Chitosan hydrogelation with a phenothiazine based aldehyde: a synthetic approach toward highly luminescent biomaterials

Andrei Bejan a, Daniela Ailincai a, Bogdan C. Simionescu ab and Luminita Marin *a
a“Petru Poni” Institute of Macromolecular Chemistry, Romanian Academy, 700487 Iasi, Romania. E-mail: lmarin@icmpp.ro
bDepartment of Synthetic and Natural Polymers, “Gh. Asachi” Technical University of Iasi, 700050 Iasi, Romania

Received 2nd October 2017 , Accepted 16th November 2017

First published on 16th November 2017


Pure organic luminescent hydrogels were synthesised using the condensation reaction of amino groups of chitosan with a photoactive aldehyde bearing a phenothiazine moiety. The hydrogels were structurally and supramolecularly characterized by FTIR and 1H-NMR spectroscopy, X-ray diffraction and polarized light microscopy. It was concluded that hydrogelation occurred due to the self-ordering of the chitosan segments grafted with pendant phenothiazine imines into ordered clusters, which play the role of crosslinking nodes. Scanning electron microscopy revealed microporous morphology with very thin pore walls. The ordered clusters immobilized in the polycationic chitosan network were responsible for high quantum efficiency, reaching the value of 51% in the xerogel state. The hydrogels formed flexible, transparent films with high mechanical toughness. Besides the remarkable optical and mechanical properties, the importance of the hydrogels is increased by their eco-design, based on chitosan from renewable resources and bio-friendly phenothiazine.


Introduction

Organic materials for opto-electronic applications have been intensely studied in the last few decades as a cheaper and more environmentally friendly alternative to inorganic ones.1 The real world applications require highly efficient luminescent materials in the solid state, a desideratum difficult to attain for organic luminogens which present the tendency to aggregation-caused quenching.2 Among organic luminescent materials, the concept of luminescent hydrogels has received increasing attention in the last few years, due to their unique physico-chemical characteristics which make them suitable not only in special biomedical applications, such as bioimaging agents, tissue engineering, molecular thermometers, 3D-printing,3–5 but also in classic photonics, as organic light emitting diodes, solar cells or fluorescent sensors.6–10 Their high content of water endows them with good transparency and mouldability, while their three-dimensional network confers them good mechanical properties.11,12 Besides this, the high surface-to-volume ratio of the micro- or nano-porous hydrogels is a promising feature to improve the optical properties,13 making them more attractive for integration in modern device architectures. A remarkable endeavour has been directed towards obtaining luminescent organic–inorganic hybrid hydrogels, mainly using lanthanide complexes as photoactive components, due to their phosphorescence properties.14 The best results reported a quantum yield up to 70%, but the stability of their efficiency is significantly affected by the non-radiative decay in aqueous environments.15,16 By contrast, luminescent hydrogels obtained using organic dyes are less reported, and their quantum efficiency is rather modest.15–18 The main difficulty in obtaining pure organic hydrogels is the low water solubility of the organic dyes, while their specific emission quenching and photobleaching discouraged research in this direction. The two important advantages of luminescent hydrogels based on biopolymers from renewable resources are their potential for bio-application, due to their biocompatibility and biodegradability, and their sustainable development, which enhances their practical value even more.19 In this regard, the latest research in the field of luminescent hydrogels has been focused on the use of bio-based materials, such as cellulose,20–22 pthalocyanines, porphyrins23 or amino acids24 as a matrix for photoactive compounds such as lanthanides, quantum dots or organic dyes. Nevertheless, the quantum yield was still low for their use in photonics, the obtained materials being especially studied for sensing applications.25–28

Chitosan is a biopolymer originating from chitin, the second most abundant natural polymer.29 Our research activity in the field of chitosan derivatives revealed a new concept of chitosan hydrogelation when reacting with monoaldehydes,30–35 experimentally reported by other authors too.36 The driving force of this unusual hydrogelation has been demonstrated to consist in the ability of the chitosan segments grafted with imine units to self-order in clusters, which act as network nodes. The new method proved to be a practical strategy towards advanced biomaterials, due to the large variety of monoaldehydes, many of them coming from natural resources, which allow the tailoring of hydrogel properties. Applying this simple principle, supramolecular hydrogels with in vivo biocompatibility and outstanding antifungal, swelling or antitumor properties were successfully obtained.32–35 The main issue of the method is to find suitable reaction conditions for the organic aldehyde with chitosan in aqueous solution.

