Urea-Functionalized Poly(ionic liquid) Photonic Spheres for Visual

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Functional Nanostructured Materials (including low-D carbon)

Urea-Functionalized Poly(ionic liquid) Photonic Spheres for Visual Identification of Explosives with a Smartphone Chengcheng Liu, Wanlin Zhang, Yang Zhao, Changxu Lin, Kang Zhou, Yan-Mei Li, and Guangtao Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04568 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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ACS Applied Materials & Interfaces

Urea-Functionalized Poly(ionic liquid) Photonic Spheres for Visual Identification of Explosives with a Smartphone Chengcheng Liu,‡,a Wanlin Zhang,‡,a Yang Zhao,b Changxu Lin,c Kang Zhou,a Yanmei Li,a Guangtao Li*,a a. Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China b. Institute of Forensic Science, Ministry of Public Security, Beijing 100038, P. R. China c. Research Institute for Biomimetics and Soft Matter, Fujian Provincial Key Laboratory for Soft Functional Materials Research, College of Physical Science and Technology, Xiamen University, Xiamen 361005, P. R. China KEYWORDS: photonic crystal, poly(ionic liquid), explosive detection, sensor array, visual identification

ABSTRACT: Current effort merging rational design of colorimetric sensor array with portable and easy-to-use hand-held readers delivers an effective and convenient method for on-site detection and discrimination of explosives. However, on the one hand, there are rare relevant reports; on the other hand, some limitations regarding direct sensing, color retention and array extendibility still remain. Herein, urea-functionalized poly(ionic liquid) photonic spheres were

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employed to construct a brand-new colorimetric sensor array for directly identifying five nitroaromatic explosives with a smartphone. It is found that the strong hydrogen bonding between the urea motifs and the nitro groups offers the spheres high affinity for binding the targets, while the existence of other abundant intermolecular interactions in poly(ionic liquid) units renders one single sphere eligible for prominent cross-responses to a broad range of analytes. Besides, in our case, opal-like photonic crystal structures other than chemical dyes are used to fabricate new style of colorimetric array. Such structural colors can be vivid and unchanged over a long period even in hazard environments. Importantly, through simply altering the preparation conditions of our PIL spheres, a pool of sensing elements could be added to the developed array for discrimination of extended target systems such as more explosives and even their mixtures in real-world context.

1. INTRODUCTION In recent years, owing to the increasing threat of bomb attacks to global security, substantial research efforts have been devoted to the detection and identification of various explosive compounds.1-2 Among them, a number of analysis techniques have been investigated, including gas chromatography,3 ion-mobility spectrometry,4 fluorimetry,5-6 surface-enhanced Raman spectroscopy,7 and electrochemical detection.8-9 Nevertheless, such methods generally require complicated measurement protocols and preconcentration procedures, and have thus greatly been restricted to laboratory application. More seriously, the inherently bulky nature and poor portability of these expensive instruments render them inappropriate for on-site applicability.10 Fortunately, the emerging colorimetric sensor array combined with hand-held readers alternatively provides an effective and portable approach for the high-throughput detection and discrimination of various explosives.11-17 Instead of using highly specific receptors, colorimetric


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sensor arrays exploit a set of easily available cross-reactive receptors to simultaneously interact with multiplex analytes, producing responsive fingerprints in colorimetric signaling channels.18-19 In combination with the widely used hand-held readers (e.g., contact image sensors, digital cameras and smart phones), rapid acquisition of color changes by these simple, inexpensive and portable instrumentations can be achieved, which is beneficial for field detection of explosives.2021

For example, Suslick et al. developed colorimetric sensor arrays composed of a series of diverse

chemoresponsive colorants to react with explosive analytes and the resulting color change profiles were recorded on a contact image sensor.22 Paixão and co-workers described the fabrication of a disposable colorimetric paper sensor array that could generate unique color patterns based on the formation of colored species from the explosive species and the chemical reagents.23 In their scenario, smartphone, a much more accessible and easy-to-operate equipment, was chosen to extract fingerprints of color changes. Despite significant progress achieved in colorimetric sensor arrays for identifying explosives,24-29 relevant reports are very scarce and some limitations still remain. First, the specific involvement of chemical reactions between explosives and dyes leads to complicated derivation processes and time-consuming experimental implementation.30-31 Second, the formed colored species are not stable enough under the influence of certain unknown environmental interferents, thus causing color diminution with time.32 Additionally, in order to obtain higher-dimensional sensing information for identifying broader analyte scope as well as complex mixtures, it is highly desirable to realize more satisfactory extendibility of sensor arrays in an efficient and ready fashion. In this work, we firstly present urea-functionalized poly(ionic liquid) (PIL) inverse opal spheres as sensing elements to construct a brand-new colorimetric sensor array, which is assisted by a smartphone to extract color change profiles for directly detecting nitroaromatic explosives. It is



