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Poly-3,4-dihydroxybenzylidenhydrazine, a different analogue of polydopamine

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Nanotechnology Reviews
From the journal Nanotechnology Reviews Volume 13 Issue 1

Abstract

In this article, 3,4-dihydroybenzylidenehydrazine is synthesized for the first time, and its properties as a dopamine analogue for polymerization are investigated. Using an oxidative polymerization reaction, the reaction mechanism as well as the coating ability of the new polymer is determined and compared to that of polydopamine. The polymerization reactions were performed in a mixture of methanol–water with NaIO4 as an oxidation reagent. The polymer was used as a coating on both glass surfaces with a thickness of ∼5 nm as determined by AFM, as well as on TiO2 nanoparticles. For the latter, SEM/TEM and the pH-dependent variation of zeta potential were measured. As a free polymer, poly-3,4-dihydroxybenzylidenhydrazine was investigated by UV-Vis, ss-NMR, and FTIR, and a variety of monomeric units were found in the polymer matrix. The solubility in methanol or DMSO of the monomer and the slight solubility of the polymer allowed us to study the fluorescence and cyclic voltammetry properties for both the monomer and polymer.

Keywords: polydopamine analogues; fluorescence; surface coating; structural analysis; cyclic voltammetry

1 Introduction

Based on the moisture-resistant adhesion of marine mussels, polydopamine (PDA) and its analogues gained much attention in the last 17 years [1,2,3,4,5,6]. Their superior adhesion to surfaces is a common property of all these polymers and is mainly based on their catechol ring and terminal amine as key units [7]. Depending on the substrate, these units might be involved in π–π stacking, van der Waals forces, hydrophobic associations, coordination bonds, various chelating and hydrogen bonds, quinhydrone charge-transfer complexes, cation–π interactions, and other electrostatic interactions in order to bind strongly to the respective surfaces [8,9,10]. Theoretically, this mechanism of universal coating can be applied to all types of natural or synthetic catecholamines, each of them with its own peculiarities. New catecholamine monomers could be designed to contain alkyl, carboxyl or carboxyl derivative groups, amino, alkylthio, azido, acetamide, etc. With such PDA analogues, one could then coat surfaces similar to PDA and provide additional functionalities or properties. If the focus is only on the synthetic analogues, substitutions to the aromatic ring, alkyl chain, or terminal amine exhibit quite different results with regard to polymerization and coating ability. For example, 6-nitrodopamine, with a nitro moiety as an electron-withdrawing group on the aromatic ring, inhibits the formation of quinone and allows the formation of a thin coating layer of ∼5 nm polymer; even so, this coating provides good biocompatibility or photodegradable property [11]. For alkyl chain-substituted dopamine analogues, L-DOPA or norepinephrine are the most common monomers described in the literature, with the respective polymer producing a thinner coating layer compared with PDA but more reaction sites for different applications [12]. Poly-2-aminomethyl-3-(3,4-dihydroxyphenyl)propionamide is a PDA analogue that provides fluorescence, a ∼70 nm thick coating on glass, and better antibacterial properties towards S. aureus than PDA [13]. For an alkyl chain length variation, poly(3,4-dihydroxybenzylamine) provides a good coating for different substrates without the formation of 5,6-dihydroxyindole as significant units in the coating process [14,15]. In other dopamine analogues with different alkyl chain lengths between the catechol and amine groups, self-polymerization occurs by the formation of different cyclization intermediates (six- or seven-membered rings) but also via different Schiff base or Michael-type additions in the case of 5- or 12-carbon chain length analogues [16]. For more conjugated systems like poly-5,6-dihydroxyindole (DHI) [17], poly-5,6-dihydroxy-1H-benzimidazole [18], or poly-5,6-dihydroxy-1H-indazole [19] the cross-linking by Michael additions or Schiff base formations is limited but still provides interesting reaction mechanisms both in the form of bulk polymer and coating. In order to develop similar conjugated catechol derivatives, we synthesized and polymerized 3,4-dihydroxybenzylidenehydrazine as a new dopamine analogue. The related polymer is poly-3,4-dihydroxybenzylidenhydrazine, hereafter called PDHBzy, in which the saturated alkyl chain is substituted with a hydrazine functionality. The substitution of one carbon by nitrogen, as well as the additional double bond, influences the polymerization mechanism and the physicochemical properties of the bulk polymer or as a coating. For the bulk polymer, structural techniques like ss-NMR, FTIR, and UV-Vis provide information about the reaction mechanism. When PDHBzy was used as a coating, SEM/TEM and confocal microscopy, zeta potential as well as AFM provide information about the adherence on TiO2 or glass. The solubility of the monomer and polymer in different solvents allowed us to study their optical or electrochemical properties using UV-Vis, fluorescence, and cyclic voltammetry (CV).

