Microporous Porphyrin Networks Mimicking a Velvet Worm Surfaceand Their Enhanced Sensitivities toward Hydrogen Chloride andAmmoniaSang Hyun Ryu,†,‡ Chang Wan Kang,†,‡ Jaewon Choi,† Yoon Myung,§ Yoon-Joo Ko,⊥ Sang Moon Lee,∥

Hae Jin Kim,∥ and Seung Uk Son*,†

†Department of Chemistry, Sungkyunkwan University, Suwon 16419, Korea§Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 05006, Korea⊥Laboratory of Nuclear Magnetic Resonance, The National Center for Inter-University Research Facilities, Seoul National University,Seoul 08826, Korea∥Korea Basic Science Institute, Daejeon 34133, Korea

*S Supporting Information

ABSTRACT: This work shows that the functions of micro-porous organic network materials can be enhanced throughengineering of the material structure. Mimicking the surfacestructure of velvet worms, we prepared the aligned 1Dstructure (rod) of microporous porphyrin networks by theSonogashira coupling of tetrakis(4-ethynylphenyl)porphyrinwith 1,4-diiodobenzene in an anodic aluminum oxide plate.The length of the 1D structure was controlled in the range of1−5 μm. The velvet worm surface-like microporous porphyrinnetworks (Velvet-MPNs) showed higher sensitivities tohydrogen chloride and ammonia gases by up to ∼14 and 4.6 times, respectively, compared with a control MPN materialwithout rods.

KEYWORDS: microporous organic polymer, Sonogashira, porphyrin, anodic aluminum oxide, sensing

Sensing is a critical function of living organisms for theirsurvival. Living organisms have adopted intriguing nano/

microsized structures for efficient sensing. One of the moststudied living organisms for their defensive performance is thevelvet worm.1 The surface of velvet worms consists of villi thatare sensitive to changes of the chemical surroundings (Figure1a).When any sign of natural enemies touches the aligned

bristles, the velvet worms recognize the situation and attack theenemies using chemical weapons. Mimicking the intriguingsurface structure of velvet worms and understanding its sensingperformance are valuable endeavors in the research anddevelopment of artificial2 systems such as robots. Our researchgroup has studied the construction of nano/microsizedmolecular structures and their functions.3−5 For example, wehave built octopus leglike structures that show an adhesiveperformance on the surface of solid supports.5

Recently, microporous organic networks (MONs) have beenprepared by the coupling of various organic building blocks.6,7

For example, Sonogashira coupling of multiethynyl arenes withmultihalo arenes resulted in various MONs.8 In addition, MONfilms have been engineered on the surface.9−11 We have shownthat the chemical performance of MONs is dependent on theirmorphological structure.4,12

In this work, we report the preparation of velvet wormsurface-like aligned 1D structures of microporous porphyrinnetworks (Velvet-MPNs) using anodic aluminum oxide (AAO)templates and their sensing behavior toward hydrogen chloride(HCl) and ammonia (NH3). As far as we are aware, MON-based porous sensing materials with an aligned 1D structurewere not reported.13

Figure 1b shows a synthetic scheme for Velvet-MPNs. AAOplates with a pore diameter of 300 nm and pore depths of 1, 2,and 5 μm were used as templates. When we used AAO plateswith a pore diameter of 40 nm, building blocks could not beincorporated into pores during a networking reaction (refer tothe corresponding SEM images in Figure S1). When we usedAAO plates with a pore diameter of 160 nm, the obtained MPNrods were too thin to stand (refer to the corresponding SEMimages in Figure S1). Thus, we used AAO plates with a porediameter of 300 nm. The MPNs filled the pores and thenformed a thin film outside the AAO plate by Sonogashiracoupling of tetrakis(4-ethynylphenyl)porphyrin with 2 equiv of1,4-diiodobenzene using (PPh3)2PdCl2 and CuI catalysts. The

Received: December 16, 2017Accepted: February 14, 2018Published: February 14, 2018

Letter

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© 2018 American Chemical Society 6815 DOI: 10.1021/acsami.7b19119ACS Appl. Mater. Interfaces 2018, 10, 6815−6819

etching of AAO plates with phosphoric acid resulted in alignedMPN rods on the MPN thin film (Velvet-MPNs). The Velvet-MPNs could be transferred onto a poly(ethylene terephthalate)(PET) film.The morphologies of Velvet-MPNs were investigated by