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found that the strong hydrogen bonding between the urea motifs and the nitro groups offers the PIL spheres high affinity for directly binding the target nitroaromatic explosives, which avoids complicated analysis procedures.33 More importantly, owing to the existence of other abundant intermolecular interactions in ionic liquids, such as π-π interactions, hydrophobic interactions, van der Waals forces, and electrostatic forces,34 one single sphere can exhibit prominent differential responses to a broad range of analytes.35 Unlike all of the above reported colorimetric sensors using chemical dyes, in our case, opal-like photonic crystal structures are used to fabricate new style of colorimetric array, which facilitate the visual detection by naked eyes. Notably, despite exposure in hazard environments, such structural colors could be vivid and unchanged over a long period of time. Besides, through simply altering the preparation conditions of our PIL spheres, unlimited sensing elements could be easily added to the developed array for discrimination of extended target systems such as more explosives and particularly their mixtures in real-world context. 2. EXPERIMENTAL SECTION 2.1 Preparation of PIL photonic spheres PIL photonic spheres were prepared using an analogous method previously reported by our group.36 The spheres were prepared by a two-step method. Firstly, monodisperse silica particles with a diameter of ca. 205 nm were synthesized by the modified Stöber method,37-39 and the monodisperse silica nanoparticles self-assembled into ordered lattices by droplet-based microfluidics. The procedure is illustrated in Figure S1. The flow rate of the dispersed phase was 1.2 mL/h, and 12 mL/h for the continuous phase. Secondly, the solution containing 0.30 g ionic liquid monomer, 0.0576 g crosslinker (the degree of crosslinking was 10%), 100 μL methanol and


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5 μL photoinitiator was infiltrated into the void spaces of the photonic spheres by capillary force, and the time of the infiltration process was about 18 hours. After UV-light induced polymerization, the silica nanoparticles was removed by using HF (5 wt. %) to afford the PIL photonic spheres. The PIL photonic spheres were kept in water in order to maintain the intact photonic structure. 2.2 Colorimetric Discrimination of Explosives in Solutions The solutions of nitroaromatics, picric acid (PA), 4-Nitrophenol (NP), 2,4,6-Trinitroresorcinol (TNR), 2,4-Dinitrophenol (DNP), and 1,3,5-Trinitrotoluene (TNT), were prepared in acetonitrile / deionized water (1:1 by volume). The PIL photonic spheres were incubated in 2 mL nitroaromatic solutions for 12 hours at room temperature under continuous stirring on an orbital shaker to achieve reaction equilibrium. To evaluate reproducibility, seven individual spheres were used to record the sensing response to each analyte. Then the optical images of spheres after binding with analytes were recorded. The RGB values were obtained by importing the photos of spheres to Adobe Photoshop and processed using principle component analysis (PCA) by Matlab. 2.3 Semiquantative analysis of Explosives and Discrimination of Mixtures in Complex Background The solutions of PA, NP and TNR with different gradient concentrations (500 μM, 100 μM, 50 μM, 10 μM and 5 μM) were prepared in deionized water. The mixtures of nitroaromatic were dissolved in acetonitrile / tap water (1:1 by volume). The concentration of individual explosive in every mixture was 100 μM. The discriminating process were performing in the same way as the part in 2.2. 2.4 Materials and Characterization



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N-(3-Aminopropyl)-imidazole and butyl isocyanate were purchased from Alfa. Hexamethylene Diisocyanate was purchased from TCI. 4-vinylbenzyl chloride and acetonitrile were purchased from J&K. Methanol and dimethylsulfoxide were purchased from Beijing Chemical Works. 1H and 13C NMR spectra were recorded on a 400 MHz NMR spectrometer (JEOL, ECS-400), and the chemical shift values were reported with respect to internal standard (tetramethylsilane). Electrospray ionization mass spectrometry (ESI-MS) was measured on a mass spectrometer (Bruker, Esquire-LC). The optical images of spheres were recorded by an optical microscope (OLYMPUS, 51M) equipped with a CCD camera (OLYMPUS, UTV0.5XC-3), and the photos were recorded with a smartphone (Honor v9). The reflection spectra of the photonic crystal template were measured by a microscope equipped with a fiber optic spectrometer (Ocean Optics, USB2000+). The UV-vis absorption spectra were obtained by an ultraviolet spectrometer (PerkinElmer Lambda35). The size and structure of nanoparticles and spheres were characterized using scanning electron microscope (SEM) (Hitachi, SU8010). 3. RESULTS AND DISCUSSION 3.1 Preparation of the urea-functionalized poly(ionic liquid) photonic sphere

Figure 1. The chemical structures of the IL monomer, IL crosslinker and target analytes.


