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Polydiacetylene Nanotubes (PDNTs): Chemistry, Structures and Properties Polydiacetylene nanotubes (PDNTs) are self-assembled diacetylene nanotubes comprised of cross-linking of conjugated double and triple bonds. These PDNTs have remarkable properties which include uniform dimensional structures and responsive chromic characteristics that can be utilized as is, within a solution, or self-assembled onto various substrates, such as silica, polymer, and metal surfaces. PDNTs have been of great interest to chemists and biologists from universities, pharmaceutical companies, paint/coating companies, and opto/electronic companies in need of cheap, pure, predictable and well-defined lipids and chromatic nanotubes. ### Polydiacetylenes (PDA) Polydiacetylenes (PDA) are conjugated polymers produced by UV (254 nm) free radical polymerization of lipid diacetylene monomers (__Figure 1__) that have self-assembling property, and their microstructures were first reported by Yager et al. in 1984.1 For polymerization, diacetylene monomers need to be well aligned, which is achieved by molecular self-assembly of the diacetylene monomers. The self-assembly process often generates interesting nanostructures, such as nano- and micro- tubes and micelles of lipidic diacetylenes.2-8 Various attempts have been made to control diameters and lengths of nanotubes by changing chemical modifications and fabrication methods of amphiphilic diacetylene lipids. Fabricating well-defined diacetylene nanotubes in high purity has been of a key interest among scientists who are studying biosensors, microelectronics, and drug delivery system (DDS).  __Figure 1.__Photopolymerization of diacetylene lipid monomer. PDA undergoes a blue-to-red color transition when they are exposed to a stimulus, such as heat, chemical exposure, extreme pH, or mechanical stress. In their ground state, polymerized diacetylenes absorb light at 650 nm, making them appear blue. When exposed to a stimulus, the absorption shifts to 550 nm, due to shortening of the p-conjugated bonds in the polymer backbone, making them appear red. Color transition of polydiacetylenes has been used for developing biosensors by introducing specific ligands to the monomers, so that the ligands can bind to the receptors, such as viruses and bacteria (__Figure 2__).9-12  __Figure 2.__Color transition mechanism of diocetylenic polymer by ligand-receptor binding. ### Polydiacetylene Nanotubes (PDNTs) One of the promising diacetylene-based synthetic nanostructures for various real world applications is the nanotube structure derived from suspensions of diacetylenic phosphatydlcholines.13 When solutions of these molecules are heated and cooled below the melting temperature of lipid, bilayer sheets form and roll up into a tubular formation with an overall helical structure (__Figure 3__). Almost all of the nano-structure forming lipids have been chiral molecules.14-18 Most derivatives of acetylcholine form spherical liposomes, but there have been discovered molecules that can form the lipid tubules, which display some interesting properties.  While there appears to be some evidence that chirality of the monomer lipid is strongly favored as a requirement for nano-structure formation, it is not an absolute necessary requirement.6 An achiral diynoic phosphatidylcholine was shown to produce helical twisted ribbon structures in which the handedness of each ribbon was constant where there were equal numbers of each type of optical isomer utilized in the preparation.19,20  Tubules formed from another achiral phosphatydlcholines were equally divided between left and right handed helices but the chirality of the whole preparation could be altered by spiking the mixture with a small amount of a chiral derivative. This observation has led to the conclusion that the organization of these structures is more influenced by the diynes in the hydrocarbon tails than the intrinsic chirality of the molecule.19 It should be noted that in most cases the structures formed by achiral diacetylenyl lipids were not homogeneous preparations of nanotubes.    In addition to chirality, there seems to be a strong preference for molecules with two lipid diacetylene chains. In light of this, it is interesting to note that a very simple single chain achiral diacetylene has been developed, contrary to the current way of thinking,  that can self- assemble into a remarkably homogeneous population of nanotubes and can be induced to form other nano structures (__Figure 3 and 4__).   __Figure 3.__Synthesis and molecular self-assembly of a single chain diacetylene monomer and polydiacetylene nanotubes (PDNT).  __Figure 4.