scanning electron microscopy (SEM; Figure 2). As shown inFigures 2 and S2, the MPNs uniformly replicated AAOtemplates to reveal the homogeneous distribution of MPN rodson the MPN thin film. The bottom view of Velvet-MPNsshowed a flat morphology without MPN rods (Figure S3). Asthe pore depth of the AAO templates increased from 1 μm to 2and 5 μm, the lengths of the MPN rods in Velvet-MPNsincreased from 1 μm to 2 and 5 μm, maintaining theirdiameters of ∼300 nm (refer to the side views of Velvet-MPNmaterials in Figure S4). The corresponding Velvet-MPNs weredenoted as Velvet-MPN-1, Velvet-MPN-2, and Velvet-MPN-5,respectively, in this work. The MPN rods in Velvet-MPN-1 andVelvet-MPN-2 overally stood well (Figure 2a−f). In compar-ison, Velvet-MPN-5 showed significant local aggregation of theend parts of the MPN rods because of the relatively high aspectratio of the rods (Figure 2g−i).To study the properties of Velvet-MPNs, we prepared a

MPN thin film without rods (C-MPN) as a control materialusing conventional thin-layer chromatography (TLC; Figure

S5a). The MPN was formed on the surface of a TLC plate bySonogashira coupling of tetrakis(4-ethynylphenyl)porphyrinwith 2 equiv of 1,4-diiodobenzene. The TLC plate in MPN/TLC could be removed easily by the treatment of a HFsolution. The thickness of C-MPN was controlled to match theQ absorption band intensity at 658 nm of Velvet-MPN-2 (videinfra) by screening the amount of building blocks and reactiontimes. As shown in Figure S5b, C-MPN had an overall flatsurface and ∼1.5 μm thickness.Porphyrin moieties in the materials can be characterized by

the Q bands in the UV−vis absorption spectroscopy.14

Especially, the Q band at 658 nm is a unique absorptionpeak of metal-free porphyrins and is sensitive to changes of thechemical surroundings of porphyrin rings. As the lengths of theMPN rods in Velvet-MPN-1, Velvet-MPN-2, and Velvet-MPN-5 increased, the absorbance of the Q band at 658 nm graduallyincreased from 0.44 to 0.88 and 1.26 (Figure 3a).In the case of Velvet-MPN-5, an additional absorption band

appeared at 700 nm, which originates from the J-typeaggregation of porphyrin materials.15 As described above, theQ-band intensity at 658 nm of C-MPN is nearly the same asthat of Velvet-MPN-2. The chemical components of Velvet-MPNs and C-MPN were further characterized by solid-state13C NMR spectroscopy. As shown in Figure 3b, the 13C peaksof alkynes appeared at 75−90 ppm. The 13C peaks of phenylrings appeared at 119, 130, and 142 ppm, respectively. The 13Cpeaks of porphyrin rings appeared at 119, 138, and 153 ppm,matching well with those in the literature.4,16

IR absorption spectroscopy of Velvet-MPNs and C-MPNshowed N−H vibration peaks at 3300 cm−1 (stretching) and970 cm−1 (bending) and no significant peaks at 1000−1010cm−1 (metal−N vibrations of metal porphyrins), the features ofwhich match well with the IR absorption spectra of metal-freeporphyrins in the literature16,17 (Figure S6). These character-ization data indicate that Velvet-MPNs and C-MPN have nearly

Figure 1. (a) Photograph (reproduced with permission from MelvynYeo) and a cartoon of the velvet worm surface structure and (b) asynthetic scheme for Velvet-MPNs on a PET film based on templatesynthesis using AAO plates.

Figure 2. SEM images of (a−c) Velvet-MPN-1, (d−f) Velvet-MPN-2,and (g−i) Velvet-MPN-5 (see Figures S2−S4 for enlarged SEMimages and bottom and side views of Velvet-MPNs, respectively).

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the same chemical structure. According to elemental analysis(N contents: 6.06, 6.01, 5.95, and 6.43 wt %,), the amounts ofporphyrins in Velvet-MPN-1, Velvet-MPN-2, Velvet-MPN-5,and C-MPN were calculated as 1.08, 1.07, 1.06, and 1.15 mmolg−1, respectively.According to analysis of the N2 isotherm curves based on the

Brunauer−Emmett−Teller (BET) theory, as the lengths ofMPN rods in Velvet-MPNs increased, the surface areasincreased from 249 m2 g−1 (Velvet-MPN-1) to 326 m2 g−1

(Velvet-MPN-2) and 528 m2 g−1 (Velvet-MPN-5), respectively

(Figure 3c). Among the studied materials, C-MPN showed thelowest surface area of 155 m2 g−1. These results indicate thatthe rod parts have higher porosity than the flat film, possiblybecause of more facile π−π stacking of the porphyrin networksin the film. The micropore volumes (Vmic) also graduallyincreased from 0.05 cm3 g−1 (C-MPN) to 0.09 cm3 g−1 (Velvet-MPN-1), 0.11 cm3 g−1 (Velvet-MPN-2), and 0.16 cm3 g−1