In this paper, we report new pure organic hydrogels obtained by the acid condensation of a photoactive monoaldehyde with chitosan polyamine. A phenothiazine bearing monoaldehyde has been chosen as the fluorogen,37 since the imine-based phenothiazine heteroaromatic ring proved to have a high quantum efficiency in the solid state.38–40 Moreover, films obtained from chitosan and phenothiazine showed green light emission.41 As a proof of concept, the synthesised hydrogels show high quantum efficiency in both the hydrogel and the xerogel state and excellent mechanical properties.

Experimental

Materials

Low molecular weight chitosan (263 kDa) with a degree of deacetylation (DA) of 83%, acetone (99%), dimethylsulfoxide (UV-spectroscopy 99.8%) and glacial acetic acid (98%) were purchased from Sigma-Aldrich Co (USA) and used as received. The ethyl alcohol (96%) from Sigma-Aldrich was dried on molecular sieves before use. The molecular sieves (2 Å) from Sigma-Aldrich were dried before use, at 80 °C under vacuum for three days. The luminescent aldehyde has been synthesised in our laboratory by following a published procedure.37 Its solubility in ethanol was 2 mg mL−1. Its structure and structural characterization are given below.

10-(-4-Hexyloxy-phenyl)-10H-phenothiazine-3-carbaldehyde (FhA), m.p. = 108–109.8 °C.


image file: c7py01678f-u1.tif
1H NMR (400.13 MHz, DMSO-d6, ppm) δ = 9.76 (s, 1H), 7.50 (s, 1H), 7.44 (d, 1H), 7.38 (d, 2H), 7.26 (d, 2H), 7.04 (d, 1H), 6.97–6.89 (superposed bands, 2H), 6.30 (d, 1H), 6.22 (d, 1H), 4.14 (t, 2H), 1.90–1.83 (m, 2H), 1.59–1.39 (superposed bands, 6H), 0.94 (t, 3H); FT-IR (KBr, cm−1): 3059, 3021 (νCHaromatic), 2945, 2920, 2855 (νCH3, νCH2), 1675 (νC[double bond, length as m-dash]O), 1595, 1504 (νC[double bond, length as m-dash]Caromatic), 1239 (νC–O–C), 836, 813, 744 (δCHaromatic); X-ray crystallography: C25H25NO2S, T = 175 K, Mr = 403.5 gmol−1; space group P21/c; cell lengths: a = 13.6446(5), b = 14.2470(5), c = 10.9711(4); cell angles: α = 90.00°, β = 94.954(3)°, γ = 90.00°; cell volume: V = 2124.76 Å3; Z = 4, R = 5.06%.