__LIG Sciences' nomenclature for their diacetylene monomers is based on the number of carbons in each section, including the hydrophic tail and hydrophilic head groups. Lee et al.7,8 have described the nanotube forming ability of diacetylene monomers (__Figure 4__), which are achiral amphiphilic diacetylenes with secondary amine salt head groups to form uniform self-assembled nanotubes. The diacetylenic monomers are composed of two different parts that contribute to the interesting assembly of microstructures while in an aqueous media. One is hydrophobic tail and the other one is hydrophilic head group. Both parts play important roles when they are self-assembled into microstructures, such as liposomes and nanotubes. __Figures 5 and 6__ are SEM images of nanostructures formed from DM-12-8-2-2-Br, which were found to be extremely pure and homogenous polydiacetylene nanotubes (PDNT) with an inner diameter (I.D.) of 34 nm, an outer diameter (O.D.) of 98 nm and length (L) of about 1 micron.  __Figure 5.__SEM image showing uniform diameter and purity of PDNT.  __Figure 6.__SEM image showing uniform nanotube openings of PDNT. These linear nanotubes have been found to be composed of five bilayers (__Figure 7__) are seamless when observed by SEM and TEM. Utilizing specific preparation conditions, it has been found that unique nanotube morphologies, such as the branched nanotubes shown in __Figure 8__, can be created. This shows that specific nanostructures and their morphologies can be created not only by altering the chemical structures of lipid monomers but also by changing the self-assembly process.  __Figure 7.__Polydiacetylene nanotubes before and after UV-polymerization.  __Figure 8.__SEM image of branched polydiacetylene nanotubes. PDNTs are polymerized by crosslinking at alkyne bond sites by exposure to ultraviolet light (__Figure 7__). After crosslinking, the nanotubes undergo a blue-to-red color transition when they are exposed to various types of stimuli, such as heat or mechanical stress. A color change from blue to red occurs around 70 oC due to the twisting of the polymer backbone when exposure to heat8. In most cases, this color changing is reversible, becoming blue again when the temperature returns below room temperature. One exception, as shown in __Figure 9__, is when the nanotubes are suspended in water, where the color change back to blue does not occur when the temperature goes back to room temperature. This color change from blue to red is also observed from applying stress on the polymerized nanostructure. It shows that these polymerized self-assembled structures have a potential to be used in sensors to detect the stress caused by enzymes, encapsulants or adsorbants that are themselves the species being sensed or that create the stress on the nanotube after coming in contact with the species being sensed. Specifically a nanotube with adsorbed/absorbed proteins may impart stress on the nanotube through protein conformational changes due to an environment change, thus creating a color change.  Another interesting result is that PDNT can be uniformly dispersed in polystyrene solution. The PDNT containing polystyrene solution was blue, indicating that the tube structures were not altered by incorporation into the polymer. By heating to 70 oC the polymer turned red (__Figure 10__) and cooling made the material back to blue, reversibly. At 110 oC, conversely, the polymer changed from red to clear, however, did not reversibly return to red when cooled below 110 oC. These reversible, irreversible chromatic changes can be utilized for practical applications, such as paints that reversibly change colors to warn of hot surfaces or that irreversible change colors for items such as frozen food or other perishables that may experience an undesired warming in transit to a store.  __Figure 10.__Color transition of polydiacetylene nanotubes in polystyrene. The reversible color change by heating up to 70 oC is also observed from the PDNT containing polyurethane elastomer. Interestingly when the elastomer is stretched, it loses its blue color, becoming red, then returns to blue upon the removal of the stress and relaxing it to the original condition. It indicates that the incorporation of non-modified PDNTs into polymers have potentials to yield smart chromatic materials and coatings that are able to monitor the changes in mechanical stress experienced on surfaces. __Figure 11__ shows the color changes of PDA in an aqueous environment before/after irradiation of light and the effect of various additives on the color of the polymerized PDA. Unpolymerized or uncrosslinked PDNT (tube 1) in water was a slightly turbid white solution which turned blue after cross-linking (tube 2) by exposing to UV radiation. Addition of bacteria (1 X 109 cells) to cross-linked PDNT caused a flocculation of the suspension within 30 minutes (tubes 3 and 4).  