Semiconducting polymer–dipeptide nanostructures by ultrasonically-assisted self-assembling

Abstract

P3HT–dipeptide nanostructures were prepared through an ultrasound-assisted self-assembling method. This new bio-organic material has the semiconducting properties of the P3HT polymer while keeping the inherent self-organization of biological systems. Benefiting from such synergy, field-effect transistors are demonstrated.

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Field effect transistors (FET), non-volatile memories, light emitting diodes (LED) and photovoltaic cells (PV) based on organic semiconductors have been widely investigated in the last decade. Their development paves the way towards flexible organic electronics, which is expected to complement silicon electronics. Organic electronics on flexible substrates may provide unique technologies and generate new applications and form factors to address the growing needs for pervasive computing and enhanced connectivity.¹

Most of the organic FETs are built from synthetic small molecules,² conjugated polymers,³ oligomeric or blended⁴ semiconducting materials. Other straightforward approaches include self-organized columnar stacks of aromatic compounds like discotic liquid crystals (DLCs),⁵ bent-core molecules and polyaromatic dendrons. In general, the polymer-based devices are easily constructed by solution processing techniques (e.g. drop-casting, spin-coating, ink-jetting, slot-die, gravure, spray) but their performance parameters and life-time are usually inferior to small-molecule ones. The field effect mobility of an organic FET is intrinsically dependent on the local intramolecular structure and the intermolecular packing between the molecules. Also, percolation by charge motion at interfaces and fluctuations in the intermolecular packing that occur on the same timescale as carrier motion are equally important for the mobility.^6,7 Besides the organic π-conjugated classes cited above a quite different alternative has been emerged in the recent years for device fabrication. Vladu, Sariciftci and Bauer called this new class as “exotic” materials and it comprehends biological or bioinspired materials like paper, leather, silk, gelatine, DNA and peptides.⁸ The motivation behind the use of such biodegradable materials as substrate, dielectrics or semiconductors is to generate more sustainable and eco-friendly electronics. In special, proteins and peptides have the intrinsic ability to self-assembly into supramolecular structures like nanotubes, nanospheres, nanowires and hydrogels. These complex structures have low dimensional order giving rise to quantum confining (QC) effects which in turn impact their optical and electronic properties. Ferroelectricity, piezoelectricity, second harmonic generation (SHG), high-capacitance and specific wettability are a few exempla of interesting properties originated from QC and the helical or chiral asymmetry of peptide nanostrucutures.⁹

In this paper, we report for the first time the preparation of a hybrid material having (L)-diphenylalanine (Phe–Phe or FF) as biological component and poly(3-hexylthiophene)-P3HT as organic semiconductor. Conventionally, FF nanostructures have been obtained by simple mixing a small amount of highly concentrate (ca. 100 mg mL⁻¹) solution of FF in fluorinated solvent (e.g. hexafluoroisopropanol or hexafluoroethanol) with deionized (DI) water. By doing so, FF molecules self-assembly mostly into bundles of nanotubes. Recently, Qin et al. studied in details the influence of driving forces in this process like peptide concentration, temperature, pressure, and self-assembly time for a β-amyloid peptide derivative.¹⁰ Our attempts using the conventional synthesis protocol were fruitless to produce FF nanostructures with P3HT. To enhance the incorporation of P3HT and also to get control over the shape and size distribution of FF structures we developed a procedure using ultrasound energy to trigger the self-assembling process. Fig. 1 shows schematically our ultrasound-assisted procedure to prepare FF:P3HT hybrid materials. In a brief, to a solution of FF in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) was added a stoichiometric amount of P3HT in 1,2-dichlorobenzene (DCB) and 100 μL of DI water.

Fig. 1 Schematic representation of the ultrasound-assisted self-assembling for FF:P3HT hybrid nanostructures.

The process of self-organization initiated spontaneously by ultrasound tip giving rise to FF:P3HT hybrid material as a light-violet precipitated. The temperature was kept at 0–4 °C with an ice-water bath during all the process. After ceased the ultrasonic energy application (15 min) the material was allowed to stand for 8 h. The crude product was washed and harvested with centrifugation–redispersion cycles (2265 g) using DCB as solvent. Then, the resulting solid was finally dried in vacuum oven at 60 °C for 24 h.

