Modulation of Luminescent Nanostructures Using Bases with Amino Acid Derivative through Co-Assembly

In this study, charming luminescent nanostructures have been obtained from two components between amino acids functionalized naphthalene derivative (IN) and bases (Adenine (A), Thymine (T), Guanine (G) and Uracil (U)) units via co-assembly. These fluorescent nanotubes, walnut-like structures, nanofibers and sponge-like structures have been investigated by means of Ultraviolet Spectra (UV), Fluorescence Spectra and Scanning Electron Microscopy (SEM). The results illustrated that these nanostructures were of strong fluorescence and the drive forces were the intermolecular hydrogen bonding and π-π stacking interactions.


Introduction
Molecular self-assembly plays an important role in controlling spatiotemporal structures for the bottom-up construction design and fabrication of functional nanoscale architectures and materials from simple molecular building blocks [1][2][3][4]. Through self-assembly of nucleic acids, peptides, phospholipids and amphiphiles, varieties of charming morphologies include nanotubes, nanovesicles, nanofibers, and nanotapes emerge [5][6][7][8][9][10][11][12][13][14]. These nanostructure morphologies have been shown to be of great importance in many research fields. For instance, nanovesicles are useful for drug or gene delivery and nanotechnology [15], nanotubes and nanofibers for electrical and medical devices [16,17], biosensor fabrication [18][19][20] and templates for processing well-defined materials [21]. The widely application of these nanostructures in area of research is due to their unique properties superior to those of their bulk counterparts. In recent years, more and more researchers devote themselves to obtaining nanomaterials with perfect optical or electronic properties [22][23][24][25]. However, luminescent nanostructures fabricated from the amino acid analogues with bases through co-assembly have less been well studied.
Amino acids, because of their biocompatibility, structural and functional diversity, have significant advantages as building blocks for the construction of self-assembled nanostructures. As a promising π-conjugated chromophore, naphthalene diimide (NDI) [26][27][28][29] has been used to build a range of organized structures in solution due to its pronounced capabilities of selfassembly by means of π-π stacking. Besides, bases [30] with low biotoxicity and good biocompatibility play an important role not only in the system of life but also in the field of creating selfassembled nanostructures.
Herein, we are aimed at acquiring luminescent nanostructures fabricated from the co-assembly of amino acid derivatives with bases. The amino acid derivatives contain an aromatic naphthalene core and two amino acid residues as terminal groups, which are able to form complementary hydrogen bonding interaction with bases. Thereafter, the synergist interaction between hydrogen bonding interaction and the π-π stacking interaction drive the formation of well-ordered nanostructures in solution.

Materials
All starting materials were purchased from commercial suppliers and used without any further processing.

Synthesis and characterization of IN
0.2682 g (1 mmol) naphthalene-1, 4, 5, 8-tetracarboxylic dianhydride, 0.2623 g (2 mmol) isoleucine and 2.0 g imidazole were mixed and then heated to 120°C for 6 hrs. under nitrogen atmosphere. Then 100 mL ethanol was poured into the hot mixture, refluxed for 6 hrs. and kept for overnight to precipitate out. Afterward using dilute HCl to acidify solution, the resulting light yellow precipitate was concentrated by decompression filters, washed with deionized water to neutral, filtrated and dried at 80°C under vacuum. The 0.3501 g yellow powder was obtained (Yield: 70.8%). The structure and purity of the product were confirmed by 1 H NMR, MS and FT-IR. 1  Characterization NMR spectra were measured on a Bruker Ultrashield 400 ( 1 H NMR 400 MHz) spectrometer. UV absorption spectra were obtained using a UV-2700 UV-Vis spectrophotometer. The luminescence spectra were measured on a LS55 fluorescence spectrophotometer. The path length of the quartz cell is 1 cm, while the emission band-width was 5 nm. The FT-IR spectra were recorded on an AVATAR 360 FTIR which were collapsed mostly after solvent evaporation under high vacuum. This phenomenon illustrated that the charming structures observed by SEM are hollow walnut-like structures. SEM image of IN/U=1.5: 1 system was exhibited in Figure 1d, in which a sponge-like nanostructure with charming appearance was found. The sponge-like nanostructure was composed of thin and curved nanosilk. All the silk are connected together through their cores and form sponge-like structures. However, no ordered structures could be observed by SEM with IN/base at certain molar ratio in diluent ammonia solution or in pure DMF, respectively. This phenomenon was presumably attributed to weakening of hydrogen bonding interactions in the process of aggregation and exaggerated solubility in pure DMF.
All these interesting nanostructures were attributed to complementary hydrogen bonds between adjacent IN and base molecules and the π-π interaction between IN units.
To explore π-π stacking of NDI chromophores within the nanostructure of IN, UV absorption spectra were obtained using a UV-Vis spectrophotometer.  Figure 3c and 3d at favourable molar ratio, a broad emission band with the peak at about 543 nm with stronger fluorescence emerged, compared to pure IN in mixed solution. This phenomenon could be ascribed to the formation of aggregates in the mixed solution, which was due to the π-π electronic coupling between NDI chromophores.