Preparation of hydrogels and xerogels

In a 20 mL vial, a chitosan solution has been prepared from 30 mg chitosan (0.147 mmol of glucosamine repeating units) dissolved in 1.5 mL solution of acetic acid 0.7% (10.5 μL glacial acetic acid in 1.489 mL bidistilled water), at room temperature. To obtain hydrogels with different contents of luminogen, a solution of aldehyde (FhA) has been prepared by dissolving various amounts of FhA in 340 μL DMSO (Table 1). Both solutions were heated at 65 °C and the aldehyde one has been slowly dropped into the chitosan one under vigorous magnetic stirring. A viscous solution, opaque yellow in colour, has been obtained through the process. Furthermore, 1 mL of acetone was slowly dropped into the reaction mixture, resulting in a transparent semisolid, which passed the inversion test used for the visual assessing of the hydrogels (Scheme 1). The hydrogels were kept uncovered for three days at room temperature to let the acetone evaporate. To remove the DMSO and the traces of unreacted aldehyde, the hydrogels were washed by successive immersions of 30 minutes each in portions of 20 mL dry ethanol, until the alcohol remained colourless, without light emission when illuminated with a UV lamp, and hydrogels were macroscopically homogeneous, with no visible aggregates inside. Furthermore, in order to rehydrate the hydrogels and to remove the ethanol, they were immersed five successive times in portions of 20 mL water. The hydrogel synthesis has been performed in triplicate, starting from three different amounts of chitosan: 30, 60 and 90 mg, respectively, for all five different molar ratios. In all cases, the hydrogelation successfully occurred. The corresponding xerogels were prepared by lyophilization.
image file: c7py01678f-s1.tif
Scheme 1 Schematic representation of the synthesis of the luminescent hydrogels.
Table 1 Reagent and product amounts used in obtaining hydrogels and their codes
Code CPA1 CPA2 CPA3 CPA4 CPA5
a Calculated with eqn (1) given in the ESI. b Calculated with eqn (2) given in the ESI. c Calculated with eqn (3) given in the ESI.
NH2/CHO molar ratioa 1[thin space (1/6-em)]:[thin space (1/6-em)]0.05 1[thin space (1/6-em)]:[thin space (1/6-em)]0.1 1[thin space (1/6-em)]:[thin space (1/6-em)]0.2 1[thin space (1/6-em)]:[thin space (1/6-em)]0.3 1[thin space (1/6-em)]:[thin space (1/6-em)]0.4
Chitosan (g) 0.03 0.03 0.03 0.03 0.03
Aldehyde (g) 0.00295 0.0059 0.0118 0.0178 0.02375
Xerogel (g) 0.03241 0.0348 0.0412 0.0475 0.04969
η% (C[H with combining low line]O → C[H with combining low line][double bond, length as m-dash]N)b 82 81 94 98 83
η% (NH2 → CH[double bond, length as m-dash]N)c 4.1 8.1 18.8 29.4 33.2


Methods

The corresponding xerogels were obtained by freezing the rehydrated hydrogels in liquid nitrogen and further lyophilization using LABCONCO FreeZone Freeze Dry System equipment, at −50 °C and 1.510 mbar for 24 hours.

FTIR spectra were recorded on xerogel samples, using a FTIR Bruker Vertex 70 spectrometer, by using the ATR technique, at room temperature, with a resolution of 2 cm−1 and an accumulation of 32 scans. They were processed using Opus6.5 and Origin8 software.

1H-NMR spectra were recorded on a Bruker Avance DRX 400 MHz spectrometer equipped with a 5 mm QNP direct detection probe and z-gradients, at room temperature, with an accumulation of 64 scans. The chemical shifts were reported as δ values (ppm) relative to the residual peak of the solvent. The hydrogel sample has been prepared in the NMR tube using deuterated solvents (D2O to solve chitosan, DMSO-d6 to solve the aldehyde, and (CD3)2CO for hydrogelation). The sample preparation succeeded only in the case of the CPA1 hydrogel. For the other samples (CPA2CPA5), in the absence of a vigorous stirring impossible to be provided into the NMR tube, the aldehyde abundantly crystallized hindering hydrogelation.

Wide angle X-ray diffraction (WXRD) was performed on xerogel pellets using a Bruker D8 Avance diffractometer with Ni-filtered Cu-Kα radiation of λ = 0.1541 nm, in the range of 2–50° (2 theta). The working conditions were 36 kV and 30 mA. Data were handled by using the FullProf 2000 program. The xerogel pellets were obtained with a manual hydraulic press, by applying a pressure of 10 N m−1.

Textures of the hydrogels were monitored on thin slices of samples placed between two clean glass slides with an Olympus BH-2 polarized light microscope equipped with a THMS 600 heating stage and a LINKAM TP92 temperature control system.

Micrometric images of the xerogels were acquired with a Scanning Electron Microscope SEM EDAX – Quanta 200 at an accelerated electron energy of 20 keV.

The UV-Vis absorption spectra were recorded with a Carl Zeiss Jena Specord M42 instrument, by using very thin hydrogel films deposited on glass plates.

