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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 5 — Feb. 27, 2012
  • pp: 5119–5126
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Low-frequency Raman scattering of bioinspired self-assembled diphenylalanine nanotubes/microtubes

Xinglong Wu, Shijie Xiong, Minjie Wang, Jiancang Shen, and Paul K. Chu  »View Author Affiliations


Optics Express, Vol. 20, Issue 5, pp. 5119-5126 (2012)
http://dx.doi.org/10.1364/OE.20.005119


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Abstract

Low-frequency Raman scattering from self-assembled bioinspired diphenylalanine (FF) nanotubes/microtubes (NTs/MTs) has been observed for the first time. Four double peaks are identified as the three-dimensional localized collective (acoustic phonon) vibrations of FF molecules in the subnanometer crystalline structure (biological building block) forming the FF NTs/MTs. The increased energy separations between two subpeaks caused by the loss of water in the nanochannel cores are due to the enhancement of vibrational couplings between the FF molecules as a result of the reduction of the influence from water on the coupling. The results provide experimental evidence of localized but still weakly coupled vibrations in organic crystalline nanostructures in the low-frequency region.

© 2012 OSA

1. Introduction

In this work, LF Raman measurements were conducted on self-assembled hierarchical FF NTs/MTs. Four double peaks are identified as the three-dimensional localized collective (acoustic phonon) vibrations of FF molecules in the SCS (biological building block). The increased energy separation between two subpeaks arises from water loss from the nanochannel cores (less influence of water on the coupling) and enhanced vibrational coupling between the FF molecules. The new results provide experimental evidence of localized but still weakly coupled vibrations in organic crystalline nanostructures in the low-frequency region.

2. Experimental

The humidity-sensitive FF NTs/MTs were prepared by evaporating a drop of fresh FF solution dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol on a glass substrate. The initial concentrations of the FF solutions were varied between 30 and 200 mg/mL and different water vapor pressure values were used in the fabrication by changing the relative humidity (RH) at 22°. The FF solution was dried in 3 min and the FF NTs/MTs were characterized after 30 min.

The LF Raman spectra were acquired at room temperature on a T64000 triple Raman system using a micro-Raman backscattering geometry without a polarization configuration using the 514.5 nm line of an Ar-ion laser as the excitation source. The resolution of the spectrometer was 0.5 cm−1. The diameter of the beam spot was 5 μm and the power was less than 4 mW in order to avoid sample degeneration caused by laser heating. The Raman spectrum acquisition time was 2 min. The high resolution and rejection rate of this measurement system allowed observation of the vibration signals close to the Rayleigh line to less than 5 cm−1 [5

5. X. L. Wu, S. J. Xiong, Y. M. Yang, J. F. Gong, H. T. Chen, J. Zhu, J. C. Shen, and P. K. Chu, “Nanocrystal-induced line narrowing of surface acoustic phonons in the Raman spectra of embedded GexSi1-x alloy nanocrystals,” Phys. Rev. B 78(16), 165319 (2008). [CrossRef]

,6

6. X. L. Wu, S. J. Xiong, L. T. Sun, J. C. Shen, and P. K. Chu, “Low-frequency Raman scattering from nanocrystals caused by coherent excitation of phonons,” Small 5(24), 2823–2826 (2009) (and references therein). [CrossRef] [PubMed]

]. The acquired spectra were the same as those taken under excitation by the 488 nm line and no similar LF signals were observed for the used glass substrate.

3. Results and discussion

The scanning electron microscopy (SEM) image of a typical hierarchical FF MT is depicted in Fig. 1(b). The MT has a “feather-like” morphology. Its head is a large MT with a mean diameter of about 3 μm and the tail consists of many small NTs with a diameter of about 360 nm. Clear nanopores can be observed easily and large quantities of SCSs make up the NT/MT walls [18

18. C. H. Görbitz, “The structure of nanotubes formed by diphenylalanine, the core recognition motif of Alzheimer’s β-amyloid polypeptide,” Chem. Commun. (Camb.) (22): 2332–2334 (2006). [CrossRef] [PubMed]

,19

19. C. H. Görbitz, “Nanotube formation by hydrophobic dipeptides,” Chemistry 7(23), 5153–5159 (2001). [CrossRef] [PubMed]

].