Formation mechanism of nanostructures
By correlating the above information, the formation of all sorts of morphologies during the co-assembly can be understood using the schematic model presented in

Preparation of nanostructures
Mixed the IN solution in DMF with the solution of bases which were dissolved in dilute ammonia in different ratios of the volume. Kept for a period of time and then characterized the properties of the aimed nanostructures.

Molecular design
The molecular structures of IN, A, T, G and U were exhibited in Scheme 1. As for the compound IN, it contains two isoleucine groups as terminal groups and a naphthalene core. The isoleucine groups have charming properties of harmless, biocompatible and biodegradable, which may contribute to biomedicine applications. Besides, as an interesting class of chromophores, 1, 4, 5, 8-naphthalenetetracarboxylic acid diimide (NDI) due to its' enhanced stability and pronounced capabilities of self-assembly by means of π-π stacking, which may play a vital role in functional devices.

Morphology and driving force of nanostructures
According to the method described in the experimental section, we got the anticipated nanostructures. The morphological properties of these assembled nanostructures were initially examined by SEM.
Interestingly, a solution of IN in DMF-diluent ammonia mixed solution generated irregular flake structures ( Figure S1a), which was ascribed to a slight weakening of hydrogen bonding interactions in the process of self-assembly. Figure 1a revealed the formation of nanotubes, which appeared as hollow tubular structures. As shown in Figure 1b, condensed entangled nanofibers generated a close-knit morphology, in which thinner and smaller fibers cross-linked with each other to form a three-dimensional network. Figure 1c     Interestingly, nanotubes and walnut-like structures were hollow while nanofibers and sponge-like nanostructures were solid. The generating of these hollow and solid nanostructures was attributed to the complementary hydrogen bonds and the π-π interaction ( Figure S3).
The conjectures to the formation of different nanostructures were below. Due to both A and G have five nitrogen atoms and  the nitrogen atoms participated in H-bond formation were diverse, the hydrogen bonds between carboxyl groups of IN and nitrogen atom of A and G were dissimilar. It speculated that the combination of carboxyl groups of IN with nitrogen atoms H1, H3 (separated by five bonds) of A through hydrogen bonds made it possible to generate tubular structures (Figure 4a). However, in consequence of steric effect, only the nitrogen atoms H1, H4 (separated by four bonds) of G participate in H-bonding, which led to forming of walnut-like structures (Figure 4a). Similarly, the nitrogen atoms involved in H-bonding in T or U were close to each other, which enabled the generation of solid nanostructures (Figure 4b).

Conclusions
In summary, we have obtained luminescent nanostructures with highly intense fluorescence fabricated from the co-assembly of amino acid derivatives IN with bases in DMF-diluent ammonia mixed solution. These components assembled into well-defined nanotubes, walnut-like structures, nanofibers and sponge-like structures based on the complementary hydrogen bonds between adjacent IN and bases molecules and the π-π interaction between IN units. These interactions are responsible for the formation of luminescent nanostructures. The nanostructures may play potential application as biomaterials in the field of biomedical.