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The structures of the urea-functionalized imidazolium-based IL monomer, crosslinker, and the target analytes are shown in Figure 1, and the synthetic routes and corresponding characterizations for the ionic liquid used are displayed in Supporting Information. We employed T-junction microfluidic device (Figure S1) to produce the droplets with narrow size dispersion. The silica colloidal crystal spheres as the template to prepare inverse opal PIL spheres were fabricated by self-assembling of silica nanoparticles in microfluidic droplets. The general process for preparation of the urea-functionalized PIL inverse opal spheres is described in Figure 2a. Figure 2b and d show the optical and SEM images of the template spheres, respectively. Ultraviolet lightinduced polymerization was utilized after the monomer solution infiltrated into the void of silica template by capillary force. The inverse opal PIL spheres were manufactured by replicating the template spheres via completely etching of silica nanoparticles with HF. In the present work, we use monodisperse silica nanoparticles to generate the photonic template with microfluidic technique. The monomer solution for UV-light induced polymerization consisted of the synthesized urea-functionalized monomer, crosslinker and the photoinitiator. Figure 2c and e show the optical and SEM images of the obtained urea-functionalized poly(ionic liquid) photonic spheres with 3D-ordered macropore structures. As we can see in Figure 2c and e, the diameter of the pores is smaller than that of silica nanoparticles we used, and we attributed this phenomenon to the shrinkage of the PIL hydrogel in dry state.40 The macropore structure makes the spheres beneficial for absorbing analyte, leading to higher sensitivity. What’s more, the porous structure could facilitate analyte diffusion throughout the hydrogel scaffold, which improves the response rate.41-42 The spatial ordered structure imparts the photonic spheres with bright and vivid structural color, providing red, green and blue (RGB) color channels. We extract RGB values from photos of PIL spheres before and after binding with analytes separately, generating the fingerprint



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information of different analytes. The three channels can reflect the binding event between PIL spheres and analytes fast and sensitively. It is worth noting that the brilliant color of the PIL spheres originates from their high-ordered structure, determined by the physical arrangement and thus is resistant to photo bleaching and anti-interference.43-49 In comparison to traditional dyes, pigments or phosphors, the photonic crystal has intrinsic superiority in acting as sensor because of stable sensing signal.41, 50-56 More critically, the single sensing element can be readily extended to sensing array by altering the proportion of crosslinker or the diameter of silica nanoparticles constructing photonic crystal templates. It is crucial that the abundant interactions involved in ionic liquid endow the PIL spheres with outstanding ability to differentiate analytes, owing to the high crossreactivity.

Figure 2. (a) Schematic illustration of the preparation of the poly(ionic liquid) photonic spheres; (b) the optical image and (d) the SEM image of the photonic crystal sphere templates; (c) the optical image and (e) the SEM image of the PIL photonic spheres; the scale bars in (b) and (c) are 300 μm, and in (d) and (e) are 500 nm.


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3.2 Discrimination for five explosives The photos and optical microscope images of the PIL spheres before and after binding with explosives are displayed in Figure 3a and b. The spheres displayed dazzling but multiple colors upon exposure to different analytes. Owing to the diverse shrinking or swelling behaviors of the PIL skeleton triggered by the five explosives with different chemical structures, giving rise to change in the periodicity of lattice, the spheres exhibited apparent structural color change to varying degrees, and the bright color will not photobleach. As we expected, the characteristic colors can be distinguished by the naked eye. The dramatic colorimetric changes of the PIL photonic spheres can be attributed to the multiple intermolecular interactions, among which hydrogen bonds have made contributions to a certain extent. Urea has been widely known because of forming strong hydrogen bonding with atoms which can act as hydrogen bonding acceptor,57-59 such as nitro,33 endowing the PIL spheres exceedingly high affinity with the explosives. In order to verify the hydrogen bonding between the PIL photonic spheres and explosives, 1H NMR titrations of IL monomer with picric acid (PA) in DMSO-d6 were conducted. Figure S3 shows the 1H

NMR spectra of the IL monomer upon addition of PA. Upon addition of PA, the Imidazole

ring-H dramatically shifted upfield from 9.44 ppm to 9.29 ppm, and the OH (triangle) of PA shifted to lower magnetic fields, from 5.33 ppm to 3.45 ppm. Urea NH (star) was dramatically shifted upfield, but the peak width became broader, so it was difficult to mark the peak in the spectra. UVvis titrations of the IL monomer and five explosives were also carried out to validate the interactions between the sensor and analytes. Figure S4 shows the UV-vis spectra of the IL monomer upon addition of five explosives respectively. The absorbance of the IL monomer changed dramatically in the presence of strongly bound explosives. Take TNR as an example: the absorbance of IL degraded from 0.135 to 0.122. The fitted curves of the five explosives are



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different from each other, reflecting the diverse responding patterns. With all the abundant diverse intermolecular interactions existing, especially the strong hydrogen bonding between nitro and urea group, we have reasonable basis to assume that colorimetric detection and discrimination of nitroalkanes

(such

as

2,3-dimethyl-2,3-dinitrobutane

(DMNB)),

Nitramines

(such

as

cyclotetramethylene-tetranitramine (HMX)), and nitrate esters (such as pentaerythritol tetranitrate (PETN)) can be realized, which are challenging to be identified for the lack of aromatic rings. Further exploration for the related issues will be conducted in our future work.

Figure 3. (a) The photos and (b) the microscope optical images of the urea-functionalized poly(ionic liquid) spheres before and after responding to five explosives at 100 μM. (c)The histogram of corresponding ΔRGB data. The scale bar in (b) is 200 μm. We extracted and calculated the RGB value from the photo to obtain ΔRGB data, which represents the variation of color in red, green and blue channels. Figure 3c shows the histogram of corresponding ΔRGB data. Due to the various intermolecular interactions involved in the urea-


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functionalized ionic liquids, including π-π interactions, van der Waals forces, hydrophobic interactions, electrostatic forces and hydrogen bonding, only one single urea-functionalized PIL sphere could respond sensitively to all the five explosives. As a majority of people know how to use a smartphone, smartphone-based sensing method don’t require training staff, and also circumvents the usage of sophisticated instruments, such as the fiber optic spectrometer, which are hard to operate and time-consuming.