We evaluated the effect of ultrasound intensity ranging from 0 to 99 W cm⁻² on the shape and mean-size of the hybrid structures obtained. The Fig. 2 shows SEM images of the FF:P3HT prepared at 0, 28.3, 56.6 and 84.7 W cm⁻², respectively from top left towards bottom right. When no ultrasound is used (0 W cm⁻²) the material produced is essentially amorphous with some predominance of barely defined threads. On the other hand, upon ultrasound energy all samples presented as bundles of nanobelts with size distribution dependent on the ultrasound intensity. For instance, using 28.3 W cm⁻² bundles having 0.5 to 2 μm width and 5 to 30 μm long are visualized in the SEM image. At higher intensity such as 56.6 and 84.7 W cm⁻² we observed the formation of somewhat wider bundles of belts peaking to 15 μm at 56.6 W cm⁻². Additionally, we performed experiments of dynamic light scattering (DLS) in diluted solution in order to get information about the size of the individual FF:P3HT nanostructures. The mean-size values presented here were estimated using the Stokes–Einstein equation where the equivalent sphere has the same contour length of the FF:P3HT nanobelts.¹¹

Fig. 2 SEM images of FF:P3HT hybrid material self-assembled upon different ultrasonic intensities@20 kHz frequency.

One can see from Table 1 a roughly linear trend of increasing mean size with ultrasound energy. Molecular structure of the assemblies formed by FF:P3HT was probed by circular dichroism (CD) and FT-IR spectroscopy. Fig. 3 displays the far-UV spectra for FF and FF:P3HT self-assembled at 28.3 W cm⁻² ultrasound intensity. Both spectra are characterized by two positive bands at 200 nm and 218 nm attributed to π → π* and n → π* transitions of the peptide bond, respectively. The positive values of ellipticity and the position of the bands (200 nm and 218 nm) strongly suggest the anti-parallel β-sheet arrangement for both FF and FF:P3HT samples. The exact origin of such bands is still unknown but it is speculated that the 200 nm band contributes to the aromatic π-stacking while the 218 nm band is an indicative of the relative degree of H-bond in the assemblies.¹² Worthwhile to notice the appearance of one extra low wavelength positive band at 205 nm in the FF:P3HT CD spectrum. Reviewing the literature we also identified the presence of this band in other papers mostly as a tiny shoulder of the 200 nm band in self-assembled FF samples.¹³ If it is correct the assumption that the 200 nm band is related to the π-stacking we believe the increasing of intensity of the 205 nm band is a indicative that P3HT molecules can be affecting the stacking among the phenyl ring from the dipeptide moieties. In infrared spectra shown in Fig. 4 we found two strong peaks at 1686 cm⁻¹ and 1614 cm⁻¹ at amide I region for both FF and FF:P3HT nanostructures. ​These absorption maxima are consistent with the predominance of β-turn and β-sheet conformations.^14,15 The characteristic bands located at frequency approximately 3253 cm⁻¹ and 1550 cm⁻¹ are due to the asymmetric stretching NH– and bending confirming the presence of the amino group.

Table 1 DLS data for ultrasound-assisted FF:P3HT self-assembling

Sample	Time [min]	Ultrasound intensity [W cm^−2]	Mean size [nm]
1	15	28.3	157
2	15	56.6	217
3	15	84.7	198
4	15	99.0	230

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Fig. 3 Far-UV circular dichroism spectrum of self-assembled FF and FF:P3HT by ultrasound energy. Conditions: c = 0.09 mg mL⁻¹ (in 1 : 1 isopropanol/ethanolamine), scan speed = 50 nm min⁻¹, 5 scans, l = 0.1 cm, T = 20 °C.

Fig. 4 Attenuated total reflectance FT-IR spectra for films of FF, P3HT and FF:P3HT.

This stretch depends on the strength of hydrogen bonding of the N–H⋯O=C chain which is sensitive to structure and its variations. The NH stretching observed, strongly suggests the presence of several hydrogen bonds present between the peptide subunits, providing information about the geometry.¹⁶ Of major importance, are the three bands at 2854, 2922 and 2952 cm⁻¹ correspond to the C–H aliphatic stretching from P3HT and along the bands at 818 cm⁻¹ (C–H out of plane), 1379 cm⁻¹ (CH₃ deformation) and 1461 cm⁻¹ (sym. C–C stretching) confirm the incorporation of the polymer in the hybrid material.