The absolute values of the quantum yield (Φfl) of the hydrogels and xerogels, respectively, were obtained with a FluoroMax-4 spectrofluorometer equipped with a Quanta-phi integrating sphere accessory Horiba Jobin Yvon, by exciting the samples with a light of wavelength corresponding to the absorption maxima from the UV-vis spectra, at room temperature. To obtain a good optical luminescence signal-to-noise ratio, the slit widths and detector parameters were optimized to maximize but not saturate the excitation Rayleigh peak. The measurements were carried out in triplicate for each sample. Close values were obtained.

Tensile strength of the hydrogels was recorded for a rectangular piece (20 × 5 × 0.5 mm3) of six different samples on TIRA test 2061 equipment Maschimnbau GmbH Ravenstein, Germany. Measurements were done on triplicate samples, at an extension rate of 50 mm min−1, at room temperature and the average value was reported. Elongation tests were performed by applying an elongation of 20, 40, 50, 60, 80 and 100%. The sample has been kept elongated for 20 minutes, and then allowed to relax for 5 minutes. The viscoelastic loss was calculated as the difference between the initial length and the length after applying the elongation test.

Results and discussion

Hydrogelation mechanism: experimental evidence

Luminescent hydrogels were prepared by grafting phenothiazine-bearing aldehydes on the chitosan backbone via an acid condensation reaction, when imine linkages were formed (Scheme 1). DMSO – a biodispersant with a high solvation power – was used as the solvent of the aldehyde, to avoid its crystallization in the aqueous medium of chitosan solution. Acetone has been used as the co-solvent to ensure a homogeneous reaction medium. Varying the molar ratio of the aldehyde and amine functionalities, a series of hydrogels with different contents of imino-phenothiazine photoactive units was obtained. The minimum amount of aldehyde necessary for chitosan gelling corresponded to NH2/CHO = 1/0.05 (CPA1). The maximum amount of aldehyde used for chitosan gelling without its abundant crystallization corresponded to NH2/CHO = 1/0.4 (CPA5). Hydrogels formation was firstly assessed by visual inspection using the inversion test (Scheme 1). Under illumination with a UV-lamp, they strongly emitted green light, indicating that the photoactivity of the phenothiazine was maintained by gelling.

The successful grafting of the aldehyde on chitosan chains by the imine linkage was demonstrated by FTIR spectra of the corresponding xerogels, which showed a vibration band at 1648 cm−1 as a slightly sharper peak as compared to the shoulder at 1639 cm−1 specific to the vibration band of the amide group of chitosan (Fig. 1).30–35,42 Taking into consideration that the vibration band of the aliphatic–aromatic imine bond is shifted to higher wavenumbers compared to aromatic–aromatic ones, this was attributed to the newly formed imine bonds.31 No evident aldehyde band was observed around 1675 cm−1,37 indicating the preponderance of the photoactive phenothiazine as imine units. Comparing the FTIR spectra of the CPA1–CPA5 xerogels with that of chitosan, changes were obvious in the 3700–2700 cm−1 region, which is characteristic of the asymmetric and symmetric N–H stretching and vibrations of the intra- and inter-molecular hydrogen bonds.30 The change involved the shifting of the overlapped bands to higher wavenumbers, suggesting morphological modifications, in agreement with the grafting of the rigid imine units on the semiflexible chitosan chains.30–35 Moreover, changes occurred in the intensity of the band maxima attributed to the stretching vibration of the C–O bond (1069 and 1031 cm−1).33 As can be seen in Fig. 1, the maximum at 1031 cm−1 increased in intensity and the maximum at 1069 cm−1 decreased for the CPA1–CPA5 samples compared to that of chitosan. Considering that this band is strongly influenced by the H-bond environment, the change was also attributed to the morphological rearrangements due to the imination.


image file: c7py01678f-f1.tif
Fig. 1 FTIR spectra of CPA1–CPA5 xerogels and chitosan.

