Hierarchical Assembly of Polydiacetylene MicrotubeBiosensors Mediated by Divalent Metal IonsHao Jiang and Raz Jelinek*[a]

Introduction

Polydiacetylenes (PDAs) are conjugated polymers that exhibitunique color and fluorescence transformations associated with

topo-polymerization of the diacetylene monomers.[1] The pho-tophysical properties of PDA systems also entail promising

chemo- and biosensing properties.[2] In particular, there has

been considerable interest in obtaining self-assembled nano-structures derived from diacetylene derivatives.[3] Indeed, PDA

systems have often displayed the spontaneous association ofmonomers into well-defined structures that are held together

by noncovalent interactions, thereby generating complexsupramolecular nanostructures.[4]

Functionalization of diacetylene monomers has been shown

to be a powerful tool for modulating self-assembly processes.[5]

Varied nanoscale architectures, including nanorods, nanowires,

nanotubes, and helical nanoribbons have been produced byemploying diacetylene derivatives that display different head

group substituents.[3b, 6] It is hypothesized that hydrogen bond-ing, hydrophobic interactions, and p–p stacking constitute theprimary driving forces of such self-assembly processes.

Herein, we demonstrate that a mixture of a carboxylic acid-displaying diacetylene monomer (10,12-tricosadyinoic acid, de-noted TrCDA) and a diacetylene monomer in which the headgroup was substituted with melamine (N-[2-(4,6-diamino-1,3,5-

triazin-2-ylamino)ethyl]tricosa-10,12-diynamide, denoted TNM;Figure 1A) formed extraordinary long microtubes upon addi-

tion of Zn2 + ions and slow evaporation. Formation of the mi-crotubes was based upon a hierarchical assembly process inwhich the initially produced vesicles assembled into elongated

nanotubes, which in turn further cross-linked into microscaletubular morphologies. Interestingly, the PDA microtubes exhib-

ited high-sensitivity fluorescence/color response in bacterialsolutions, thus pointing to possible sensing applications.

Results and Discussion

Figure 1 depicts the experimental scheme and images of the

microtubes. As outlined in Figure 1A, the PDA microtubeswere formed upon slow evaporation (to result in approximate-

ly 50 % water volume loss within 30 days) of an aqueous mix-ture of the two diacetylene monomers at a ratio of 9:1 (TrCDA/

TNM) and exposed to room lighting. Importantly, Zn2+ ions (at

a concentration of 2 ppm), which were added to the diacety-lene mixture one day after starting the incubation, had a critical

role in microtube assembly. The PDA microtubes slowly self-as-sembled in solution and appeared to the naked eye as bundles

of visible fibers within approximately 30 days (Figure 1B). ThePDA microtubes were flexible; the filament bundles shown in

Polydiacetylenes (PDAs) constitute a family of conjugated am-phiphilic polymers that display unique color and fluorescence

properties. Described is the hierarchical assembly of microme-

ter-sized tubes that comprise a melamine-substituted PDA de-

rivative. The nano- to microscale transformations of the amphi-philic building blocks were induced by addition of divalent

metal ions. The PDA microtubes facilitated the high-sensitivity

fluorescence detection of bacteria.

Figure 1. PDA microtubes. (A) Experimental scheme. The two monomerswere incubated for one day followed by the addition of Zn2 + ions, which in-duced self-assembly of the microtubes. (B) Photograph showing the visiblemicrotube bundles. (C) SEM images of the PDA microtubes; the two insetsshow magnified microtube areas. (D) Optical microscopy and (E) correspond-ing fluorescence microscopy images of the PDA microtubes. The magnifiedarea in the inset highlights the tubular organization.

[a] Dr. H. Jiang, Prof. R. JelinekDepartment of Chemistry and Ilse Katz Institute for NanotechnologyBen Gurion University of the NegevBeer Sheva 84105 (Israel)E-mail : razj@bgu.ac.il

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cplu.201500405.

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Figure 1B could be bent without fracturing while immersed inwater.

Figure 1C–E presents microscopy images that highlight themorphology and organization of the PDA microtubes. The

scanning electron microscopy (SEM) image in Figure 1C depictsthe abundant, extremely long microtubes that formed. Note

the macroscopic dimensionalities of the tubes, which exhibitthicknesses of around 3–5 mm and are several millimeters long

(thus making them visible to the eye). The insets in Figure 1C

show magnified areas within the microtube network, thus un-derscoring a uniform surface area (top inset in Figure 1C) and

the occurrence of “branching” within some of the microtubes(Figure 1C, bottom inset).