Note that all of the colored material was precipitated by the bacteria. Addition of SDS results in the color change to yellow (tube 5) which is unusual as most polydiacetylenes show only a blue to red color change. Other surfactants showed various results; CTAB had no effect (tube 6) while CTAC, which differs only in the counter ion (chloride verses bromide), caused the classic blue to red conversion (tube 7). Pluronic had no effect but exposure to triton-X100 made a purple color (tubes 8 and 9, respectively). Finally, the effect of salt concentration (by addition of NaCl, tube 10) or pH (by addition of HCl or NaOH) on the color change has not been observed. This broad variety of color changes for the different reactions that was found with PDNT is unusual. It is anticipated that this unique feature of PDA can be exploited in the design of colorimetric biosensors that utilize PDNTs that are composed of PDA.  __Figure 11.__Effect of various additions of polymerized PDA. Tube: 1) Non-polymerized PDA; 2) Polymerized PDA; 3) E. Coli.; 4) B. subtilis; 5) sodium dodecyl sulfate (SDS); 6) cetyltrimethylammonium bromide (CTAB); 7) cetyltrimethylammonium chloride (CTAC); 8) Pluronic; 9) Triton-X 100; 10) NaCl. ## 2. Features and Potential Applications of PDNTs The mechano chromism property of PDNTs can be utilized in future products. Any mechanical stress will induce microstructural changes in the nanotube/polymer composite coatings, resulting in the color changes. This color changing paint could be utilized in situations where unwanted yielding to mechanical stress needs to be monitored and detected such as in the aluminum skins of aircraft, building components, critical bridge members, etc. In addition, the thermo chromism properties can also be utilized in these nanotube/polymer composite coatings to provide visual temperature warnings or indications of hot surfaces which are painted with this composite coating. Also, there is a potential to utilize PDNTs as a support platform of enzyme delivery to facilitate biochemical and biological reactions. This platform could be used in biosensors where enzymes tailored for interacting with specific chemical or biological materials might create stresses on the nanotube resulting in a color change. It could be utilized in a microassay card for onsite identification of unknown substances whether it is narcotics, pollutants, biological/chemical warfare agents, etc. Table 1 summarizes salient features of the PDNTs. FeaturesPotential applications - Deep blue nanotubes powder - Extremely uniform inner and outer diameter of nanotubes - High purity - π bond electrons within polymer chain enables greater electron conductivity - Photoconductivity - Electroconductivity - Thermo-and mechanochromism - No adverse reaction observed with fibroblast cells - Price competitive with carbon nanotubes - Stimuli-sensitive smart nanomaterial composites - Drug delivery systems (DDS) - Biosensors for viruses, bacteria, and glucose - Colorimetric pressure sensors - Template for making conductive nanorods - Langmuir-Blodgett films - Lab-on-a-chip and MEMS - UV-based sensors - Nanoelectronics - Optics and Artificial retinas - Neuronal networks - Membrane filtration - Gas separation Table 1Unique features of polydiacetylene nanotubes and potential applications ## 3. Polydiacetylene Nanotubes (PDNTs) available Listed in __Table 2__ are the high-purity PDNTs ([773484](https://www.sigmaaldrich.com/PL/en/product/aldrich/773484) and [773492](https://www.sigmaaldrich.com/PL/en/product/aldrich/773492)) offered in our catalog. These PDNTs are extremely pure and well-polymerized due to the excellent alignment after the self-assembly of diacetylene monomers. SEM images of PDNT-12-8-2-2-Br ([773484](https://www.sigmaaldrich.com/PL/en/product/aldrich/773484)) in __Figures 5 and 6__ show the purity and nanotube lumen openings. Product No.Product NameAppearance and Structures [773484](https://www.sigmaaldrich.com/PL/en/product/aldrich/773484)PDNT-12-8-2-2-Br - Deep blue compact powder - O.D x I.D x Length: 100 nm x 30 nm x 1 µm - Monomer: N-Ethylethylenediamino-10,12-pentacosadiyneamide hydrogen bromide [773492](https://www.sigmaaldrich.com/PL/en/product/aldrich/773492.html)PDNT-12-8-2-3-Br - Deep blue compact powder - O.D x I.D x Length: 110 nm x 40 nm x 1 µm - Monomer: N-propylethylenediamino-10,12-pentacosadiyneamide hydrogen bromide Table 2PDNTs available in our catalog ### References 1\. Yager P, Schoen PE. 1984. Formation of Tubules by a Polymerizable Surfactant. Molecular Crystals and Liquid Crystals. 