Figure 2(a)
Fig. 2 (a) LF Raman spectra of the FF NT/MT samples fabricated with the FF concentrations ranging from 30 to 200 mg/mL at the RH value of 1. (b) LF Raman spectra of the FF NT/MT samples fabricated with the FF concentration of 160 mg/mL at the RH values varying from 0.33 to 1.0.
shows the LF Raman spectra acquired from the FF NT/MT samples with FF concentrations between 30 and 200 mg/mL at RH (relative humidity) of 1. To investigate the LF Raman spectra, we simply divide each one into four paired peaks marked as DP1 at 41.5 and 56.5 cm−1, DP2 at 76.5 and 86.5 cm−1, DP3 at 107 and 122 cm−1, and DP4 due to the dual structure in FF molecule. DP1 and DP3 have the same energy separation of about 15 cm−1 and the peak intensity changes with FF concentration. The double-peak structure is pronounced in DP2 produced with an FF concentration of 200 mg/mL. In particular, a relatively small energy separation of ~10 cm−1 is observed. The peak observed from DP4 is localized at 168.5 cm−1 with a linewidth close to that of DP2. Its position and linewidth do not vary with FF concentration although the intensity varies slightly. For DP4, it is still a double-peak, but cannot be resolved well. Our theory also shows this feature (see below). These spectral features can also be observed from other samples fabricated at different RH values, as shown in Fig. 2(b) that shows the LF Raman spectra of the samples fabricated at an FF concentration of 160 mg/mL and RH values from 0.33 to 1.0. The energy separation between the subpeaks observed from DP1 and DP3 is almost the same whereas that of DP4 still appears at 168.5 cm−1. The only change is that DP2 with a double-peak feature can be observed from the three samples fabricated at RH values of 0.67, 0.83, and 1.0. Hence, it is reasonable to regard DP2 as a paired peak.

To explore the role of water weakly bound to FF molecules via hydrogen bonds in the nanochannel core [20

20. J. B. Kim, T. H. Han, Y. I. Kim, J. S. Park, J. Choi, D. G. Churchill, S. O. Kim, and H. Ihee, “Role of water in directing diphenylalanine assembly into nanotubes and nanowires,” Adv. Mater. (Deerfield Beach Fla.) 22(5), 583–587 (2010). [CrossRef] [PubMed]

], the LF Raman spectra acquired for different times are presented in Fig. 3
Fig. 3 LF Raman spectra of the FF NT/MT samples with different light illumination times. The initial NT/MT sample was fabricated with the FF concentration of 180 mg/mL at the RH value of 1 and 22 °C.
. With increasing illumination time (corresponding to more water loss [21

21. M. J. Wang, S. J. Xiong, X. L. Wu, and P. K. Chu, “Effects of water molecules on photoluminescence from hierarchical peptide nanotubes and water probing capability,” Small 7(19), 2801–2807 (2011). [CrossRef] [PubMed]

]), all the peak intensity is reduced. The energy separation between the two subpeaks also increases for DP2 and DP3. Since the LF modes stem from the collective vibrations of some large molecular groups in the FF molecule, water loss weakens the vibration coupling between these groups with water, leading to strengthening vibration coupling between the large groups themselves, thereby creating significant energy separation between the two subpeaks.