Figure 4. The urea-functionalized poly(ionic liquid) photonic spheres for the discrimination of five explosives at 100 μM. (a) 2D PCA score plot; (b) hierarchical cluster analysis (HCA) dendrogram. The principle component analysis (PCA) was performed to evaluate the discriminatory capacity of the spheres. The RGB data was analyzed by using MATLAB program to reduce data and achieve information classification (3 dimensional information × 5 analytes ×7 trials = a total of 105 samples). As the 2D PCA score plot illustrates (Figure 4a), every explosive formed clear cluster with an obvious separation from the others, revealing the extraordinary differentiation power of the urea-functionalized PIL photonic spheres. The data in every cluster reflects the high reproducibility of the relevant set of trails. The contribution of the first two primary component



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accounts for 85.83% of the total variance (PC1 = 46.63%, PC2 = 39.2%). Additionally, hierarchical cluster analysis (HCA) was also employed to analyze the generated information. The dendrogram of HCA is shown as Figure 4b, and it is observed that every explosive has been discriminable clearly with no confusion. The inter-cluster distance (representing the magnitude of distinction between different clusters) as well as the connectivity (describing which clusters are most similar with each other) are demonstrated intuitively in the dendrogram. The analysis result confirms the excellent discriminatory power of the PIL photonic spheres. 3.3 Semiquantitative assay for explosives In the present work, the PIL photonic spheres not only can be applied to qualitative analysis for nitroaromatic explosives, but also are suitable for semiquantitative assay. We take three explosives, PA, NP and TNR for example. The urea-functionalized PIL photonic spheres were incubated in the explosive solutions at different gradient concentrations (500 μM, 100 μM, 50 μM, 10 μM and 5 μM). Figure S5 shows the photo of the PIL spheres before and after response. The colors of the spheres can be clearly differentiated by the naked eye. Figure S6-8 demonstrate the optical microscope images of the spheres and the histogram of corresponding ΔRGB data. As we can see in the photos and the optical images, when the concentration of the analytes was higher, the PIL photonic spheres displayed more obvious color change after responding. The phenomenon could be ascribed to two reasons: Firstly, the environment with higher concentration of analytes provided stronger interactions between the PIL spheres and explosives, and the skeleton of the spheres would swell or shrink more drastically. This was consistent with the volume change of the PIL spheres binding with analytes, as the photos shows. The structural color would change consequently. Secondly, the different concentration would cause different osmotic pressure, which also affect the shrinking or swelling of the PIL spheres. PCA was employed to analyze the


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collected ΔRGB data, and Figure 5 present the 2D PCA score plots for the three analytes with gradient concentrations. The clear clusters in the score plot elucidate that the spheres are capable of detecting and discriminating explosive at different concentrations. Moreover, the result implies that the potential to ascertain the concentration of the sample with unknown concentration, as long as the training set has been set up in advance.60

Figure 5. The 2D PCA score plots for semiquantitative analysis for (a) PA, (b) NP and (c) TNR. 3.4 Application in complicated situation To further demonstrate the practical value of our PIL spheres, we applied the spheres to detect and differentiate samples simulating real-world situations. Six kinds of explosive solutions were prepared in tap water, and the components of each solution are as follows: DNP, PA+TNT, PA+TNR, TNT+DNP, TNR+DNP and PA+TNR+DNP +TNT (the concentrations for each explosive are 100 μM). As the photos and optical microscope images of the PIL spheres before



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and after binding with explosives illustrate (Figure S9), the clear differences between the colors exhibited by the spheres ensure the feasibility to detect and discriminate mixtures of analytes. Figure S9c shows the histogram of corresponding ΔRGB data. The well-separated clusters shown in the 2D PCA score plot (Figure 6) indicate that the PIL spheres can successfully discriminate multicomponent analytes. In addition, the result reveals that our sensor system can be applied to predict and identify the unknown samples, as long as the training sets have been set up previously.60 By the way, we also tested the anti-interference to non-nitro compound (such as phenol) of our sensing system. As the PCA score plot (Figure S10) indicates, the clear clusters of phenol (PHE), hydroquinone (HQ) and 4-iodophenol (IDP) formed a larger cluster with distinct separation from the ones of explosives. The result could be ascribed to that phenol do not have the strong hydrogen bond between nitro and urea group.