The successful grafting of the phenothiazine moieties onto chitosan chains by imine linkages was further proved by the 1H-NMR spectrum recorded on the CPA1 freshly prepared hydrogel, before washing. It revealed the chemical shift of the imine proton at 8.5 ppm as two bands corresponding to α and β conformers (Fig. 1s).33,43 The chemical shift of the aldehyde proton was also present in the NMR spectrum, with an integral ratio of C[H with combining low line]O/C[H with combining low line][double bond, length as m-dash]N of 1/3.7. This indicated the shifting of the reaction equilibrium to the imino-chitosan product (a glycodynamer due to the reversible imine linkages) with a conversion rate of the aldehyde into imine units of 79%. This means that in the case of CPA1, about 4% from the amine groups on chitosan were transformed in imine linkages. The estimation of the conversion rate from the 1H-NMR spectrum agrees well with the calculation from the weighting measurements (see the Experimental section), displaying an average conversion of the aldehyde into imine units at around 90%, i.e. the percent of the amine units converted into the imine ones on the chitosan chains varied from 4.2 to 33.2% (Table 1). The shifting of the reaction equilibrium to the imine products was attributed to the formation of the phenothiazine–imine donor–acceptor system with high stability.38,39

To gain insight into the morphological changes suggested by FTIR spectroscopy, X-ray diffraction has been performed on xerogel pellets. As can be seen in Fig. 2, the semicrystalline chitosan transformed by imination into a more ordered material, characterised by a sharp reflection in the medium angle domain, at 6.4°, sharper reflection peaks on the top of the broad reflection in the wide angle region, around 20° and a sharp band at 29.4°, and a slight broad band around 12°. The corresponding d-spacing, as calculated using Bragg's law was around 13.8, 4.4, 3.15, and 7.4 Å, respectively. Compared to other phenothiazine imine derivatives, the position of the reflection bands is quite similar, signifying a similar ordering pattern. As an example, an imine bearing a phenothiazine residue showed in single crystal X-ray diffraction a packing of the molecules in ribbons with an intermolecular distance of 8.2 Å, further packed in layers with an inter-ribbon distance of 3.8 Å and an inter-layer distance of 15.3 Å.38,44 Similarly, a layered packing of the chitosan segments grafted with phenothiazine imine units can be foreseen. Compared to other imino-chitosan derivatives,30–35,45 the sharper peaks at a lower angle were of lower intensity or even missing, in agreement with the low amount of imine units on the chitosan backbones. However, the diffraction pattern was quite similar in the case of CPA2, CPA3, CPA4 samples, with sharper reflection bands around 6 and 20° and a broad reflection around 12° – attributed to the inter-layer, inter-chain and inter-molecular distances of a supramolecular layered architecture resulted due to (i) the hydrophobic/hydrophilic segregation of the hydrophobic imine units and hydrophilic chitosan chains, and (ii) self-ordering of the hydrophobic imines. The hydrogelation can be explained considering that the same chitosan chain can pass through different ordered clusters, thus forming a network in which the ordered clusters play the role of crosslinking nodes, as represented in Scheme 1. The lower density of imine units (CPA1) led to a relatively lower amount of crosslinking nodes as compared to the higher amount of chitosan, below the detection limit of the diffractometer. On the other hand, the formation of smaller ordered clusters in the case of the CPA5 sample, with a weak positional order and a long-range directional order, also led to the absence of specific reflections in the X-ray diffraction. The broad reflection of the chitosan X-ray pattern became slightly broader into those of the xerogels, in agreement with the linking of the chitosan chains between the ordered clusters, a fact which hinders their H-bonding leading to a more disordered chitosan phase. In light of these data, the chitosan crosslinking appears to be the result of the self-ordering process of the chitosan segments grafted with imine units, which represents the driving force of hydrogelation.


image file: c7py01678f-f2.tif
Fig. 2 X-ray diffraction of chitosan and CPA1–CPA5 xerogels.