The localized vibration modes in different FF molecules at the same frequency will couple together producing acoustic waves propagating throughout the whole NT/MT. Since the coupling between different FF molecules is usually very weak, the resulting peak broadening and frequency shifts are also small as shown in Fig. 2. This implies that the coupling between FF molecules mainly occur in a SCS. If we consider a mode ωi in a FF molecule and pay attention to the nearest neighbor (NN) coupling between FF molecules in the tube, the kinetic energy is
Ki=nMiωi2Xin22,
(1)
where Xin is the displacement of the ωi mode in the nth FF molecule and Mi is the generalized mass corresponding to the ωi mode in every molecule. The potential energy due to the coupling between molecules is
Vi=12<n,n'>λi(XinXin')2,
(2)
where 〈……〉 denotes the NN molecules and λi is the coupling strength of the NN modes. From the total energy Ki + Vi and dynamic equation for all Xin, we can obtain the eigenfrequencies for a nanochannel as [28

28. X. L. Wu, S. J. Xiong, Z. Liu, J. Chen, J. C. Shen, T. H. Li, P. H. Wu, and P. K. Chu, “Green light stimulates terahertz emission from mesocrystal microspheres,” Nat. Nanotechnol. 6(2), 103–106 (2011). [CrossRef] [PubMed]

]
ωi(k1,k2)=ωi2+ηi2(2cosk1cosk2),
(3)
where ηi = (2λi/Mi)1/2, k1 and k2 are the wave vectors in the channel direction and plane perpendicular to channel, respectively. Because the channel is formed by a ring of 6 FF molecules in the perpendicular plane (red circle in Fig. 1(a)), k2 is discretized as k2m = /3 with m = 0, 1, 2, 3, 4, 5. We suppose that in the nanochannel direction, the system is infinitely extended and so k1 is continuous in the range of [0, 2π]. Considering the different Raman activities of the modes, the whole Raman spectrum can be calculated as
I(ω)=i=18m=05Ai02πδ[ωωi(k1,k2m)]dk1,
(4)
where Ai is the Raman activity for mode ωi obtained in the above DFT calculation for a single FF molecule.

According to the above analysis, we can expect that due to the coupling between FF molecules, the discrete LF phonon modes in one FF molecule should be extended to bands with widths depending on the size of the SCS. Such a reformation is determined by the values of ηi which may be affected by water molecules in the SCS core. Figures 2 and 3 show that ω1,2,7,8 (DP1 and DP4) has a smaller linewidth change with FF concentration, RH, and amount of water in the nanochannel than ω3-6 (DP2 and DP3). Hence, we only consider the influence of water molecules on η3-6. Figure 5
Fig. 5 Calculated Raman intensity in the LF range of a SCS. ωi and Ai with i = 1,2,…,8 are obtained from the DFT calculation for FF molecule. Other parameters are selected in a SCS as: η1 = η2 = 10 cm−1, η7 = η8 = 30 cm−1. For black curve 1: η3 = η4 = 30 cm−1, η5 = η6 = 40 cm−1. For red curve 2: η3 = η4 = 35 cm−1, η5 = η6 = 45 cm−1. For blue curve 3: η3 = η4 = 40 cm−1, η5 = η6 = 50 cm−1.
plots the calculated Raman intensity for different values of η3-6 in a SCS. The curves reproduce the behavior of the measured Raman spectrum depicted in Fig. 3. DP1 and DP4 show almost the same linewidths and line shape, but not DP2 and DP3. This indicates that the acoustic vibrations of a SCS determine the LF Raman scattering. The samples under longer optical radiation yield larger coupling strength η3-6. This is because under radiation, water molecules escape and their effects on the coupling between FF molecules is reduced. Consequently, the coupling strength is increased. The enhanced coupling also widens the bands and reduces the peak heights. This coupling change caused by water addition and loss changes the SCS sizes and it has been confirmed by our DFT calculation and x-ray diffraction [21

21. M. J. Wang, S. J. Xiong, X. L. Wu, and P. K. Chu, “Effects of water molecules on photoluminescence from hierarchical peptide nanotubes and water probing capability,” Small 7(19), 2801–2807 (2011). [CrossRef] [PubMed]

].