Optical microscopy and the corresponding fluorescence mi-croscopy images (Figure 1D, E, respectively) attest to the tubu-lar organization and confirm that the microtubes were indeed

composed of PDA. Specifically, the optical microscopy imagein Figure 1D clearly shows the thick edges of the microtubes.

The tubular organization was also apparent in the fluorescencemicroscopy image in Figure 1E (lex = 490 nm). The intense fluo-

rescence emission from the microtubes is due to the PDA scaf-fold, which is fluorescent in the purple/red phases.[7] Impor-

tantly, polymerization of the diacetylene network was likely in-

duced by residual light reaching the microtubes during the30 day incubation.

To elucidate the kinetic/structural details of microtube for-mation, we examined the diacetylene solution using SEM at

different times during incubation of the two monomers(Figure 2).

The SEM data underline the hierarchical assembly of the mi-

crotubes. Specifically, nanosized vesicular particles wereformed upon dissolving the monomers in water and before ad-

dition of Zn2 + (Figure 2A, panel denoted as “0 day”). Vesicle-like particulates have been observed in numerous supramolec-

ular PDA systems.[2b, 6a] Notably, six days after the addition ofZn2+ , thin nanofibers were observed (Figure 2B). The nanofib-

ers grew longer and became the predominant morphology

after 12 days (Figure 2C).A dramatic morphological transformation of the diacetylene

assemblies was apparent 18 days after the addition of Zn2 +

(Figure 2D). Specifically, the SEM image in Figure 2D revealsthat the individual nanofibers coalesced into a condensed net-work, thereby giving rise to the formation of a microscale elon-

gated filament. The magnified area shown in the inset (Fig-ure 2D) illuminates the cross-linked organization of the diacety-lene nanofibers that comprise the scaffolding of the microscalefilament. Additional SEM and transmission electron microscopy(TEM) images in Figure S1 of the Supporting Information attest

to the cross-linked network. AFM analysis indicates that the fil-aments exhibited thicknesses of around 200 nm (Figure S2 in

the Supporting Information). The diacetylene filament becamesignificantly denser, thicker, and rounded after 24 days (Fig-ure 2E and Figure S2); it adopted a tubular morphology

30 days after the addition of the Zn2 + ions (Figure 2F).To further explore the mechanism of PDA microtube forma-

tion, we compared the Fourier transform infrared (FTIR) spectraof the microtubes (formed after 30 days of incubation) and the

vesicular nanoparticles assembled initially in the aqueous solu-tion before the addition of the Zn2 + ions (Figure 3A). Inspec-

tion of the spectra reveals significant differences in peak posi-tions and intensities associated with the melamine headgroupof TNM, whereas no changes were recorded for the backbone

spectral features. Specifically, Figure 3A demonstrates that thevibration band that corresponds to the C=O unit was shiftedfrom 1682 cm¢1 in the vesicles to 1658 (amide I in TNM) and1608 cm¢1 (uCOO¢ in TrCDA) in the microtubes sample.[8] Simi-larly, the C=N band in the vesicles sample was shifted from1657 to 1695 cm¢1.[9] Moreover, the spectral shifts that occur

upon the transformation from vesicles to microtubes were ac-companied by significant attenuation of the intensities of thevibration bands (Figure 3A). In contrast to melamine spectral

shifts, the peaks at 2919 and 2850 cm¢1, assigned to asymmet-ric and symmetric vibrations, respectively, of the CH2 group

within the alkyl chain,[10] did not exhibit significant differencesbetween vesicles and microtubes, thus indicating similar struc-

tural features in the alkyl backbones of the diacetylene units in

vesicles and microtubes.Figure 3B depicts a proposed assembly route based upon

the microscopy and spectroscopy data in Figures 2 and 3.Nanosized vesicles are initially formed upon dispersing TrCDA

and TNM in water. We hypothesize that the addition of diva-lent metal cations and the gradual increase in concentration of

Figure 2. Formation of PDA microtubes. SEM images showing the kinetic/structural details of microtube formation. Images were taken in the indicat-ed times after addition of Zn2 + ions; the image in (A) denoted as “0 day”was recorded one day after incubating the two monomers in water andprior to the addition of Zn2+ . The scale bar of the magnified SEM imageshowing the cross-linked organization of the diacetylene nanofibers in (D)corresponds to 500 nm.