Figure 6. The 2D PCA score plots for discrimination of explosives in complicated situation. 3.5 Strategies to extend the number of sensing elements of the array


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Figure 7. The two strategies for sensor array modulation. (a) the photos and (c) the microscope optical images of the PIL spheres with different degree of crosslinking before and after responding to PA or NP at 100 μM. (b) the photos and (d) the microscope optical images of the PIL spheres generated from different photonic crystal templates before and after responding to PA or NP at 100 μM. The scale bars in (c) and (d) are 200 μm. Higher-dimensional information is quite necessary for high-throughput screening or the situation where the analyte components are complex or the structures are similar. Under the circumstances, it is eagerly anticipated that a sensor array possesses favorable extendibility. For instance, the amount of the types of sensing elements can be increased to generate cross-responsive pattern, enriching the information density.61 Consequently, the discrimination accuracy may be improved dramatically. In the present work, two strategies are proposed to modulate sensor array. Firstly, degree of crosslinking can be altered. Figure 7a and c show the photos and microscope optical images of the PIL spheres with different degree of crosslinking (10%, 20%, 30%, 40% and 100%). The PIL spheres displayed different colors after binding the same analyte. The



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phenomenon can be explained as follows: the higher the degree of crosslinking, the stiffer the PIL sphere skeleton. The extent of the shrinking or swelling of the sphere skeleton varies with the stiffness. No matter how much the degree of the crosslinking is, the PIL spheres show bight and obvious color change with bound analyte (Figure 7). The histogram of corresponding ΔRGB data are shown as Figure S11. This strategy succeeded in producing multidimensional information in an easy way. Secondly, we can also change the diameters of silica nanoparticles constituting the photonic crystal spheres (opal structure) used as templates. Accordingly, the inverse opal PIL spheres replicating different templates will possess macropores with different diameters. Figure S12a-f shows the SEM images of the silica colloidal crystal spheres we chose, and the diameters are 156 nm, 205 nm, 245 nm, 256 nm, 286 nm, and 295 nm. Figure S12g displays the reflection spectra, and each template sphere has different spectra position of the stop band from others, which is determined by the structural period. The resultant PIL photonic spheres have diverse structural colors due to structure, (Figure 7b and d), and every kind of PIL spheres changed the color apparently after binding analytes. The histogram of corresponding ΔRGB data are shown as Figure S13. As we anticipate, this strategy can also be applied to realize higher information density. Above result reveals that our sensor array is designable and can be simply modulated to achieve more precise discrimination. By the way, anion exchange can also be adopted as a strategy to extend the sensor array. For instance, Cl- could be replaced with PF6- and so on. Different anion are expected to induce diverse intermolecular interactions, affording more available sensing signals to enrich the fingerprint database. 4. Conclusions


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In summary, we designed and prepared the urea-functionalized poly(ionic liquid) photonic spheres to construct a new-type colorimetric sensor array combined with smartphone for detection and discrimination of explosives. Five explosives were identified with no confusion or error. Owing to the different swelling or shrinking performance of the PIL photonic spheres triggered by different analytes, the vivid structural color of the photonic spheres changed to various extent, which is bright and obvious enough for detection with the naked eye. Besides, not only colorimetric qualitative discrimination can be achieved, but also the semiquantitative assay and even analysis for analyte mixtures in real-world circumstance. More than that, to demonstrate the great extendibility of our sensing system, two strategies were employed (changing ratio of the crosslinker or using different template spheres). Although several research efforts have been made to develop a high-efficient and easy-to-use colorimetric sensor array for explosives, there are still some limitations. To solve the existing problems, our sensing system showed its unique advantages. Firstly, instead of taking advantage of reaction between indicators and analytes, the direct capturing and binding analytes without the tedious experimental operation and complex derivation procedure are conductive to improve the sensing efficiency; Secondly, different from the common dyes or pigments, the bright color of the PIL photonic spheres is determined by the physical structural period, thus can remain for ages, providing steady sensing signal, which is crucial for a sensing system; Thirdly, infinite sensing element can be added to the sensor array to obtain more valuable information by simply changing the preparation condition. Altogether, we believed that our high-performance sensing system would hold great promise for identifying explosives on site. ASSOCIATED CONTENT Supporting Information. Schematic illustration of the preparation of photonic crystal sphere templates by droplet-based microfluidics, details of synthesis, 1H NMR spectra, 13C NMR spectra,



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ESI-MS spectra, 1H NMR titrations, UV-vis titrations, optical images and the histogram of corresponding ΔRGB data for semiquantitative analysis, related data for discrimination of mixtures, histogram of corresponding ΔRGB data of the PIL spheres with different degree of crosslinking, SEM images and the reflection spectra of the photonic crystal sphere template, relevant data for PIL spheres generated from different photonic crystal templates. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the NSFC (No. 21773135, 21473098,

21121004

and

21421064),

MOST

(2013CB834502)

and

the

Deutsche

Forschungsgemeinschaft DFG (TRR61). REFERENCES (1) Germain, M. E.; Knapp, M. J. Optical Explosives Detection: from Color Changes to Fluorescence Turn-on. Chem. Soc. Rev. 2009, 38, 2543-2555.