4. Conclusion

Four double peaks are observed from LF Raman spectra acquired from self-assembled bioinspired FF NTs/MTs. As water is lost from the nanochannel cores, the relative peak intensity diminishes and the separation between the double peak increases. DFT calculation reveals that the observed peaks stem from 3D collective vibrations of FF molecules in the SCS. The loss of water molecules decreases significantly the effects of water on the coupling between FF molecules. Consequently, the band width and energy separation increase. Our results provide for the first time evidence of local collective vibrations in inorganic NC systems.

Acknowledgments

This work was supported by the National Basic Research Program of China under Grant No. 2011CB922102 and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Partial support was also from the National Natural Science Foundations (Nos. 60976063 and 10874071), Hong Kong Research Grants Council (RGC) General Research Grants (GRF) CityU 112510, and City University of Hong Kong Strategic Research Grant (SRG) No. 7008009.

References and links

1.

E. Duval, A. Boukenter, and B. Champagnon, “Vibration eigenmodes and size of microcrystallites in glass-Observation by very-low-frequency Raman-scattering,” Phys. Rev. Lett. 56(19), 2052–2055 (1986). [CrossRef]

2.

L. Saviot and D. B. Murray, “Long lived acoustic vibrational modes of an embedded nanoparticle,” Phys. Rev. Lett. 93(5), 055506 (2004). [CrossRef] [PubMed]

3.

A. Courty, A. Mermet, P. A. Albouy, E. Duval, and M. P. Pileni, “Vibrational coherence of self-organized silver nanocrystals in f.c.c. supra-crystals,” Nat. Mater. 4(5), 395–398 (2005). [CrossRef] [PubMed]

4.

H. K. Yadav, V. Gupta, K. Sreenivas, S. P. Singh, B. Sundarakannan, and R. S. Katiyar, “Low frequency Raman scattering from acoustic phonons confined in ZnO nanoparticles,” Phys. Rev. Lett. 97(8), 085502 (2006). [CrossRef] [PubMed]

5.

X. L. Wu, S. J. Xiong, Y. M. Yang, J. F. Gong, H. T. Chen, J. Zhu, J. C. Shen, and P. K. Chu, “Nanocrystal-induced line narrowing of surface acoustic phonons in the Raman spectra of embedded GexSi1-x alloy nanocrystals,” Phys. Rev. B 78(16), 165319 (2008). [CrossRef]

6.

X. L. Wu, S. J. Xiong, L. T. Sun, J. C. Shen, and P. K. Chu, “Low-frequency Raman scattering from nanocrystals caused by coherent excitation of phonons,” Small 5(24), 2823–2826 (2009) (and references therein). [CrossRef] [PubMed]

7.

M. Reches and E. Gazit, “Casting metal nanowires within discrete self-assembled peptide nanotubes,” Science 300(5619), 625–627 (2003). [CrossRef] [PubMed]

8.

M. Reches and E. Gazit, “Controlled patterning of aligned self-assembled peptide nanotubes,” Nat. Nanotechnol. 1(3), 195–200 (2006). [CrossRef] [PubMed]

9.

M. Reches and E. Gazit, “Formation of closed-cage nanostructures by self-assembly of aromatic dipeptides,” Nano Lett. 4(4), 581–585 (2004). [CrossRef]

10.

N. Kol, L. Adler-Abramovich, D. Barlam, R. Z. Shneck, E. Gazit, and I. Rousso, “Self-assembled peptide nanotubes are uniquely rigid bioinspired supramolecular structures,” Nano Lett. 5(7), 1343–1346 (2005). [CrossRef] [PubMed]

11.

L. Adler-Abramovich, M. Reches, V. L. Sedman, S. Allen, S. J. B. Tendler, and E. Gazit, “Thermal and chemical stability of diphenylalanine peptide nanotubes: implications for nanotechnological applications,” Langmuir 22(3), 1313–1320 (2006). [CrossRef] [PubMed]

12.