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the diacetylene vesicles (brought about through slow water

evaporation) are both critical parameters in the assembly pro-cess. The zinc ions, in particular, mediate formation of the

longer-range diacetylene assemblies through cooperative inter-actions with the nitrogen and oxygen atoms within the mela-

mine headgroup, as well as interactions with the carboxylicacid moieties of PDA.[11] These interactions are pivotal in thetransformations of vesicles to nanofibers (e.g. , Figure 2B–C),

nanofibers to microscale filaments (Figure 2D), and filamentsto microtubes (Figure 2E). Indeed, the gradual increase in re-agent concentrations (induced by water evaporation) likelypromoted formation of the interlinked diacetylene networks

that comprise the microtubes (see, for example, the SEMimages in Figure 2D–F) as the self-assembled diacetylene

building blocks (vesicles, nanofibers) become more abundant

and the Zn2 + linking among them becomes more feasible.The FTIR results in Figure 3A and SEM analysis in Figure 2

attest to the critical role of the Zn2 + cations in the hierarchicalassembly process. Notably, microtubes were not formed in

TrCDA/TNM solutions to which Zn2+ was not added (Fig-ure 4A). Similar disruption of microtube formation was appar-

ent when a buffer solution rather than distilled water was used

for incubation, as buffers contain different ionic species that in-terfere with the assembly process. It should be also empha-

sized that long microtubes similar in appearance to the onesdepicted in Figures 2 and 3 were observed when Pb2 + ions in-

stead of Zn2+ were added to the diacetylene monomer solu-tion (Figure 4B). Moreover, the addition of higher concentra-

tions of Zn2 + to the diacetylene mixture (for example 5 ratherthan 2 ppm) prevented microtube assembly and instead result-

ed in aggregate formation (data not shown). These observa-

tions confirm the prominent role of divalent metal cations inpromoting the assembly of the PDA microtubes.

The significance of Zn2 + interactions with the melaminemoieties is further reflected by the dependence of microtube

formation upon the molar ratio between TrCDA and TNM. Infact, the 9:1 molar ratio (TrCDA:TNM) was found to be critical

for microtube assembly; a 4:1 molar ratio, for example, result-

ed in unstable vesicle solution, likely due to aggregation in-duced by the relatively abundant melamine. Similarly, when

vesicle aggregation was observed when we used a 7:1 molarratio.

PDA systems have been widely used as biosensing plat-forms.[1a, d, 2b, 12] Accordingly, we examined application of the

newly produced PDA microtubes as conduits for bacterial de-

tection (Figure 5). Previous studies have shown that PDA as-semblies—vesicles, thin films, PDA/gel matrixes—undergo

chromatic transformations induced by bacterial cells and mole-cules secreted by bacteria to their environments.[13] In the ex-

periment depicted in Figure 5 we incubated the PDA micro-tubes with different concentrations of Pseudomonas aerugino-sa, a widely studied Gram negative bacterial strain, and record-

ed fluorescence microscopy images of the microtubes. Impor-tantly, the fluorescence emission threshold in the microscopyimages in Figure 5 was set to display no fluorescence signalfrom the control samples (i.e. , no fluorescence in the case of

microtubes incubated with a solution not containing bacterialcells).

The fluorescence microscopy images in Figure 5 reveal

a direct correlation between bacterial concentration and theintensity of fluorescence signals. This relationship confirms that

the fluorescence transformations of the PDA fibers wereindeed induced by bacterial cells and/or bacterially secreted

substances. However, the significantly low detection thresholdobtained—fewer than 103 cells mL¢1—is quite striking as it is

orders of magnitude smaller than other reports that utilized

PDA for bacterial detection,[2b, 12] and also relative to other fluo-rescence-based bacterial detection assays.[14] The high sensitivi-

ty might be ascribed to the unique ion-mediated interlinkedPDA network that results in the induction of chromatic

changes even upon small external stimuli by analytes. Theblue-to-red transition of the microtubes following incubation

Figure 3. Hierarchical assembly of the microtubes. (A) FTIR spectra of TrCDA/TNM vesicles (black spectrum) and microtubes (red spectrum). (B) Proposedmodel of Zn2 +-induced microtube formation. The Zn2 + ion mediates forma-tion of nanofibers, filaments, and microtubes by cross-linking the melamineresidues of TNM.