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


(2) Salinas, Y.; Martínez-Máñez, R.; Marcos, M. D.; Sancenón, F.; Costero, A. M.; Parra, M.; Gil, S. Optical Chemosensors and Reagents to Detect Explosives. Chem. Soc. Rev. 2012, 41, 12611296. (3) Håkansson, K.; Coorey, R. V.; Zubarev, R. A.; Talrose, V. L.; Håkansson, P. Low-Mass Ions Observed in Plasma Desorption Mass Spectrometry of High Explosives. J. Mass Spectrom. 2000, 35, 337-346. (4) Buryakov, I. A. Detection of Explosives by Ion Mobility Spectrometry. J. Anal. Chem. 2011, 66, 788-809. (5) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chemical Sensors Based on Amplifying Fluorescent Conjugated Polymers. Chem. Rev. 2007, 107, 1339-1386. (6) Rochat, S.; Swager, T. M. Conjugated Amplifying Polymers for Optical Sensing Applications. ACS Appl. Mater. Interfaces 2013, 5, 4488-4502. (7) Sylvia, J. M.; Janni, J. A.; Klein, J. D.; Spencer, K. M. Surface-Enhanced Raman Detection of 2,4-Dinitrotoluene Impurity Vapor as a Marker to Locate Landmines. Anal. Chem. 2000, 72, 5834-5840. (8) Krausa, M.; Schorb, K. Trace Detection of 2,4,6-Trinitrotoluene in the Gaseous Phase by Cyclic Voltammetry.1Dedicated to Professor W. Vielstich on the occasion of his 75th birthday.1. J. Electroanal. Chem. 1999, 461, 10-13. (9) Ho, M. Y.; D’Souza, N.; Migliorato, P. Electrochemical Aptamer-Based Sandwich Assays for the Detection of Explosives. Anal. Chem. 2012, 84, 4245-4247. (10) Sun, X.; Wang, Y.; Lei, Y. Fluorescence Based Explosive Detection: From Mechanisms to Sensory Materials. Chem. Soc. Rev. 2015, 44, 8019-8061.



Page 20 of 26

(11) Rakow, N. A.; Suslick, K. S. A Colorimetric Sensor Array for Odour Visualization. Nature 2000, 406, 710-713. (12) Askim, J. R.; Mahmoudi, M.; Suslick, K. S. Optical Sensor Arrays for Chemical Sensing: The Optoelectronic Nose. Chem. Soc. Rev. 2013, 42, 8649-8682. (13) Li, Z.; Askim, J. R.; Suslick, K. S. The Optoelectronic Nose: Colorimetric and Fluorometric Sensor Arrays. Chem. Rev. 2018, 119, 231-292. (14) Geng, Y.; Peveler, W. J.; Rotello, V. Array-Based “Chemical Nose” Sensing in Diagnostics and Drug Discovery. Angew. Chem. Int. Ed. 2019, 58, 5190-5200. (15) Tang, N.; Mu, L.; Qu, H.; Wang, Y.; Duan, X.; Reed, M. A. Smartphone-Enabled Colorimetric Trinitrotoluene Detection Using Amine-Trapped Polydimethylsiloxane Membranes. ACS Appl. Mater. Interfaces 2017, 9, 14445-14452. (16) Xie, Z.; Ge, H.; Du, J.; Duan, T.; Yang, G.; He, Y. Compartmentalizing Incompatible Tandem Reactions in Pickering Emulsions to Enable Visual Colorimetric Detection of Nitramine Explosives Using a Smartphone. Anal. Chem. 2018, 90, 11665-11670. (17) Meng, Q.; Mo, S.; Ruobing, B.; Yiqi, M.; Xiaoshi, Q.; Dipika, S.; Yuan, Z.; Hang Jerry, Q.; Zhigang, S.; Ximin, H. Bioinspired Hydrogel Interferometer for Adaptive Coloration and Chemical Sensing. Adv. Mater. 2018, 30, 1800468-1800474. (18) Diehl, K. L.; Anslyn, E. V. Array Sensing Using Optical Methods for Detection of Chemical and Biological Hazards. Chem. Soc. Rev. 2013, 42, 8596-8611. (19) Jia, R.; Tian, W.; Bai, H.; Zhang, J.; Wang, S.; Zhang, J. Amine-Responsive CelluloseBased Ratiometric Fluorescent Materials for Real-Time and Visual Detection of Shrimp and Crab Freshness. Nat. Commum. 2019, 10, 795-802.


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


(20) Zhang, C.; Suslick, K. S. A Colorimetric Sensor Array for Organics in Water. J. Am. Chem. Soc. 2005, 127, 11548-11549. (21) Askim, J. R.; Suslick, K. S. Hand-Held Reader for Colorimetric Sensor Arrays. Anal. Chem. 2015, 87, 7810-7816. (22) Lin, H.; Suslick, K. S. A Colorimetric Sensor Array for Detection of Triacetone Triperoxide Vapor. J. Am. Chem. Soc. 2010, 132, 15519-15521. (23) Salles, M.; Meloni, G.; Araujo, W.; Paixão, T. Explosive Colorimetric Discrimination Using a Smartphone, Paper Device and Chemometrical Approach. Anal. Methods 2014, 6, 20472052. (24) Askim, J. R.; Li, Z.; LaGasse, M. K.; Rankin, J. M.; Suslick, K. S. An Optoelectronic Nose for Identification of Explosives. Chem. Sci. 2016, 7, 199-206. (25) Mosca, L.; Karimi Behzad, S.; Anzenbacher Jr., P. Small-Molecule Turn-on Fluorescent Probes for RDX. J. Am. Chem. Soc. 2015, 137, 7967-7969. (26) Sheykhi, S.; Mosca, L.; Anzenbacher Jr., P. Toward Wearable Sensors: Optical Sensor for Detection of Ammonium Nitrate-Based Explosives, ANFO and ANNM. Chem. Commun. 2017, 53, 5196-5199. (27) Kartha, K. K.; Babu, S. S.; Srinivasan, S.; Ajayaghosh, A. Attogram Sensing of Trinitrotoluene with a Self-Assembled Molecular Gelator. J. Am. Chem. Soc. 2012, 134, 48344841. (28) Kartha, K. K.; Sandeep, A.; Praveen, V. K.; Ajayaghosh, A. Detection of Nitroaromatic Explosives with Fluorescent Molecular Assemblies and π-Gels. Chem. Rec. 2015, 15, 252-265.