A. Kholkin, N. Amdursky, I. Bdikin, E. Gazit, and G. Rosenman, “Strong piezoelectricity in bioinspired peptide nanotubes,” ACS Nano 4(2), 610–614 (2010). [CrossRef] [PubMed]

13.

K. Biswas and C. N. R. Rao, “Nanostructured peptide fibrils formed at the organic-aqueous interface and their use as templates to prepare inorganic nanostructures,” ACS Appl. Mater. Interfaces 1(4), 811–815 (2009). [CrossRef] [PubMed]

14.

N. Amdursky, M. Molotskii, D. Aronov, L. Adler-Abramovich, E. Gazit, and G. Rosenman, “Blue luminescence based on quantum confinement at peptide nanotubes,” Nano Lett. 9(9), 3111–3115 (2009). [CrossRef] [PubMed]

15.

N. Amdursky, M. Molotskii, E. Gazit, and G. Rosenman, “Self-assembled bioinspired quantum dots: Optical properties,” Appl. Phys. Lett. 94(26), 261907 (2009). [CrossRef]

16.

N. Amdursky, E. Gazit, and G. Rosenman, “Quantum confinement in self-assembled bioinspired peptide hydrogels,” Adv. Mater. (Deerfield Beach Fla.) 22(21), 2311–2315 (2010). [CrossRef] [PubMed]

17.

J. K. Ryu, S. Y. Lim, and C. B. Park, “Photoluminescent peptide nanotubes,” Adv. Mater. (Deerfield Beach Fla.) 21(16), 1577–1581 (2009). [CrossRef]

18.

C. H. Görbitz, “The structure of nanotubes formed by diphenylalanine, the core recognition motif of Alzheimer’s β-amyloid polypeptide,” Chem. Commun. (Camb.) (22): 2332–2334 (2006). [CrossRef] [PubMed]

19.

C. H. Görbitz, “Nanotube formation by hydrophobic dipeptides,” Chemistry 7(23), 5153–5159 (2001). [CrossRef] [PubMed]

20.

J. B. Kim, T. H. Han, Y. I. Kim, J. S. Park, J. Choi, D. G. Churchill, S. O. Kim, and H. Ihee, “Role of water in directing diphenylalanine assembly into nanotubes and nanowires,” Adv. Mater. (Deerfield Beach Fla.) 22(5), 583–587 (2010). [CrossRef] [PubMed]

21.

M. J. Wang, S. J. Xiong, X. L. Wu, and P. K. Chu, “Effects of water molecules on photoluminescence from hierarchical peptide nanotubes and water probing capability,” Small 7(19), 2801–2807 (2011). [CrossRef] [PubMed]

22.

M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, Jr., J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, Ö. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, P. Salvador, J. J. Dannenberg, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komáromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle, and J. A. Pople, Gaussian 03, Revision A.1 (Gaussian, Inc., Pittsburgh, PA, 2003).

23.

J. P. Perdew, K. Burke, and M. Ernzerhof, “Generalized gradient approximation made simple,” Phys. Rev. Lett. 77(18), 3865–3868 (1996). [CrossRef] [PubMed]

24.

J. B. Foresman, M. Head-Gordon, J. A. Pople, and M. J. Frisch, “Toward a systematic molecular orbital theory for excited states,” J. Phys. Chem. 96(1), 135–149 (1992). [CrossRef]

25.

C. Peng, P. Y. Ayala, H. B. Schlegel, and M. J. Frisch, “Using redundant internal coordinates to optimize equilibrium geometries and transition states,” J. Comput. Chem. 17(1), 49–56 (1996). [CrossRef]

26.

R. Krishnan, H. B. Schlegel, and J. A. Pople, “Derivative studies in configuration-interaction theory,” J. Chem. Phys. 72(8), 4654–4655 (1980). [CrossRef]

27.