2 Experimental

2.1 Materials

The chemicals used were 3,4-dihydroxybenzaldehyde (Alfa Aesar), tert-butylcarbazate (Alfa Aesar), TiO2 (titanium (IV) oxide, NanoArc, anatase, nanopowder, 99.9%, 32 nm, Alfa Aesar), trifluoroacetic acid (Alfa Aeasr), and cover glasses (square, borosilicate glass 18 mm × 18 mm, thickness 0.13–0.17 mm, No. 1, Brand GMBH + CO KG, Germany). Other chemical reagents such as sodium acetate, sodium chloride, disodium hydrogen phosphate, sodium hydroxide, hydrochloric acid, methanol, ethanol, acetone, dichloromethane, and dimethyl sulfoxide (DMSO) were purchased from Merck. Deionized water (DW) was utilized throughout the study.

2.2 Polymer preparation

2.2.1 (E)-Tert-butyl-(3,4-dihydroxybenzylidene)hydrazinecarboxylate 2

3,4-Dihydroxybenzaldehyde (3.00 g, 21.7 mmol) was dissolved in 30 ml of water at 50°C and tert-butyl carbazate (3.44 g, 26 mmol) was added and left to stir at this temperature for 5 h. TLC (pentane/ethyl acetate = 1:2) shows the consumption of the aldehyde (R f = 0.37). The compound was purified chromatographically in the same elution system, obtaining 85% yield. 1H-NMR (500 MHz, d 6-DMSO): δ (ppm) 10.64 (s, 1H), 7.79 (s, 1H), 7.07 (s, 1H), 6.77–6.72 (d, 2H), 1.44 (s, 9H). 13C-NMR (125 MHz, d 6-DMSO): δ (ppm) 152.37 (NH–CO–O), 147.20 (arC–OH), 145.53 (C═N), 143.75 (arC–OH), 126.03 (arC═C), 119.65 (arC═C), 115.41 (arC═C), 112.20 (arC═C), 79.03 (O–C–CH3), 28.08 (–CH3).

2.2.2 3,4-Dihydroxybenzylidenehydrazine (DHBzy, 3)

To 4.55 g of (E)-tert-butyl 2-(3,4-dihydroxybenzylidene)hydrazinecarboxylate (2), 20 ml of DCM was added, forming a suspension. Afterward, 5 ml of TFA was added and the precipitate dissolved; in 15 min an intensely yellow precipitate was formed. The reaction mixture was stirred for another 2 h at r.t., filtered, and the residue was washed with DCM. The yellow solid was dried at 60°C overnight to obtain a yield of 81% (m.p. = 210–212°C).

1H-NMR (500 MHz, d 6-DMSO): δ (ppm) 8.46 (s, 1H), 7.30 (d, J = 2.0 Hz, 1H), 7.09 (dd, J = 2.0 Hz, J = 8.0 Hz, 1H), 6.81 (d, J = 8.0 Hz, 1H). 13C-NMR (125 MHz, d 6-DMSO): δ (ppm) 160.54 (arC–OH), 148.94 (arC–OH), 145.61 (C═N), 125.37 (arC–C═N), 121.99 (arC), 115.57 (arC), 113.84 (arC).