(Velvet-MPN-5), respectively. The pore-size distributiondiagrams obtained by the density functional theory (DFT)method revealed the microporosities of Velvet-MPNs and C-MPN. While porphyrins are very versatile and functionalmolecules, they are subject to facile packing because of theintrinsic planar molecular geometry, resulting in the blocking offunctional porphyrin sites. Thus, the networking-inducedmicroporosities of Velvet-MPNs and C-MPN can enhancethe functionalities of porphyrins. The powder X-ray diffraction(PXRD) studies showed amorphous characteristics of all MPNmaterials, which is the common property of MONs prepared bythe Sonogashira coupling of organic building blocks8 (FigureS7). Thermogravimetric analysis (TGA) showed that Velvet-MPNs and C-MPN are thermally stable up to ∼195 °C (FigureS8).Because living organisms have utilized porphyrins for various

functions, artificial porphyrin systems have also beenextensively synthesized and applied.18,19 Metal-free porphyrinrings have reactivity toward acid because of the basic pyrrolerings.20−24 Also, the acidified metal-free porphyrins can beneutralized by the reaction with bases such as NH3.

25−27 Theporphyrin rings are sensitive to changes of the chemicalsurroundings, followed by changes of the optical proper-ties.18−27 To understand any beneficial behavior of the velvetworm surface-like structure of Velvet-MPNs, we studied theirsensing performance toward HCl and NH3 gases (Scheme S1).Figure 4 and Table S1 summarize the results.Through the reaction of Velvet-MPNs with HCl, a new

absorption band appeared at 691 nm, which is attributed to theformation of acidified pyrrolinium salts (Figure 4a). Unfortu-nately, the sensing performance of Velvet-MPN-5 could not bestudied because of the additional absorbance at 700 nm evenbefore the reaction with HCl.15 Velvet-MPN-2 with longerMPN rods was generally more sensitive than Velvet-MPN-1(Figure 4b). While Velvet-MPN-1 under 50 ppm of HClshowed 56% of the ΔA observed at 500 ppm of HCl, Velvet-MPN-2 under 21 ppm of HCl showed 55% of the ΔA observedat 500 ppm of HCl. Velvet-MPN-1 and Velvet-MPN-2 showedinitial Δ(ΔA)/ΔC (ΔA change at 691 nm)/(concentrationchange of HCl in ppm units) values of 7.5 × 10−3 ppm−1 (R2 =0.99; standard deviation = 2.4 × 10−4) and 0.019 ppm−1 (R2 =0.99; standard deviation = 3.2 × 10−4) toward HCl,respectively. The limits of detection (LODs)28 of Velvet-MPN-1 and Velvet-MPN-2 for HCl were calculated as 0.097and 0.050 ppm, respectively (Figure S9). It is worth noting thatthe recent porphyrin materials developed for the optical sensingof HCl showed LODs in the range of 6.7−0.1 ppm20−24 (TableS2).The control C-MPN showed an initial Δ(ΔA) (at 691 nm)/

ΔC (ppm) value of 3.5 × 10−3 ppm−1 (R2 = 0.98; standarddeviation = 8.0 × 10−4) and a LOD of 0.69 ppm toward HCl.These observations indicate that the 1D structure of Velvet-MPNs (especially, Velvet-MPN-2 with longer rods) is muchmore sensitive in the sensing performance than C-MPN by upto ∼14 times, possibly because of the enhanced surface area,the enhanced utilization of porphyrin sites, and the resulting

Figure 3. (a) UV−vis absorption spectra and photographs. (b) Solid-state 13C NMR spectra. (c) N2 adsorption−desorption isothermcurves at 77 K. Inset: Pore-size distribution diagrams (based on theDFT method) of Velvet-MPNs and C-MPN.

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more efficient contract of small molecules with sensing speciesin the Velvet-MPNs.NH3 is a useful but toxic gas. It is known that humans can

smell NH3 at concentrations higher than 50 ppm.29 Interna-tional environmental departments such as the U.S. Occupa-tional Safety and Health Administration (OSHA) set apermissible exposure limit of 50 ppm for NH3 and regulatethe working time (8 h day−1).29,30 Thus, the sensing of NH3 atconcentrations lower than 50 ppm is important.The reaction of the salt forms of Velvet-MPNs with NH3

regenerated the original metal-free porphyrin moieties (Figure4a). The sensitivities of Velvet-MPNs toward NH3 were muchhigher than those toward HCl (Figure 4c). Velvet-MPN-1 andVelvet-MPN-2 were very sensitive toward NH3 and showed ΔA