Page 22 of 26

(29) Philips, D. S.; Ghosh, S.; Sudheesh, K. V.; Suresh, C. H.; Ajayaghosh, A. An Unsymmetrical Squaraine-Dye-Based Chemical Platform for Multiple Analyte Recognition. Eur. Respir. J. 2017, 23, 17973-17980. (30) Dasary, S. S.; Singh, A. K.; Senapati, D.; Yu, H.; Ray, P. C. Gold Nanoparticle Based Label-Free Sers Probe for Ultrasensitive and Selective Detection of Trinitrotoluene. J. Am. Chem. Soc. 2009, 131, 13806-13812. (31) Ular, N.; Üzer, A.; Durmazel, S.; Erçağ, E.; Apak, R. DiaminocyclohexaneFunctionalized/Thioglycolic Acid-Modified Gold Nanoparticle-Based Colorimetric Sensing of Trinitrotoluene and Tetryl. ACS Sensors 2018, 3, 2335-2342. (32) Pablos, J. L.; Trigo-López, M.; Serna, F.; García, F. C.; García, J. M. Water-Soluble Polymers, Solid Polymer Membranes, and Coated Fibres as Smart Sensory Materials for the Naked Eye Detection and Quantification of Tnt in Aqueous Media. Chem. Commun. 2014, 50, 2484-2487. (33) Azhdari Tehrani, A.; Esrafili, L.; Abedi, S.; Morsali, A.; Carlucci, L.; Proserpio, D. M.; Wang, J.; Junk, P. C.; Liu, T. Urea Metal-Organic Frameworks for Nitro-Substituted Compounds Sensing. Inorg. Chem. 2017, 56, 1446-1454. (34) Hayes, R.; Warr, G. G.; Atkin, R. Structure and Nanostructure in Ionic Liquids. Chem. Rev. 2015, 115, 6357-6426. (35) Zhang, W.; Gao, N.; Cui, J.; Wang, C.; Wang, S.; Zhang, G.; Dong, X.; Zhang, D.; Li, G. AIE-Doped Poly (ionic liquid) Photonic Spheres: A Single Sphere-Based Customizable Sensing Platform for the Discrimination of Multi-Analytes. Chem. Sci. 2017, 8, 6281-6289. (36) Cui, J.; Zhu, W.; Gao, N.; Li, J.; Yang, H.; Jiang, Y.; Seidel, P.; Ravoo, B. J.; Li, G. Inverse Opal Spheres Based on Polyionic Liquids as Functional Microspheres with Tunable Optical Properties and Molecular Recognition Capabilities. Angew. Chem. Int. Ed. 2014, 53, 3844-3848.


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


(37) Giesche, H. Synthesis of Monodispersed Silica Powders I. Particle Properties and Reaction Kinetics. J. Eur. Ceram. Soc. 1994, 14, 189-204. (38) Giesche, H. Synthesis of Monodispersed Silica Powders II. Controlled Growth Reaction and Continuous Production Process. J. Eur. Ceram. Soc. 1994, 14, 205-214. (39) Hartlen, K. D.; Athanasopoulos, A. P. T.; Kitaev, V. Facile Preparation of Highly Monodisperse Small Silica Spheres (15 to >200 nm) Suitable for Colloidal Templating and Formation of Ordered Arrays. Langmuir 2008, 24, 1714-1720. (40) Zhao, Y.; Zhao, X.; Hu, J.; Li, J.; Xu, W.; Gu, Z. Multiplex Label-Free Detection of Biomolecules with an Imprinted Suspension Array. Angew. Chem. Int. Ed. 2009, 121, 7486-7488. (41) Ge, J.; Yin, Y. Responsive Photonic Crystals. Angew. Chem. Int. Ed. 2011, 50, 1492-1522. (42) Phillips, K. R.; Vogel, N.; Burgess, I. B.; Perry, C. C.; Aizenberg, J. Directional Wetting in Anisotropic Inverse Opals. Langmuir 2014, 30, 7615-7620. (43) Deng, J.; Chen, S.; Chen, J.; Ding, H.; Deng, D.; Xie, Z. Self-Reporting Colorimetric Analysis of Drug Release by Molecular Imprinted Structural Color Contact Lens. ACS Appl. Mater. Interfaces 2018, 10, 34611-34617. (44) Kim, T. H.; Lee, B. Y.; Jaworski, J.; Yokoyama, K.; Chung, W.-J.; Wang, E.; Hong, S.; Majumdar, A.; Lee, S.-W. Selective and Sensitive TNT Sensors Using Biomimetic Polydiacetylene-Coated CNT-FETs. ACS Nano 2011, 5, 2824-2830. (45) Lee, J. H.; Fan, B.; Samdin, T. D.; Monteiro, D. A.; Desai, M. S.; Scheideler, O.; Jin, H.E.; Kim, S.; Lee, S.-W. Phage-Based Structural Color Sensors and Their Pattern Recognition Sensing System. ACS Nano 2017, 11, 3632-3641.