2.2.3 Poly-3,4-dihydroxybenzylidenhydrazine (PDHBzy)

Two different conditions were employed for polymerization, using a concentration of 4 mg/ml monomer for 20 h in both cases. The solvents that were used are A – an aqueous solution of TrisCl (10 mM, pH = 8.5) and B –methanol/water. The classical way of polymerization (A, using only an aqueous solution of TrisCl without any oxidation reagent except atmospheric oxygen) was unsuccessful in our case. For this reason, we also added NaIO4 (2 eq.) as an oxidant for method B. For the polymerization according to A, in TrisCl (10 mM, pH = 8.5), a clear reddish solution without any precipitate was formed, which after 20 h of reaction time was dialyzed against water to obtain less than 10 mg solid from 100 mg of the starting material. For the polymerization B in methanol/water (20/80 volume ratio, DHBzy [200 mg, 1 eq.], NaIO4 [560 mg 2 eq.]), the polymer started to precipitate relatively fast; after several trials, a reaction time of 2 h was deemed sufficient in order to stop the reaction and work-up the formed precipitate. The work-up involves centrifugation of the reaction mixture and washing of the formed precipitate three times with water in order to remove NaIO4. The polymer was then dried in an oven (60°C overnight) and used for further analysis.

For glass coating, the borosilicate glass disks were washed with water and ethanol, left to dry, and placed into five different beakers (25 ml). A solution was made from 200 mg DHBzy and 50 ml water/methanol (40/10 ml). After the monomer dissolution, 10 ml from this stock solution was added to each beaker, respectively. The five recipients were shaken at 200 rpm at r.t. for 20 h. Subsequently, each disk was removed from the beaker, washed with water and methanol and left to dry. The samples were then subjected to different surface analysis methods.

In the case of TiO2 coating, 100 mg of TiO2 (commercial particles) and 100 mg of DHBzy were dispersed in 25 ml of a water/methanol mixture and stirred at r.t. for 20 h. After centrifugation of the particles from the reaction mixture, the work-up of the particles consisted of washing three times with water and three times with methanol, removing all the uncoated polymer or unreacted monomer. The coated particles were dried at 60°C for 48 h.

2.3 Characterization methods

The size and shape of the coated TiO2 particles were examined by scanning transmission electron microscopy (STEM) with a Hitachi HD2700 equipped with a cold field emission gun (Dual EDX System, X-Max N100TLE Silicon Drift Detector) from Oxford Instruments. For the analysis, a suspension of the samples was sonicated (<10 s) with a UP100H ultrasound finger and deposited by the droplet method on a 400-mesh copper grid coated with a thin carbon layer. For both types of analyses, the nominal operating tension was 200 kV.

The surface morphology and topography of PDHBzy films were assessed by atomic force microscopy (AFM), using a Cypher S microscope (Asylum Research-Oxford Instruments, Santa Barbara, CA, USA). AFM images were acquired in amplitude-modulated AC mode (tapping mode), in air, with silicon probes AC160TS-R3 (Olympus, Japan), with a typical spring constant of 26 N/m (8.42–57.0 N/m) and a resonance frequency of 300(±100) kHz. AFM images were recorded with a resolution of 512 × 512 pixels and a scan rate of less than 1 Hz. Multiple zones of the sample surface were analyzed at different scan sizes (20, 10, and 2 µm). Data processing was performed using the Asylum Research SPM software (AR SPM) based on Igor Pro software package (Igor Pro 6, WaveMetrics, Inc., Lake Oswego, OR, USA).

The film thickness was evaluated by scratching the film with a blunt stainless-steel needle, paying attention to not digging into the glass substrate. For film thickness evaluation, the sample was positioned on the stage with the scratch perpendicular to the fast scan direction, and, when necessary, small scan angle adjustments were made using AR SPM software. Following the acquisition, AFM images were first flattened by masking features using an appropriate high threshold and then applying first-order flattening to the unmasked region. When calculating the step height, a subsequent plane fit in the fast scan direction was applied. The layer thickness was measured at different sites along a scratch as a difference in height between the quasi-flat PDHBzy film surface and the flat bottom of the scratch left clean after removing the polymeric layer by scratching.