Figure 4. Effect of divalent cations. The morphologies of TrCDA/TNM solu-tions (A) without addition of metal ions and (B) following addition of Pb2 +

(2 ppm).

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with bacteria could also be demonstrated by the Raman spec-

tra (Figure 6). The presence of the peak at 1517 cm¢1 is associ-ated with the red PDA phase, and the emergence of this peak

following incubation with the bacteria (Figure 6B) confirms theinduction of the structural transformation of PDA by interac-tions with the bacteria.

Conclusion

This study demonstrates the formation of microscale tubular

PDA morphologies that are induced by the addition of Zn2 +

ions and complemented by slow water evaporation in a mix-ture that comprises two diacetylene monomers, one of whichdisplays a melamine headgroup. The experimental data showthat the addition of Zn2 + ions induced a gradual hierarchicaltransformation of vesicles to nanofibers to microfilaments tomicrotubes that occurs over a 30 day incubation period. The

polydiacetylene microtubes exhibit useful biosensing proper-ties and undergo fluorescence transformations in the presence

of bacterial cells in low concentrations. In a broader contextthis study demonstrates that the hierarchical assembly of am-

phiphilic substances such as diacetylene monomers can be

modulated through interactions between their functional headgroups and ionic species in solution.

Experimental Section

Materials

The diacetylene monomer 10,12-tricosadiynoic acid (TrCDA) waspurchased from Alfa Aesar. 1,2-Ethylenediamine, N-hydroxysuccini-mide (NHS), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydro-chloride (EDC), 2-chloro-4,6-diamino-1,3,5-triazine, and anhydroussodium hydrogen carbonate were purchased from Sigma–Aldrichand were used as received.

The synthesis of N-(2-aminoethyl)tricosa-10,12-diynamide (TEA) wascarried out according to published methods.[15] NHS (3.0 mmol)was added to a solution of TrCDA (2.7 mmol) in CH2Cl2 (10 mL) fol-lowed by the addition of EDC (3.1 mmol). The solution was stirredat room temperature for two hours followed by rotary evaporationof the CH2Cl2. The residue was extracted with diethyl ether andwater. The organic layer was dried with MgSO4, filtered, and thesolvent was removed by rotary evaporation to give a white solid.The residue (1.28 mmol) in CH2Cl2 (20 mL) was added to a solutionof 1,2-ethylenediamine (6.60 mmol) in CH2Cl2 (25 mL) dropwise,with stirring, over a period of 30 min. The reaction was stirred foran additional 30 min before removal of the solvent by rotary evap-oration. The residue was dissolved in ethyl acetate and extractedtwice with water; the organic layer was dried with MgSO4, and thesolvent was removed by rotary evaporation. The extract was puri-fied by means of silica gel chromatography (20:1 CHCl3/MeOH) togive TEA as a white solid.

The synthesis of N-[2-(4,6-diamino-1,3,5-triazin-2-ylamino)ethyl]tri-cosa-10,12-diynamide (TNM) was carried out according to pub-lished methods.[16] A mixture of 2-chloro-4,6-diamino-1,3,5-triazine(2 mmol), TEA (2 mmol), and anhydrous sodium hydrogen carbon-ate (2 mmol) in 1,4-dioxane (20 mL) was heated to reflux under anargon atmosphere for 6 h. The reaction mixture was then cooledand poured into water (100 mL). The precipitate was removed byfiltration and washed with water. The product was purified bysilica-gel chromatography and eluted with a dichloromethane/methanol 20:1 mixture.

Figure 5. Bacterial detection with the PDA microtubes. Fluorescence micros-copy images in bright field (left) and fluorescence mode (right) of the PDAmicrotubes induced by solutions containing P. aeruginosa in different con-centrations: (A) 105 cells mL¢1, (B) 104 cells mL¢1, (C) 103 cells mL¢1,(D) 102 cells mL¢1. Scale bars correspond to 5 mm.

Figure 6. Raman spectra of PDA microtubes. (A) As-prepared microtubesafter 30 day incubation. (B) After induction of the red PDA phase followingincubation with bacteria.

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Preparation of TNM/TrCDA hybrid vesicles

TNM and TrCDA monomers with a molar ratio of 1:9 (a totalamount of 5.2 mg) were dissolved in a minimum amount of etha-nol and then injected into deionized water (200 mL). Then the mix-ture was sonicated for 60 min above 75 8C to form a clear solution.The solution was cooled to room temperature and conserved at4 8C.