Page 24 of 26

(46) England, G. T.; Russell, C.; Shirman, E.; Kay, T.; Vogel, N.; Aizenberg, J. The Optical Janus Effect: Asymmetric Structural Color Reflection Materials. Adv. Mater. 2017, 29, 16068761606881. (47) Kou, D.; Ma, W.; Zhang, S.; Lutkenhaus, J. L.; Tang, B. High-Performance and Multifunctional Colorimetric Humidity Sensors Based on Mesoporous Photonic Crystals and Nanogels. ACS Appl. Mater. Interfaces 2018, 10, 41645-41654. (48) Chen, J.; Liu, P.; Du, X.; Xie, Z. Clickable Colloidal Photonic Crystals for Structural Color Pattern. Langmuir 2018, 34, 13219-13224. (49) Meng, Y.; Qiu, J.; Wu, S.; Ju, B.; Zhang, S.; Tang, B. Biomimetic Structural Color Films with a Bilayer Inverse Heterostructure for Anticounterfeiting Applications. ACS Appl. Mater. Interfaces 2018, 10, 38459-38465. (50) Leng, Y.; Sun, K.; Chen, X.; Li, W. Suspension Arrays Based on Nanoparticle-Encoded Microspheres for High-Throughput Multiplexed Detection. Chem. Soc. Rev. 2015, 44, 5552-5595. (51) Phillips, K. R.; England, G. T.; Sunny, S.; Shirman, E.; Shirman, T.; Vogel, N.; Aizenberg, J. A Colloidoscope of Colloid-Based Porous Materials and Their Uses. Chem. Soc. Rev. 2016, 45, 281-322. (52) Ye, B.; Rong, F.; Gu, H.; Xie, Z.; Cheng, Y.; Zhao, Y.; Gu, Z. Bioinspired AngleIndependent Photonic Crystal Colorimetric Sensing. Chem. Commun. 2013, 49, 5331-5333. (53) Qin, M.; Huang, Y.; Li, Y.; Su, M.; Chen, B.; Sun, H.; Yong, P.; Ye, C.; Li, F.; Song, Y. A Rainbow Structural-Color Chip for Multisaccharide Recognition. Angew. Chem. Int. Ed. 2016, 55, 6911-6914. (54) Tian, E.; Wang, J.; Zheng, Y.; Song, Y.; Jiang, L.; Zhu, D. Colorful Humidity Sensitive Photonic Crystal Hydrogel. J. Mater. Chem. 2008, 18, 1116-1122.


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


(55) Ding, H.; Liu, C.; Ye, B.; Fu, F.; Wang, H.; Zhao, Y.; Gu, Z. Free-Standing Photonic Crystal Films with Gradient Structural Colors. ACS Appl. Mater. Interfaces 2016, 8, 6796-6801. (56) Braun, P. V. Colour without Colourants. Nature 2011, 472, 423-424. (57) Amemiya, S.; Bühlmann, P.; Umezawa, Y.; Jagessar, R. C.; Burns, D. H. An Ion-Selective Electrode for Acetate Based on a Urea-Functionalized Porphyrin as a Hydrogen-Bonding Ionophore. Anal. Chem. 1999, 71, 1049-1054. (58) Amendola, V.; Fabbrizzi, L.; Mosca, L. Anion Recognition by Hydrogen Bonding: UreaBased Receptors. Chem. Soc. Rev. 2010, 39, 3889-3915. (59) Busschaert, N.; Caltagirone, C.; Van Rossom, W.; Gale, P. A. Applications of Supramolecular Anion Recognition. Chem. Rev. 2015, 115, 8038-8155. (60) Anzenbacher Jr, P.; Lubal, P.; Buček, P.; Palacios, M. A.; Kozelkova, M. E. A Practical Approach to Optical Cross-Reactive Sensor Arrays. Chem. Soc. Rev. 2010, 39, 3954-3979. (61) Zhou, C.; Xu, W.; Zhang, P.; Jiang, M.; Chen, Y.; Kwok, R. T. K.; Lee, M. M. S.; Shan, G.; Qi, R.; Zhou, X.; Lam, J. W. Y.; Wang, S.; Tang, B. Z. Engineering Sensor Arrays Using Aggregation-Induced Emission Luminogens for Pathogen Identification. Adv. Funct. Mater. 2019, 29, 1805986-1805995.



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Table of Contents Graphic


Urea-Functionalized Poly(ionic liquid) Photonic Spheres for Visual

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