The zeta potential of the colloidal particles was determined using a Malvern Zetasizer Nano ZS-900 equipped with a He–Ne laser (633 nm, 5 mW).

Solid-state 13C spectra were acquired at 125.73 MHz Larmor frequencies with a Bruker Avance III wide-bore spectrometer operated at room temperature. Standard 13C cross-polarization under magic angle spinning (CP-MAS) pulse sequence was used for both nuclei at a spinning frequency of 14/7 kHz, a contact time of 2/4 ms , and proton decoupling under TPPM. The 13C CP-MAS spectra were obtained by averaging 10k/50k transients at 1 s relaxation delay, as optimized on the 1Hss-NMR spectrum. The recorded 13C spectra were calibrated relative to the CH3 line in TMS (tetramethylsilane), through an indirect procedure, which uses glycine (COOH line at 176.5 ppm for 13C) as an external reference.

Fourier transform infrared (FTIR) spectra were recorded using a JASCO FTIR-4600 spectrophotometer.

Fluorescence images were obtained using a confocal system from Nikon (model AX R). The system is made up of an inverted microscope Ti2-E equipped with Galvano scanning at 8k resolution with several excitation laser lines. For acquiring the images of PDHBzy@TiO 2 , various excitations were used, such as the 405, 445, and 488 nm laser lines.

The UV-Vis absorption spectra were recorded at room temperature using a Cecil Super Aquarius spectrophotometer. The fluorescence spectra were collected using a FS5 spectrofluorometer (Edinburgh Instruments), equipped with a 150 W CW ozone-free xenon arc lamp, Czerny-Turner with plane grating monochromators, and a PMT-900 emission detector. The quantum yield (QY) was measured indirectly using quinine sulfate in 0.05 M sulfuric acid as a fluorescence standard.

The electrochemical investigations (CV and differential pulse voltammetry [DPV]) were carried out using an Autolab Potentiostat/Galvanostat 302 N (Metrohm Autolab), controlled by Nova 1.11 software. For all the measurements, a typical three-electrode electrochemical cell comprising a platinum working electrode (Pt) with a 0.07 cm2 geometric area, an Ag/AgCl (KCl, 3 M) reference electrode, and a platinum plate as the auxiliary electrode was employed. As a supporting electrolyte, a 10−3 M solution of DHBzy/PDHBzy at pH 7.4 phosphate buffer was used. The CVs and DPVs were recorded by sweeping the potential between −0.2 and 0.6 V vs Ag/AgCl for DHBzy, while for PDHBzy, the potential was between 0.3 and 0.8 V vs Ag/AgCl. For CVs, the scan rate was 50 mV/s, while for DPVs, it was 10 mV/s was used.

3 Results and discussion

The new dopamine analogue 3,4-dihydroxybenzylidenehydrazine (DHBzy, 3) was obtained in a two-step procedure starting from 3,4-dihydroxybenzaldeyde (1), which was converted into the Boc-derivative 2 by a condensation reaction with tert-butyl carbazate followed by a deprotection reaction in the TFA/DCM mixture to give 3 in 81% yield as a yellow precipitate (Scheme 1).

Scheme 1 Synthesis of 3,4-dihydroxybenzylidenehydrazine (DHBzy, 3).
Scheme 1

Synthesis of 3,4-dihydroxybenzylidenehydrazine (DHBzy, 3).

The polymerization reaction of DHBzy was attempted employing three different methods using 4 mg/ml monomer 3 each time, in Tris buffer for 20 h (bulk, method A), in water/methanol using NaIO4 as an oxidant for 2 h (bulk, method B) and 20 h for surface coating. The bulk polymerization, according to method A, only produced a small amount of polymer, so method B was chosen subsequently, which might be due to the instability of hydrazines in a basic environment. PDHBzy was obtained in bulk as a brown-red precipitate and as an almost transparent coating for glass or in a gray color for TiO2 (Scheme 2).

