The interplay of chemical structure, physical properties, and structural design as a tool to modulate the properties of melanins within mesopores

Abstract

The design of modern devices that can fulfil the requirements for sustainability and renewable energy applications calls for both new materials and a better understanding of the mixing of existing materials. Among those, surely organic-inorganic hybrids are gaining increasing attention due to the wide possibility to tailor their properties by accurate structural design and materials choice. In this work, we'll describe the tight interplay between porous Si and two melanic polymers permeating the pores. Melanins are a class of biopolymers, known to cause pigmentation in many living species, that shows very interesting potential applications in a wide variety of fields. Given the complexity of the polymerization process beyond the formation and structure, the full understanding of the melanins' properties remains a challenging task. In this study, the use of a melanin/porous Si hybrid as a tool to characterize the polymer's properties within mesopores gives new insights into the conduction mechanisms of melanins. We demonstrate the dramatic effect induced on these mechanisms in a confined environment by the presence of a thick interface. In previous studies, we already showed that the interactions at the interface between porous Si and eumelanin play a key role in determining the final properties of composite materials. Here, thanks to a careful monitoring of the photoconductivity properties of porous Si filled with melanins obtained by ammonia-induced solid-state polymerization (AISSP) of 5,6-dihydroxyindole (DHI) or 1,8-dihydroxynaphthalene (DHN), we investigate the effect of wet, dry, and vacuum cycles of storage from the freshly prepared samples to months-old samples. A computational study on the mobility of water molecules within a melanin polymer is also presented to complete the understanding of the experimental data. Our results demonstrate that: (a) the hydration-dependent behavior of melanins is recovered in large pores (≈ 60 nm diameter) while is almost absent in thinner pores (≈ 20 nm diameter); (b) DHN-melanin materials can generate higher photocurrents and proved to be stable for several weeks and more sensitive to the wet/dry variations.

Figure 1

Pictorial view of the main properties of the melanin polymers obtained from DHI and DHN. In detail, (A) shows the main structural features, (B) shows the UV–visible absorption profiles, (C) shows the different robustness and (D) shows the EPR spectra and main data^–.

Figure 2

SEM image of the cross section of a typical sample, where the pore shape and size are visible.

Figure 3

Schematic of a PSi sample ready for the photocurrent measurements. The various elements are indicated.

Figure 4

Schematic of the experimental steps of this study. (1) Fabrication of the porous layer, (2) impregnation of the porous layer using an alcoholic monomer solution, (3) polymerization of the monomers by keeping the samples in an ammonia-saturated environment, (4) deposition of the electrical contact, (5) wet or dry storage of the samples, (6) measure of the photocurrent values.

Figure 5

Evolution of the ratio R between the unmodified polymer volume V2 to the full pore volume Vtot as a function of the pore diameter. The symbols are defined in the figure inset. Larger pore diameters lead to higher R, with the highest sensitivity in the 5–30 nm diameter range.

Figure 6

Behavior of PSi:melanin samples when stored in wet (light blue areas) and dry (grey areas) environment for (a) D1-DHI sample and (b) W2-DHI sample in a timespan of three weeks after formation.

Figure 7

Behavior of PSi samples with DHN-based melanin: (a) W1-DHN sample and (b) D1-DHN sample. The grey and light blue areas indicate dry and wet storage environment, respectively.

Figure 8

Temporal evolution of the photocurrent generation in a D3-DHN sample. The grey and light blue areas indicate dry and wet storage environment, respectively.

Figure 9

Temporal evolution of a D2-DHN sample photocurrent. The grey and light blue areas indicate dry and wet storage environment, respectively. The light red area indicates the temporal interval corresponding to the Covid-19 lockdown restrictions to laboratory access.

Figure 10

Long-term evolution of the (a) D1-DHN and (b) W1-DHN samples shown in Fig. 6. The grey and light blue areas indicate dry and wet storage environment, respectively.

Figure 11

Estimated MSD versus time at T = 300 K for 100 TIP3P water molecules initially placed inside the polymer matrix in a region R1 between z > 0 and z < 1.3 nm (blue circles) and in a region R2 corresponding to z > 2 and < 4 nm (red diamonds).