Thermotropic behavior

To further confirm the formation of the ordered clusters, the hydrogel samples were investigated by polarized light microscopy (POM). As can be seen in Fig. 3, all samples showed strong birefringence, typical of ordered systems, confirming the X-ray data. In the case of hydrogels containing lower amounts of phenothiazine units (CPA1, CPA2), rare, birefringent, rhombohedral geometric shapes could be detected, in line with the weak reflection at a small angle in the X-ray pattern. As the amount of phenothiazine units increased, the geometric shapes decreased in size and became denser, giving a continuous optical texture in the case of CPA5, due to the superposing of the birefringent shapes across the sample.46,47 By heating the hydrogels, the birefringent shapes melted in the isotropic state at temperatures around 75 °C in the case of the hydrogels with a lower density of imine units (CPA1, CPA2, CPA3) and transformed into a Schlieren-like texture specific to the nematic mesophases in the case of the hydrogels with a higher density of imine units (CPA4, CPA5).48 The stability range of the texture exceeded the temperature of thermal decomposition of chitosan.49 The melting point was lower as compared to the one of the aldehyde precursors (108–109.8 °C). This suggests a less stiff supramolecular architecture of the ordered clusters, which can be attributed to the low mobility of the phenothiazine imine units on the semiflexible chitosan chains, hindering their close packing. Representative POM images are given in Fig. 3.
image file: c7py01678f-f3.tif
Fig. 3 Representative POM images of the hydrogels acquired, at 400× magnification (RT: room temperature, C: cooling).

Combining X-ray diffraction and polarized light microscopy data, the supramolecular architecture of the hydrogels could be envisioned as follows. When the concentration of the phenothiazine aldehyde was lower (CPA1, CPA2), the chitosan segments grafted with imine units segregated to form big, rare, layered clusters, similar to crystallization from diluted solutions. Their formation is favored by the carbonyl-amine/imine interconversion on the chitosan polyamine, acting as a molecular truck.50,51 Due to their smaller amount as compared to the larger amount of chitosan (approx. 0.8/10; 1.28/10, weight ratio), their characteristic reflections could not be detected through the X-ray diffraction experiment. By increasing the amount of phenothiazine aldehyde (CPA3, CPA4), smaller crystals are formed due to the higher density of imine units and of such a density that their sharper reflections in the X-ray diffractograms prevailed over the broad bands specific to the chitosan. With the the highest amount of aldehyde (CPA5), denser and smaller crystals were formed, with a weak positional order and a long-range directional order, undetectable under X-ray diffraction.52–54 Thus, the driving force of hydrogelation appears to be the formation of ordered clusters of variable sizes, anchored in the chitosan network. The size of the crystals decreases with increasing phenothiazine aldehyde amount, from CPA1 to CPA5.

Microstructure

The hydrogel morphology at the micrometric level, monitored by scanning electron microscopy of their corresponding xerogels, revealed a sponge like morphology with interconnected pores of diameter around 50 μm (Fig. 4, Fig. 2s). The remarkable part is the extreme thinness of pore walls, attributed to the low density of the crosslinking nodes, leading to an elastic, hydrophilic network, able to hold large amounts of water and thus to form large pores during the lyophilization process. No crystals were observed in the pores, indicating that the crystals evidenced by XRD and POM measurements were placed into the solid walls of the pores.
image file: c7py01678f-f4.tif
Fig. 4 SEM images of the hydrogels.

Photophysical behavior of the hydrogels/xerogels

The chitosan based hydrogels crosslinked with the phenothiazine aldehyde were designed as luminescent materials for optoelectronics and sensing applications. It was expected that combining the high surface-to-volume ratio (characteristic of microporous materials) with the emission ability of the phenothiazine fluorophore would result in efficient luminescent materials.

In this regard, the photophysical properties of both the hydrogels and the corresponding xerogels were monitored by absorption/emission spectroscopy, measuring the absorption and emission maxima, the Stokes shift, the quantum yield and the chromaticity coordinates. Besides this, to better understand the origin of the photophysical properties, comparison with the aldehyde precursor and other imines bearing a phenothiazine fluorophore was done.

