrication of the photonic nanochains. W.L., Q.C., K.F., and K.C. performed optical measurement experiments. W.L. H.M., and J.G. engaged in the discussion and analysis of results. W.L. H.M., and J.G. assisted in manuscript preparation and interpretation discussions. J.G. supervised the whole project. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21474078, 51573144 and 51521001), the Natural Science Foundation (2015CFA003) and Top


20

Page 21 of 26 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

Nano Letters

Talents Lead Cultivation Project of Hubei Province, and the Yellow Crane talent plan of Wuhan municipal government. W.L. and J. G. highly appreciate Prof. P. V. Braun and Graduate student Cong Xu in University of Illinois at Urbana−Champaign for their helpful discussion. REFERENCES 1.

Yang, Z.; Cao, J.; He, Y.; Yang, J. H.; Kim, T.; Peng, X.; Kim, J. S. Chem Soc Rev 2014,

43, 4563-4601. 2.

Simon, A. P.; Stefan, N. Methods and Applications in Fluorescence 2015, 3, 034003.

3.

Schäferling, M. Angew Chem Int Edit 2012, 51, 3532-3554.

4.

Wolfbeis, O. S. Chem Soc Rev 2015, 44, 4743-4768.

5.

Wencel, D.; Abel, T.; McDonagh, C. Anal Chem 2014, 86, 15-29.

6.

Han, J.; Burgess, K. Chem Rev 2010, 110, 2709-2728.

7.

Wang, X.; Wolfbeis, O. S.; Meier, R. J. Chem Soc Rev 2013, 42, 7834-7869.

8.

von Bultzingslowen, C.; McEvoy, A. K.; McDonagh, C.; MacCraith, B. D.; Klimant, I.;

Krause, C.; Wolfbeis, O. S. Analyst 2002, 127, 1478-1483. 9.

Wang, X.; Wolfbeis, O. S. Chem Soc Rev 2014, 43, 3666-3761.

10.

Shiwen, S.; Birgit, U.; Torsten, M. Methods and Applications in Fluorescence 2015, 3,

034002. 11.

Santos-Figueroa, L. E.; Moragues, M. E.; Climent, E.; Agostini, A.; Martinez-Manez, R.;

Sancenon, F. Chem Soc Rev 2013, 42, 3489-3613. 12.

Moragues, M. E.; Martinez-Manez, R.; Sancenon, F. Chem Soc Rev 2011, 40, 2593-2643.

13.

Zhu, H.; Fan, J.; Du, J.; Peng, X. Accounts Chem Res 2016, 49, 2115-2126.

14.

Kim, H.; Ge, J.; Kim, J.; Choi, S.; Lee, H.; Lee, H.; Park, W.; Yin, Y.; Kwon, S. Nature

Photonics 2009, 3, 534-540.


21

Nano Letters 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

Page 22 of 26

15.

McDonagh, C.; Burke, C. S.; MacCraith, B. D. Chem Rev 2008, 108, 400-422.

16.

Bamfield, P., Chapter 1 Phenomena Involving a Stimulated Colour Change. In Chromic

Phenomena: Technological Applications of Colour Chemistry (2), The Royal Society of Chemistry: 2010; pp 9-140. 17.

Phillips, K. R.; England, G. T.; Sunny, S.; Shirman, E.; Shirman, T.; Vogel, N.;

Aizenberg, J. Chem Soc Rev 2016, 45, 281-322. 18.

Dumanli, A. G.; Savin, T. Chem Soc Rev 2016, 45, 6698-6724.

19.

Ge, J.; Yin, Y. Angew Chem Int Edit 2011, 50, 1492-1522.

20.

Fenzl, C.; Hirsch, T.; Wolfbeis, O. S. Angew Chem Int Edit 2014, 53, 3318-35.

21.

Yetisen, A. K.; Butt, H.; Volpatti, L. R.; Pavlichenko, I.; Humar, M.; Kwok, S. J. J.; Koo,

H.; Kim, K. S.; Naydenova, I.; Khademhosseini, A.; Hahn, S. K.; Yun, S. H. Biotechnol Adv 2016, 34, 250-271. 22.

Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829-832.

23.

Ye, B.; Ding, H.; Cheng, Y.; Gu, H.; Zhao, Y.; Xie, Z.; Gu, Z. Adv Mater 2014, 26, 3270-

3274. 24.

Goponenko, A. V.; Asher, S. A. J Am Chem Soc 2005, 127, 10753-10759.

25.

Lim, H. S.; Lee, J.; Walish, J. J.; Thomas, E. L. ACS Nano 2012, 6, 8933-8939.

26.

Sharma, A. C.; Jana, T.; Kesavamoorthy, R.; Shi, L.; Virji, M. A.; Finegold, D. N.; Asher,

S. A. J Am Chem Soc 2004, 126, 2971-2977. 27.

Zhang, C.; Cano, G. G.; Braun, P. V. Adv Mater 2014, 26, 5678-5683.

28.

Asher, S. A.; Alexeev, V. L.; Goponenko, A. V.; Sharma, A. C.; Lednev, I. K.; Wilcox, C.

S.; Finegold, D. N. J Am Chem Soc 2003, 125, 3322-3329.


22

Page 23 of 26 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

Nano Letters

29.

Zhao, Y. J.; Zhao, X. W.; Hu, J.; Li, J.; Xu, W. Y.; Gu, Z. Z. Angew Chem Int Edit 2009,

48, 7350-7352. 30.

MacConaghy, K. I.; Geary, C. I.; Kaar, J. L.; Stoykovich, M. P. J Am Chem Soc 2014,

136, 6896-6899. 31.

Cai, Z.; Kwak, D. H.; Punihaole, D.; Hong, Z.; Velankar, S. S.; Liu, X.; Asher, S. A.

Angew Chem Int Edit 2015, 54, 13036-40. 32.

Zhang, J. T.; Cai, Z.; Kwak, D. H.; Liu, X.; Asher, S. A. Anal Chem 2014, 86, 9036-41.

33.

Lee, K.; Asher, S. A. J Am Chem Soc 2000, 122, 9534-9537.

34.

Lee, Y. J.; Braun, P. V. Adv Mater 2003, 15, 563-566.

35.

Xu, X.; Goponenko, A. V.; Asher, S. A. J Am Chem Soc 2008, 130, 3113-3119.

36.

Zhang, J. T.; Wang, L.; Luo, J.; Tikhonov, A.; Kornienko, N.; Asher, S. A. J Am Chem

Soc 2011, 133, 9152-5. 37.

Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959-963.

38.

Matsubara, K.; Watanabe, M.; Takeoka, Y. Angew Chem Int Edit 2007, 46, 1688-1692.

39.

Jeon, S.; Chiappelli, M. C.; Hayward, R. C. Adv Funct Mater 2016, 26, 722-728.

40.

Ma, H.; Zhu, M.; Luo, W.; Li, W.; Fang, K.; Mou, F.; Guan, J. J Mater Chem C 2015, 3,

2848-2855. 41.

Zhang, Y.; Fu, Q.; Ge, J. Nat Commun 2015, 6, 7510.

42.

Burgess, I. B.; Koay, N.; Raymond, K. P.; Kolle, M.; Lončar, M.; Aizenberg, J. ACS

Nano 2012, 6, 1427-1437. 43.

Luo, W.; Yan, J.; Tan, Y.; Ma, H.; Guan, J. Nanoscale 2017, 9, 9548-9555.

44.

Kobler, J.; Lotsch, B. V.; Ozin, G. A.; Bein, T. ACS Nano 2009, 3, 1669-1676.


23

Nano Letters 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

45.

Page 24 of 26

Wu, Y.; Li, F.; Zhu, W.; Cui, J.; Tao, C.-a.; Lin, C.; Hannam, P. M.; Li, G. Angew Chem

Int Edit 2011, 50, 12518-12522. 46.

Xie, Z.; Cao, K.; Zhao, Y.; Bai, L.; Gu, H.; Xu, H.; Gu, Z.-Z. Adv Mater 2014, 26, 2413-

2418. 47.