Scheme 2 Polymerization of PDHBzy both as a bulk polymer and as a coating for glass.
Scheme 2

Polymerization of PDHBzy both as a bulk polymer and as a coating for glass.

Assumptions about the polymerization mechanism of the new derivative involve a series of oxidation, cyclization, or condensation processes similar to PDA or other derivatives of this class [2,20,21]. Possible oligomer formation is based on derivatives, as shown in Scheme 3.

Scheme 3 Possible intermediates in the polymerization process of DHBzy.
Scheme 3

Possible intermediates in the polymerization process of DHBzy.

3.1 Polymerization kinetics

The DHBzy polymerization kinetics in water/methanol were investigated in real time by UV-Vis spectroscopy. As shown in Figure 1, the reaction proceeds quickly, and after 5 min the disappearance of the two absorption maxima specific to the monomer (285 and 310 nm) can be observed together with the formation of the other two bathochromically shifted peaks (340 and 430 nm). The appearance of these two maxima can be explained by the formation of oxidation or condensation species that can be semiquinone, Michael addition, or Schiff base formation products as intermolecular arrangements of different monomeric units. After 10 min, a decrease in intensity of the two new absorption maxima and a hypsochromic shift can be observed (Figure 1, red dotted line), which otherwise does not change their shape too much. This blue shift and the decrease in intensity of the absorption maxima may indicate the formation of more complex species in the polymer matrix that are insoluble in methanol/water. The reaction was left for 20 h, showing similar absorption patterns as PDA and other analogues.

Figure 1 Time-dependent polymerization in UV−Vis PDHBzy in water/methanol.
Figure 1

Time-dependent polymerization in UV−Vis PDHBzy in water/methanol.

3.2 Surface characterization: zeta potential, TEM/SEM, confocal microscopy, and AFM measurements

The PDHBzy coating on TiO2 nanoparticles was investigated by TEM/SEM and zeta potential. The coating of PDHBzy on TiO2 nanoparticles was not quite visible compared to the unmodified TiO2 (Figure S1). Although in TEM, the polymer layer is invisible, the zeta potential and confocal microscopy measurements confirm its presence on the surface of the nanoparticles (Figure S2). The fluorescence of PDHBzy@TiO 2 in confocal microscopy was not observed for the initial TiO2 nanoparticles. The zeta potential of TiO2 nanoparticles and PDHBzy@TiO 2 coated with polymer were measured at different pH values. Figure 2 shows the variation of zeta potential of TiO2 and PDHBzy@TiO 2 nanoparticles with pH increasing from 3 to 10. TiO2 nanoparticles that have been coated with polymer demonstrate a negative zeta potential of −3 mV at a pH of 3. In contrast, uncoated TiO2 nanoparticles display a potential of 38 mV at the same pH level. A comparison of the zeta potentials of both nanoparticle types at different pH levels reveals a stark difference in value. However, it can also be observed that the zeta potential of both TiO2 and polymer-coated nanoparticles drops gradually with an increase in pH. Both of these observations firmly confirmed that the polymer PDHBzy effectively covered the TiO2 nanoparticles. The results of the study indicate that pure TiO2 nanoparticles exhibit a zeta potential ranging from +38 to −30 mV, with an isoelectric point at pH 6.8. Conversely, PDHBzy@TiO 2 particles also demonstrated a significant negative shift in zeta potential, from −3 to −37 mV, but without an isoelectric point in the range of pH measured, which might be due to the abundance of hydroxy moieties of the catechol units. Based on the analysis of the results, it is evident that the increase in the negative charge on the surface of PDHBzy@TiO 2 can be partly attributed to the enhanced adsorption of hydroxide ions by the polymer present on the surface. This phenomenon can be attributed to the chemical properties of the polymer, which facilitate the adsorption of ions and cause a corresponding increase in the negative charge on the nanoparticle surface. This finding highlights the potential of surface chemistry modifications in enhancing the performance of nanoparticles in various applications, including drug delivery, catalysis, and imaging.