A. Komornicki and R. L. Jaffe, “Ab initio investigation of the structure, vibrational frequencies, and intensities of HO2 and HOCl,” J. Chem. Phys. 71(5), 2150–2155 (1979). [CrossRef]

28.

X. L. Wu, S. J. Xiong, Z. Liu, J. Chen, J. C. Shen, T. H. Li, P. H. Wu, and P. K. Chu, “Green light stimulates terahertz emission from mesocrystal microspheres,” Nat. Nanotechnol. 6(2), 103–106 (2011). [CrossRef] [PubMed]

OCIS Codes
(120.7280) Instrumentation, measurement, and metrology : Vibration analysis
(300.6450) Spectroscopy : Spectroscopy, Raman
(160.1435) Materials : Biomaterials

ToC Category:
Materials

History
Original Manuscript: November 14, 2011
Revised Manuscript: December 29, 2011
Manuscript Accepted: January 12, 2012
Published: February 16, 2012

Virtual Issues
Vol. 7, Iss. 4 Virtual Journal for Biomedical Optics

Citation
Xinglong Wu, Shijie Xiong, Minjie Wang, Jiancang Shen, and Paul K. Chu, "Low-frequency Raman scattering of bioinspired self-assembled diphenylalanine nanotubes/microtubes," Opt. Express 20, 5119-5126 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-5-5119