Water-evaporation-induced nanovesicle-to-microtube transi-tion

The vesicle solution was placed into 20 mL vials, and 2 ppm Zn2 +

ions were added to the above solution. The vials were open toallow for water evaporation and kept at room temperature. A fila-mentous suspension could be seen with the naked eye after30 days. We did not apply direct irradiation but rather left the vialopen to residual lighting, which induced polymerization of the di-acetylene monomers over the 30 day incubation period. Indeed,this polymerization process particularly over the long (30 day) incu-bation period is difficult to control. However, we found that thevariation in residual polymerization during this incubation periodhad a much less significant effect upon microtube formation andmorphology.

Scanning electron microscopy

For SEM analysis, samples with different aging time starting fromthe vesicle solution with 2 ppm Zn2 + ions were drop-cast onto sili-con wafers. After air-drying, the dehydrated specimens were fur-ther desiccated at 25 8C for 24 h and coasted with gold in an EMI-TECH K575x (Emitech Ltd, UK) sputtering device. SEM images wererecorded using a Jeol JSM-7400F scanning electron microscope(JEOL LTD, Tokyo, Japan) that was operated and analyzed using theinstrument software.

Bacterial growth

Bacteria (Pseudomonas aeruginosa) were grown aerobically at 37 8Cin a sterilized solid Luria Bertani (LB) medium composed of 13.5 %yeast extract, 27 % peptone, 27 % NaCl, and 32.5 % agar at pH 7.4.After overnight growth, a colony from each bacterial strain wastaken and added to sterilized LB growth medium (10 mL). Bacterialconcentrations in the solutions were measured by means of UV/Visspectroscopy (600 nm). The bacteria solutions were centrifugedand spread into the same amount of water for the bacteria sensingexperiments.

Fluorescence microscopy

Fluorescence images of the PDA microtubes were obtained withan Olympus IX70 microscope (Japan) equipped with a Roper Scien-tific Inc. MicroMAX camera. The experiments employed a narrow-band filter cube UPlanFI, excitation (470–490 nm), beam splitter(505 nm), and outgoing filter (510 nm). The PDA microtubes wereplaced onto a glass slide and dried in the darkness at room tem-perature. Then the PDA microtubes were subjected to fluorescencemicroscopy for bacteria sensing. The in situ fluorescence images ofthe PDA microtubes before and after adding bacteria solutionswith different concentrations (102–105 cells ml¢1) were obtainedwith a response time of 30 min.

Fourier-transform infrared (FTIR) microscopy

PDA microtubes were placed onto gold-coated glass, and the FTIRspectrum was collected with a Nicolet iN10 FTIR microscope MXspectrometer (Thermo Scientific) fitted with a narrow-band liquid-nitrogen-cooled MCT detector. The microscope was operated inthe reflectance mode. In all cases, the incident infrared beam wasfocused at a sample surface. All single-beam spectra were mea-surement against a background recorded from the reflectance offa gold-coated disk. Appropriate backgrounds were obtained foreach series of measurements. The spectra were recorded in thewavenumber range of 700–4500 cm¢1.

Raman measurements

Raman spectra were recorded with a Jobin–Yvon LabRam HR 800micro-Raman system equipped with a liquid-nitrogen-cooled de-tector. The excitation source was an Argon laser (514 nm) witha power of 5 mW on the sample. To protect the samples the laserpower was reduced by 1000 using ND filters. The laser was focusedwith 100 Õ long-focal-length objective to a spot of about 4 mm.Measurements were taken with the 600 grating per mm and a mi-croscope confocal hole setting of 100 mm with a typical exposuretime of 1 min.

Transmission electron microscopy (TEM)

TEM experiments were carried out with an FEI Tecnai 12 G2 TWINTEM at an acceleration voltage of 120 kV. Images were recorded ona 1k Õ 1k CCD camera (Gatan model 794). The samples were pre-pared on 400-mesh copper formvar/carbon grids by drop-castingthe vesicle solution with 2 ppm Zn2 + ions after incubation of18 days.

Acknowledgements

We are grateful to the Israel–US BARD Foundation for generous

financial support.

Keywords: amphiphiles · polymers · self-assembly · sensors ·zinc

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Manuscript received: September 9, 2015Revised: October 7, 2015Accepted Article published: October 9, 2015Final Article published: October 30, 2015

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