The absorption spectra of the hydrogels exhibit overlapped bands with two maxima in the UV domain, around 360 and 395 nm, and a maximum of lower energy in the visible domain, around 480 nm, giving a broad absorption profile (Fig. 5a), reminiscent of the absorption spectra of inorganic55 or organic nanocrystals.40,56–58 Compared to the absorption spectrum of the aldehyde precursor37 and other phenothiazine–imine derivatives,38,39,59 the electronic absorption maxima appear highly bathochromically shifted by around 80 nm. This is a characteristic behavior of organic aggregates stabilized in emulsions,60 similar to the studied hydrogels in which phenothiazine based clusters are immobilized in the polycationic chitosan network (Fig. 5b). (i) The absorption maximum around 360 nm has been attributed to the π–π* benzenoid transitions of the local aromatic units, (ii) the maximum around 395 nm to the electronic transition of the conjugated skeleton and (iii) the maximum around 480 nm to the D → A intramolecular charge transfer.37,38,61 There is a close correlation between the supramolecular architecture of the hydrogels and the position of the absorption peaks that are almost 10 nm bathochromically shifted, as the size of the ordered cluster network nodes increases. This effect, well documented in the literature, was attributed to the enhanced intermolecular forces as the cluster size increases.56,57


image file: c7py01678f-f5.tif
Fig. 5 UV-vis absorption of the (a) hydrogels and (b) hydrogel CPA1 compared to the THF solution of the FhA aldehyde precursor37 and a phenothiazine–imine model compound (DP10).38

All hydrogels are transparent and formed flexible self-standing films when cast on a support (Fig. 6g and h). Under UV illumination, they emitted strong green-yellow light (Fig. 6i and j). Their photoluminescence properties were investigated by measuring the quantum yield and chromaticity diagrams when exciting the samples with the light of wavelength corresponding to the absorption maxima.


image file: c7py01678f-f6.tif
Fig. 6 Emission spectra and chromaticity diagrams of the hydrogels when excited with a light of (a, c) 360 nm; (b, d) around 395 nm and (c, f) around 480 nm (the exact wavelength is given in the inset of the graph) and images of the CPA1 hydrogel (g, h) under normal light and (i, j) when illuminated with a UV lamp.

All hydrogels emitted light in the green domain, with an emission maximum red shifted from 522 to 532 nm as the wavelength of the exciting light increased from 360 to 480 nm (Fig. 6a–c). When excited with higher wavelength light, the emission maxima slightly red shifted upon increasing the size of the ordered clusters; the effect considered to result from the diminishing of the Stokes shift (Table 1s) due to the configuration reorganization induced by the larger intermolecular interactions.58

The quantum yield has high values, from 24.1 to 32.8% (Table 2). When excited with higher energy light (UV light of 360 or 390–406 nm) only CPA1 and, CPA1 and CPA2, respectively, emitted light in the gamut of human vision (Fig. 6d and e). Decreasing the energy of the exciting light (green light of 479–487 nm) all the samples emitted pure light in the gamut of human vision close to the spectral locus (Fig. 6f).

Table 2 Values of the absolute quantum yield (Φfl) of the hydrogels and xerogels, excited with the absorption maxima
Code Hydrogels Xerogels
λ ex 1 λ ex 2 λ ex 3 λ ex 1 λ ex 2 λ ex 3
λ ex 1: 360 nm; λex2: 395; 393; 395; 390; 406; λex3: 481; 487; 484; 480; 479.
CPA1 25.4 29.1 32.8 43 25.7 11.4
CPA2 25.9 27.0 25.0 48.1 26.8 13.3
CPA3 29.3 25.2 26 40.4 24.7 14.3
CPA4 29.5 27 24.11 44.5 21.2 11.9
CPA5 29.8 25.3 32.1 51 25.8 9.8


The quantum yield of the films obtained by air drying of the hydrogels gave similar results, while the chromaticity diagrams exhibited a similar color of the emitted light.

Xerogels exhibited a different behavior. The emission maximum had a red shifting trend, from 514 to 544 nm as the size of the ordered clusters decreased (from CPA1 to CPA5), in agreement with literature reports.56,58,62 The quantum yield of the xerogels had values on a larger range, from 9.8 to 51%, strongly related to exciting light energy. The highest values from 40.4 to 51% were recorded when excited with UV light of higher energy (360 nm), while the lowest values from 9.8 to 14.3% were obtained when excited with the light of lower energy attributed to D → A intramolecular charge transfer (480 nm). Almost similar values for both hydrogels and xerogels were obtained when excited with UV light of energy corresponding to the π–π* electron transition of the conjugated skeleton. The color of the light emitted by the xerogels laid in the gamut of the human vision in almost all cases, spanning from blue-green to yellowish-green (Fig. 7d–f), as can also be seen when samples were illuminated with a UV lamp (Fig. 7g–k).


image file: c7py01678f-f7.tif
Fig. 7 Emission spectra and the corresponding chromaticity diagrams of xerogel samples excited with a light of (a, d) 360 nm, (b, e) around 390 nm, (c, f) around 395 nm (the exact wavelength is given in the inset of the graph) and (g–h) images of the xerogels illuminated with a UV-lamp.