106(3-4):371-381. [https://doi.org/10.1080/00268948408071454](https://doi.org/10.1080/00268948408071454) 2\. Selinger JV, Spector MS, Schnur JM. 2001. Theory of Self-Assembled Tubules and Helical Ribbons. J. Phys. Chem. B. 105(30):7157-7169. [https://doi.org/10.1021/jp010452d](https://doi.org/10.1021/jp010452d) 3\. Schnur JM. 1993. Lipid Tubules: A Paradigm for Molecularly Engineered Structures. Science. 262(5140):1669-1676. [https://doi.org/10.1126/science.262.5140.1669](https://doi.org/10.1126/science.262.5140.1669) 4\. Thomas BN, Safinya CR, Plano RJ, Clark NA. 1995. Lipid Tubule Self-Assembly: Length Dependence on Cooling Rate Through a First-Order Phase Transition. Science. 267(5204):1635-1638. [https://doi.org/10.1126/science.267.5204.1635](https://doi.org/10.1126/science.267.5204.1635) 5\. Singh A, Wong EM, Schnur JM. 2003. Toward the Rational Control of Nanoscale Structures Using Chiral Self-Assembly:  Diacetylenic Phosphocholines?. Langmuir. 19(5):1888-1898. [https://doi.org/10.1021/la026337r](https://doi.org/10.1021/la026337r) 6\. Singh A, Schoen PE, Schnur JM. 1988. Self-assembled microstructures from a polymerizable ammonium surfactant: di(hexacosa-12,14-diynyl)dimethylammonium bromide. J. Chem. Soc., Chem. Commun..(18):1222. [https://doi.org/10.1039/c39880001222](https://doi.org/10.1039/c39880001222) 7\. Lee SB, Koepsel R, Stolz DB, Warriner HE, Russell AJ. 2004. Self-Assembly of Biocidal Nanotubes from a Single-Chain Diacetylene Amine Salt. J. Am. Chem. Soc.. 126(41):13400-13405. [https://doi.org/10.1021/ja048463i](https://doi.org/10.1021/ja048463i) 8\. Lee SB, Koepsel RR, Russell AJ. 2005. Surface Dispersion and Hardening of Self-Assembled Diacetylene Nanotubes. Nano Lett.. 5(11):2202-2206. [https://doi.org/10.1021/nl0513582](https://doi.org/10.1021/nl0513582) 9\. Charych D, Nagy J, Spevak W, Bednarski M. 1993. Direct colorimetric detection of a receptor-ligand interaction by a polymerized bilayer assembly. Science. 261(5121):585-588. [https://doi.org/10.1126/science.8342021](https://doi.org/10.1126/science.8342021) 10\. Cheng Q, Stevens RC. 1997. Coupling of an induced fit enzyme to polydiacetylene thin films: Colorimetric detection of glucose. Adv. Mater.. 9(6):481-483. [https://doi.org/10.1002/adma.19970090605](https://doi.org/10.1002/adma.19970090605) 11\. Morigaki K, Baumgart T, Jonas U, Offenhäusser A, Knoll W. 2002. Photopolymerization of Diacetylene Lipid Bilayers and Its Application to the Construction of Micropatterned Biomimetic Membranes. Langmuir. 18(10):4082-4089. [https://doi.org/10.1021/la0157420](https://doi.org/10.1021/la0157420) 12\. Ahn DJ, Chae E, Lee GS, Shim H, Chang T, Ahn K, Kim J. 2003. Colorimetric Reversibility of Polydiacetylene Supramolecules Having Enhanced Hydrogen-Bonding under Thermal and pH Stimuli. J. Am. Chem. Soc.. 125(30):8976-8977. [https://doi.org/10.1021/ja0299001](https://doi.org/10.1021/ja0299001) 13\. Robinson JK, Bollinger MJ, Birks JW. 1999. Luminol/H2O2Chemiluminescence Detector for the Analysis of Nitric Oxide in Exhaled Breath. Anal. Chem.. 71(22):5131-5136. [https://doi.org/10.1021/ac990646d](https://doi.org/10.1021/ac990646d) 14\. Spector MS, Singh A, Messersmith PB, Schnur JM. 2001. Chiral Self-Assembly of Nanotubules and Ribbons from Phospholipid Mixtures. Nano Lett.. 1(7):375-378. [https://doi.org/10.1021/nl015554u](https://doi.org/10.1021/nl015554u) 15\. Thomas BN, Lindemann CM, Corcoran RC, Cotant CL, Kirsch JE, Persichini PJ. 2002. Phosphonate Lipid Tubules II. J. Am. Chem. Soc.. 124(7):1227-1233. [https://doi.org/10.1021/ja012137a](https://doi.org/10.1021/ja012137a) 16\. Wang G, Hollingsworth R. 2000. Adv. Mater.. 12871-874. 17\. Lim MD, Lorkovic IM, Wedeking K, Zanella AW, Works CF, Massick SM, Ford PC. 2002. Reactions of Nitrogen Oxides with Heme Models. Characterization of NO and NO2Dissociation from Fe(TPP)(NO2)(NO) by Flash Photolysis and Rapid Dilution Techniques:  Fe(TPP)(NO2) as an Unstable Intermediate. J. Am. Chem. Soc.. 124(33):9737-9743. [https://doi.org/10.1021/ja020653a](https://doi.org/10.1021/ja020653a) 18\. Spasojevi? I, Batini?-Haberle I, Fridovich I. 2000. Nitrosylation of Manganese(II) Tetrakis(N-ethylpyridinium-2-yl)porphyrin: A Simple and Sensitive Spectrophotometric Assay for Nitric Oxide. Nitric Oxide. 4(5):526-533. [https://doi.org/10.1006/niox.2000.0303](https://doi.org/10.1006/niox.2000.0303) 19\. Lindsell WE, Preston PN, Seddon JM, Rosair GM, Woodman TAJ. 2000. Macroscopic Helical and Cylindrical Morphologies from Achiral 1,3-Diynes. Chem. Mater.. 12(6):1572-1576. [https://doi.org/10.1021/cm000172v](https://doi.org/10.1021/cm000172v) 20\. Spector MS, Easwaran KRK, Jyothi G, Selinger JV, Singh A, Schnur JM. 1996. Chiral molecular self-assembly of phospholipid tubules: A circular dichroism study. Proceedings of the National Academy of Sciences. 93(23):12943-12946. 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