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References

  1. E. Duval, A. Boukenter, and B. Champagnon, “Vibration eigenmodes and size of microcrystallites in glass-Observation by very-low-frequency Raman-scattering,” Phys. Rev. Lett.56(19), 2052–2055 (1986). [CrossRef]
  2. L. Saviot and D. B. Murray, “Long lived acoustic vibrational modes of an embedded nanoparticle,” Phys. Rev. Lett.93(5), 055506 (2004). [CrossRef] [PubMed]
  3. A. Courty, A. Mermet, P. A. Albouy, E. Duval, and M. P. Pileni, “Vibrational coherence of self-organized silver nanocrystals in f.c.c. supra-crystals,” Nat. Mater.4(5), 395–398 (2005). [CrossRef] [PubMed]
  4. H. K. Yadav, V. Gupta, K. Sreenivas, S. P. Singh, B. Sundarakannan, and R. S. Katiyar, “Low frequency Raman scattering from acoustic phonons confined in ZnO nanoparticles,” Phys. Rev. Lett.97(8), 085502 (2006). [CrossRef] [PubMed]
  5. X. L. Wu, S. J. Xiong, Y. M. Yang, J. F. Gong, H. T. Chen, J. Zhu, J. C. Shen, and P. K. Chu, “Nanocrystal-induced line narrowing of surface acoustic phonons in the Raman spectra of embedded GexSi1-x alloy nanocrystals,” Phys. Rev. B78(16), 165319 (2008). [CrossRef]
  6. X. L. Wu, S. J. Xiong, L. T. Sun, J. C. Shen, and P. K. Chu, “Low-frequency Raman scattering from nanocrystals caused by coherent excitation of phonons,” Small5(24), 2823–2826 (2009) (and references therein). [CrossRef] [PubMed]
  7. M. Reches and E. Gazit, “Casting metal nanowires within discrete self-assembled peptide nanotubes,” Science300(5619), 625–627 (2003). [CrossRef] [PubMed]
  8. M. Reches and E. Gazit, “Controlled patterning of aligned self-assembled peptide nanotubes,” Nat. Nanotechnol.1(3), 195–200 (2006). [CrossRef] [PubMed]
  9. M. Reches and E. Gazit, “Formation of closed-cage nanostructures by self-assembly of aromatic dipeptides,” Nano Lett.4(4), 581–585 (2004). [CrossRef]
  10. N. Kol, L. Adler-Abramovich, D. Barlam, R. Z. Shneck, E. Gazit, and I. Rousso, “Self-assembled peptide nanotubes are uniquely rigid bioinspired supramolecular structures,” Nano Lett.5(7), 1343–1346 (2005). [CrossRef] [PubMed]
  11. L. Adler-Abramovich, M. Reches, V. L. Sedman, S. Allen, S. J. B. Tendler, and E. Gazit, “Thermal and chemical stability of diphenylalanine peptide nanotubes: implications for nanotechnological applications,” Langmuir22(3), 1313–1320 (2006). [CrossRef] [PubMed]
  12. A. Kholkin, N. Amdursky, I. Bdikin, E. Gazit, and G. Rosenman, “Strong piezoelectricity in bioinspired peptide nanotubes,” ACS Nano4(2), 610–614 (2010). [CrossRef] [PubMed]
  13. K. Biswas and C. N. R. Rao, “Nanostructured peptide fibrils formed at the organic-aqueous interface and their use as templates to prepare inorganic nanostructures,” ACS Appl. Mater. Interfaces1(4), 811–815 (2009). [CrossRef] [PubMed]
  14. N. Amdursky, M. Molotskii, D. Aronov, L. Adler-Abramovich, E. Gazit, and G. Rosenman, “Blue luminescence based on quantum confinement at peptide nanotubes,” Nano Lett.9(9), 3111–3115 (2009). [CrossRef] [PubMed]
  15. N. Amdursky, M. Molotskii, E. Gazit, and G. Rosenman, “Self-assembled bioinspired quantum dots: Optical properties,” Appl. Phys. Lett.94(26), 261907 (2009). [CrossRef]
  16. N. Amdursky, E. Gazit, and G. Rosenman, “Quantum confinement in self-assembled bioinspired peptide hydrogels,” Adv. Mater. (Deerfield Beach Fla.)22(21), 2311–2315 (2010). [CrossRef] [PubMed]
  17. J. K. Ryu, S. Y. Lim, and C. B. Park, “Photoluminescent peptide nanotubes,” Adv. Mater. (Deerfield Beach Fla.)21(16), 1577–1581 (2009). [CrossRef]
  18. C. H. Görbitz, “The structure of nanotubes formed by diphenylalanine, the core recognition motif of Alzheimer’s β-amyloid polypeptide,” Chem. Commun. (Camb.) (22): 2332–2334 (2006). [CrossRef] [PubMed]
  19. C. H. Görbitz, “Nanotube formation by hydrophobic dipeptides,” Chemistry7(23), 5153–5159 (2001). [CrossRef] [PubMed]
  20. J. B. Kim, T. H. Han, Y. I. Kim, J. S. Park, J. Choi, D. G. Churchill, S. O. Kim, and H. Ihee, “Role of water in directing diphenylalanine assembly into nanotubes and nanowires,” Adv. Mater. (Deerfield Beach Fla.)22(5), 583–587 (2010). [CrossRef] [PubMed]
  21. M. J. Wang, S. J. Xiong, X. L. Wu, and P. K. Chu, “Effects of water molecules on photoluminescence from hierarchical peptide nanotubes and water probing capability,” Small7(19), 2801–2807 (2011). [CrossRef] [PubMed]
  22. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, Jr., J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, Ö. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, P. Salvador, J. J. Dannenberg, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komáromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle, and J. A. Pople, Gaussian 03, Revision A.1 (Gaussian, Inc., Pittsburgh, PA, 2003).
  23. J. P. Perdew, K. Burke, and M. Ernzerhof, “Generalized gradient approximation made simple,” Phys. Rev. Lett.77(18), 3865–3868 (1996). [CrossRef] [PubMed]
  24. J. B. Foresman, M. Head-Gordon, J. A. Pople, and M. J. Frisch, “Toward a systematic molecular orbital theory for excited states,” J. Phys. Chem.96(1), 135–149 (1992). [CrossRef]
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