The notable emission properties of the studied hydrogels and xerogels have been mainly attributed to the formation of phenothiazine based ordered clusters within the chitosan network which significantly improved the quantum efficiency due to the high surface-to-volume ratio aspect. The formation of some isolated phenothiazine–imine units on chitosan chains or some traces of unreacted aldehyde must have an important influence on the improvement of the quantum yield too. On the other hand, the immobilization of the ordered clusters within the chitosan network has to lower the diffusion quenching, favouring photoluminescence intensification. The lower luminescence of the hydrogels as compared to xerogels could be ascribed to the presence of water which promotes luminescence quenching by a non-radiative process through O–H vibrations of the water molecules. An important factor in improving the quantum yield appears to be the porous morphology of the xerogels, probably by additional increasing of the surface-to-volume ratio aspect.

The luminescence of both hydrogels and xerogels was well preserved over time. Measurements of the quantum yield over 3 months, on the same samples held under laboratory conditions at room temperature, gave similar values with those measured on freshly prepared hydrogels and xerogels, proving the good photoluminescence stability of the samples, an important feature in optoelectronic applications.

Mechanical properties

An important advantage of organic materials versus the inorganic ones for optoelectronics application is their facile processability as thin films with good mechanical properties. A preliminary investigation of the processability and mechanical properties of the studied hydrogels showed that they were able to give thin, flexible films with good mechanical toughness under the pressure of an external stress such as bending, elongation or even attaching of a balance weight; a film of 5 cm/1 cm/0.2 cm was able to sustain a balance weight of 5 g over one week, without breaking (Fig. 8). When applying different elongation percentages between 20 and 100%, the hydrogels suffered a viscoelastic loss between 5 and 35% (Fig. 8a). Their breaking occurred at a high tensile strength of 0.22 MPa and at a strain of 92% (Fig. 8b).
image file: c7py01678f-f8.tif
Fig. 8 Mechanical properties of the representative CPA1 hydrogel: (a) creep-elongation curve with the image of the hydrogel sustaining a balance weight; (b) stress–strain curve with the image of the bent hydrogel under UV light.

Conclusions

This paper brings into attention a new type of eco-friendly luminescent hydrogel prepared by the acid condensation reaction of the amine groups of chitosan with aldehyde bearing phenothiazine. The hydrogelation takes place due to the self-ordering of the chitosan segments grafted with rigid imine units, facilitated by the ability of the carbonyl-amine/imine interconversion system to afford ordered clusters acting as network nodes. The hydrogels have a microporous morphology with very thin pore walls and good mechanical toughness. The use of the phenothiazine aldehyde to crosslink the chitosan polyamine proved to be an excellent route toward highly luminescent stable materials; a quantum yield of 32.8% was reached for the hydrogels and 51% for the xerogels. The high quantum efficiency has been attributed to the synergistic influence of (i) the phenothiazine heteroaromatic ring; (ii) formation of ordered clusters with a high surface-to-volume ratio; (iii) microporous morphology which further enhances the active surface.

All these results demonstrate a new, original approach in exploiting chitosan biopolymers, paving the way towards efficient luminescent biomaterials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This paper is part of a project financed through a Romanian National Authority for Scientific Research MEN – UEFISCDI grant, project number PN-II-RU-TE-2014-4-2314 and through the European Union's Horizon 2020 research and innovation programme under grant agreement 667387.

The authors are thankful to Professor Mihai Barboiu, Institut Européen des Membranes, Montpellier, France for useful discussions related to the reversibility of the imine linkage on chitosan.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7py01678f

This journal is © The Royal Society of Chemistry 2018