values of 0.15 and 0.25, respectively, even at 0.76 ppm of NH3.These ΔA values are 55 and 53% of the ΔA values observed at200 ppm of NH3. Moreover, Velvet-MPN-1 and Velvet-MPN-2showed ΔA values of 0.24 and 0.44 at 10 ppm of NH3,respectively. These ΔA values are 89 and 92% of the ΔA valuesobserved at 200 ppm of NH3. Velvet-MPN-1 and Velvet-MPN-2 showed initial Δ(ΔA) (at 691 nm)/ΔC (ppm) values of−0.086 ppm−1 (R2 = 0.98; standard deviation = 4.0 × 10−3) and−0.21 ppm−1 (R2 = 0.99; standard deviation = 4.7 × 10−3)toward NH3, respectively. The LODs of Velvet-MPN-1 andVelvet-MPN-2 for NH3 were calculated as 0.14 and 0.070 ppm,respectively (Figure S9). In comparison, the control C-MPNshowed an initial Δ(ΔA) (at 691 nm)/ΔC (ppm) value of−0.018 ppm−1 (R2 = 0.90; standard deviation = 1.9 × 10−3) andLOD of 0.32 ppm toward NH3, indicating that the Velvet-MPNs are more sensitive toward NH3 than C-MPN by up to∼4.6 times. In the literature,25−27 the LODs of recentporphyrin materials for the optical sensing of NH3 werereported in the range of 7−0.16 ppm (Table S2).The repeatability of the sensing performance was inves-

tigated. As shown in Figure 4d,e, Velvet-MPN-1 and Velvet-MPN-2 showed good repeatability in the five successive sensingcycles of HCl (50 ppm) and NH3 (50 ppm). According to SEManalysis of the Velvet-MPNs recovered after five sensing cycles,the aligned 1D morphology was completely retained (FigureS10).In the literature (refer to Table S2), most porphyrin-

incorporated materials20,22,24−27 for the optical sensing of HCland NH3 were fabricated through the mixing or grafting ofsingle molecular porphyrins with organic/inorganic polymermatrixes, resulting in nonporous materials. In these cases, itmight be difficult to utilize the inner porphyrin moieties in thesensing. Recently, Wu et al.21,23 reported the fabrication ofnanostructured porphyrin polyimide materials, showing a LODof 5 ppm for HCl. In comparison, the enhanced sensitivities(LODs of 0.050 and 0.070 ppm for HCl and NH3, respectively)of Velvet-MPN-2 in this work are attributable to its 1Dnanostructure and inner microporosity.In conclusion, this work shows that MON materials can be

applied to the construction of nano/microsized molecularstructures. By template synthesis, porphyrin network materialscould be engineered to the aligned rods biomimicking thesurface structure of velvet worms. The aspect ratios of MPNrods could be controlled by changing the AAO templates. TheVelvet-MPNs showed much higher sensitivities than C-MPN toHCl and NH3 by ∼14 and ∼4.6 times, respectively, because ofthe efficient contact of sensing species with gases (Figure S11).Velvet-MPN-2 with longer MPN rods showed better sensitivitythan Velvet-MPN-1. Especially, Velvet-MPN-2 showed a ΔA(at 691 nm) of 0.25 at 0.76 ppm of NH3, an initial Δ(ΔA)/ΔCvalue of −0.21 ppm−1, and a LOD of 0.070 ppm for NH3. Webelieve that a variety of functional materials with an aligned 1Dstructure can be engineered by the synthetic strategy in thiswork.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.7b19119.

Experimental procedures, SEM images of MPN materialsprepared by various AAO templates, a scheme for controlC-MPN, IR spectra, PXRD patterns, TGA curves of

Figure 4. (a) Changes of the UV−vis absorption spectra of Velvet-MPN-2 by reaction with HCl and NH3 gases. Changes of theabsorbance at 691 nm (the average values of three independentsamples) of Velvet-MPN-1, Velvet-MPN-2, and C-MPN by reactionwith (b) HCl and (c) NH3 gases. Repeatability (the retention of ΔA at691 nm upon repeated exposures to 50 ppm of HCl and 50 ppm ofNH3) of the sensing performance of (d) Velvet-MPN-1 and (e)Velvet-MPN-2.

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Velvet-MPNs and C-MPN, SEM images of Velvet-MPNsrecovered after sensing cycles, and additional analysis ofthe sensing performance (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: sson@skku.edu.ORCIDYoon Myung: 0000-0002-5774-6183Seung Uk Son: 0000-0002-4779-9302Author Contributions‡These authors contributed equally.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Basic Science ResearchProgram (2016R1E1A1A01941074) through the NationalResearch Foundation of Korea funded by the Ministry ofScience, ICT and Future Planning.

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