Diao, Y. Y.; Liu, X. Y.; Toh, G. W.; Shi, L.; Zi, J. Adv Funct Mater 2013, 23, 5373-5380.

48.

Xuan, R.; Wu, Q.; Yin, Y.; Ge, J. J Mater Chem 2011, 21, 3672-3676.

49.

Hu, H.; Chen, Q.; Cheng, K.; Tang, J. J Mater Chem 2012, 22, 1021-1027.

50.

Reese, C. E.; Mikhonin, A. V.; Kamenjicki, M.; Tikhonov, A.; Asher, S. A. J Am Chem

Soc 2004, 126, 1493-1496. 51.

Ge, J.; Hu, Y.; Yin, Y. Angew Chem Int Edit 2007, 46, 7428-31.

52.

Luo, W.; Ma, H.; Mou, F.; Zhu, M.; Yan, J.; Guan, J. Adv Mater 2014, 26, 1058-1064.

53.

Wang, M.; Yin, Y. J Am Chem Soc 2016, 138, 6315-6323.

54.

Hu, Y.; He, L.; Yin, Y. Angew Chem Int Edit 2011, 50, 3747-3750.

55.

Wang, H.; Chen, Q. W.; Sun, Y. B.; He, M. Y. Journal of Physical Chemistry C 2010,

114, 19660-19666. 56.

Zhou, J.; Meng, L.; Feng, X.; Zhang, X.; Lu, Q. Angew Chem Int Ed Engl 2010, 49,

8476-9. 57.

Bannwarth, M. B.; Kazer, S. W.; Ulrich, S.; Glasser, G.; Crespy, D.; Landfester, K.

Angew Chem Int Ed Engl 2013, 52, 10107-11. 58.

Kim, Y.; Choi, Y. S.; Lee, H. J.; Yoon, H.; Kim, Y. K.; Oh, M. Chem Commun 2014, 50,

7617-7620. 59.

Ma, M. L.; Ma, Y.; Zhang, B. L.; Zhang, H. P.; Geng, W. C.; Zhang, Q. Y. J Mater Sci

2015, 50, 3083-3090.


24

Page 25 of 26 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

Nano Letters

60.

Amnuaypanich, S.; Ratpolsan, P. J Appl Polym Sci 2009, 113, 3313-3321.

61.

Połowiński, S. Prog Polym Sci 2002, 27, 537-577.

62.

Feng, Z.; Wang, Z.; Gao, C.; Shen, J. Chem Mater 2007, 19, 4648-4657.

63.

Rainaldi, I.; Cristallini, C.; Ciardelli, G.; Giusti, P. Macromol Chem Phys 2000, 201,

2424-2431. 64.

Chun, M.-K.; Cho, C.-S.; Choi, H.-K. J Control Release 2002, 81, 327-334.

65.

Ge, J.; Lee, H.; He, L.; Kim, J.; Lu, Z.; Kim, H.; Goebl, J.; Kwon, S.; Yin, Y. J Am Chem

Soc 2009, 131, 15687-15694. 66.

Shin, J.; Braun, P. V.; Lee, W. Sensors and Actuators B: Chemical 2010, 150, 183-190.

67.

Zhao, Y.; Shang, L.; Cheng, Y.; Gu, Z. Accounts Chem Res 2014, 47, 3632-3642.

68.

Lee, S. Y.; Kim, S.-H.; Hwang, H.; Sim, J. Y.; Yang, S.-M. Adv Mater 2014, 26, 2391-

2397. 69.

Dreyfus, R.; Baudry, J.; Roper, M. L.; Fermigier, M.; Stone, H. A.; Bibette, J. Nature

2005, 437, 862-5. 70.

Vutukuri, H. R.; Demirors, A. F.; Peng, B.; van Oostrum, P. D.; Imhof, A.; van Blaaderen,

A. Angew Chem Int Edit 2012, 51, 11249-53. 71.

van Blaaderen, A. Science 2003, 301, 470-471.


25

Nano Letters 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

Page 26 of 26

Table of Contents


26

Responsive Hydrogel-based Photonic Nanochains for

Recommend Documents