Powder XRD data shown in Fig. 5 reveal that the hexagonal six-fold symmetry (space group P6₁) of the self-assembled FF is kept to FF:P3HT sample. The diffraction peaks and hkl indexes attribution were done by comparing the data found in the literature.^17,18 The electrochemical properties of the pristine and hybrid bio-organic materials were investigated by cyclic voltammetry (CV) as drop-casted film, Fig. 6. The CV of pure self-assembled FF agrees perfectly with vapour-deposited FF nanotubes on carbon electrodes.¹⁹ On the other hand, when FF is self-assembled in the presence of P3HT to render FF:P3HT the semiconducting properties of the polymer are completely transferred. The FF:P3HT's voltammogram is quite similar to pure polymer with a slightly displacement of 0.12 V and 0.08 V for first reduction and oxidation potentials, respectively. Taking the redox potential of the Fc/Fc⁺ couple as −5.1 eV relative to vacuum²⁰ and using an empirical relation we estimated the HOMO/LUMO levels for the materials (ESI†). The HOMO and LUMO levels of the FF:P3HT nanostructures were found to be respectively 5.0 eV and 3.0 eV giving rise to a electrochemical bandgap of E_g = 2.0 eV.

Fig. 5 XRD patterns of self-assembled FF and FF:P3HT.

Fig. 6 Cyclic voltammograms of self-assembled FF and FF:P3HT and P3HT films.

Just for comparison, Santhanamoorthi et al. calculated the bandgap for pure FF nanotubes by using density functional theory (DFT) method. They found values of E_g = 5.59 eV and E_g = 6.41 eV for simulated linear and cyclic structures, respectively.²¹ Chemical stability of FF:P3HT was determined by thermogravimetric analysis (TGA and DTGA). From Fig. 7, it can be seem the FF:P3HT nanostructures are completely decomposed at 415 °C that is somewhat about 50 °C higher than the unmodified self-assembled FF. Weight loss observed up to 100 °C (−4.2 wt%) is attributed to evaporation of water molecules that interact with the cavity of the nanostructures.¹⁸ The TGA trace also presents an additional weight-loss step (−5.1 wt%) at about 195 °C. About 83 wt% of sample is wasted at 263 °C through the release of phenylalanine building blocks.^13,22,23 At last, the decomposition of the P3HT polymer takes place in the range of 348–412 °C. Once this represents 7.1% of the initial mass we estimated such percentage as the amount of P3HT incorporated in FF:P3HT material. We evaluated the application of the hybrid material FF:P3HT as semiconductor in FETs. A dispersion of FF:P3HT nanobelts in 1-butanol (1.88 g mL⁻¹) prepared as described above using 28.3 W ultrasound intensity was drop-casted on bottom-gate/bottom-contact device structure having n⁺⁺-Si as gate, SiO₂ as gate dielectric and Au/ITO as source and drain (ESI†). The Fig. 8 shows the output and transfer characteristics of the FET. From the data above, the calculated threshold voltage and hole field-effect mobility in the saturation region are V_th = 18.5 V and μ_FE = 8.57 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively. As comparison, a standard device was made using only the P3HT polymer (ESI†) in ambient conditions and it presented V_th = 12.5 V and μ_FE = 2.44 × 10⁻⁵ cm² V⁻¹ s⁻¹. Such increasing in the mobility value indicates that FF nanostructure is increasing in some degree the charge transport through the polymer. Additional experiments are being undertaken to understand exactly this phenomenon.

Fig. 7 TGA traces for FF and FF:P3HT.

Fig. 8 (a) Output and (b) transfer characteristics for FF:P3HT FET. W/L = 500.

Additionally, we investigate the ultrasonically-assisted self-assembling of FF with other conjugated polymer looking for applications beyond FETs. From the PCDTBT, PFO, and PFOG were prepared (ESI†) the correspondent hybrid materials with FF as shown in Fig. 9.

Fig. 9 Ultrasound-assisted self-assembling of FF with different conjugated polymers at 28.3 W cm⁻².

By the SEM images we can see that PCDTBT result in nanorods of FF:PCDTBT while PFO and PFOG produced nanotubes and nanoflowers of FF:PFO and FF:PFOG, respectively. That result indicates there is some molecular recognition between the conjugated polymer and the dipeptide which in turn leads to different nanostructures.

Conclusions

In conclusion, we have demonstrated a simple and efficient method to prepare hybrid bio-organic nanostructures composed by dipeptides and semiconducting polymers. The size of the nanostructures can be controlled by the solvent and applied ultrasound intensity during the self-assembly process. The hybrid material has the semiconducting p-type properties from the original polymer (P3HT) and can be used in FET structures. The protocol can be extended to other conjugated polymers as demonstrated by the preparation of FF:PCDTBT, FF:PFO and FF:PFOG owing applications for instance in OPV and OLEDs.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and additional characterization. See DOI: 10.1039/c6ra03013k

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