Figure 2 Comparison of zeta potential variation with pH for PDHBzy@TiO 2 (blue) and TiO 2 nanoparticles (black).
Figure 2

Comparison of zeta potential variation with pH for PDHBzy@TiO 2 (blue) and TiO 2 nanoparticles (black).

AFM images of PDHBzy show a continuous and uniform film with very rare scratches from manipulation (Figure 3). The granular aspect of the PDHBzy-coated surface (Figure 3a1 and a2) is preserved over the entire surface of the coating layer, similar to what was found for PDA and other derivatives such as norepinephrine [22], adrenaline, l-DOPA [23], or 6-nitrodopamine [24]. The film thickness was evaluated using sections across the scratches as a difference in height between the quasi-flat PDHBzy film surface and the flat bottom of the scratch (Figure 3b1). The roughness of a PDA film (deposited in the presence of O2 as an oxidant agent for 24 h deposition time) is considerably higher (R a 5.1 nm for PDA vs R a 0.6 nm for the PDHBzy film), and aggregates of tens or hundreds of nm (λ max 227 nm) are randomly distributed across the surface [25]. As expected, the PDHBzy film is much thinner (5.0 ± 0.5 nm) as compared with PDA (40.0 ± 1.6 nm). The roughness of the polymer film from the previously reported lower homolog of dopamine, PDHBA [14], on a glass substrate also manifests higher roughness (R a 5.0 nm, 10 µm scan size) as compared to PDHBzy, with aggregates still present (λ max 214 nm) and a quasi-flat layer thickness of about 20 nm (20.5 ± 0.9 nm) [14].

Figure 3 2D (a1, b1) and 3D (a2, b2) AFM topographic images of the PDHBzy film on the glass substrate. The red line highlighted in (b1) indicates a section used for the film thickness evaluation and its corresponding cross-section profile (white). Scan size is 2 µm (a) and 10 µm (b).
Figure 3

2D (a1, b1) and 3D (a2, b2) AFM topographic images of the PDHBzy film on the glass substrate. The red line highlighted in (b1) indicates a section used for the film thickness evaluation and its corresponding cross-section profile (white). Scan size is 2 µm (a) and 10 µm (b).

3.3 Solid-state NMR measurements (ss-NMR)

Solid-state 13C ss-NMR spectra were recorded at a 125.73 MHz Larmor frequency with a Bruker Avance III wide-bore NMR spectrometer at room temperature, using a 4 mm double resonance (1H/X) MAS probe. Standard 13C CP-MAS pulse sequence with a contact pulse of 2 ms and high-power proton decoupling (100 kHz) with a TPPM (two-pulse phase modulation) sequence were used for both experiments. For the DHBzy sample, 16,200 transients were acquired with a recycle delay of 5 s, and for the PDHBzy sample, the number of acquired transients was 20,500 with a recycle delay of 3 s. The recorded spectra are calibrated relative to the CH3 line in TMS (tetramethylsilane) through an indirect procedure that uses α-glycine (C═O of glycine at 176.5 ppm) as an external reference.

Figure 4 shows a comparison between the 13C CP-MAS spectra recorded on the monomer DHBzy (blue) and the polymer PDHBzy (red). The monomer DHBzy shows seven C atoms in the order C4-156.2 ppm, C3-145.9 ppm, C7-133.3 ppm, C1-122.3 ppm, C6-119.1 ppm, C5-117.0 ppm, and C2-110.0 ppm (Table S1). The polymer structure in the 13C-NMR spectrum provides both similar and different arrangements of the monomeric units compared with the starting material. In general, the signals are specific for different types of carbon atoms found in similar compounds with some particularities. The simulated structures with their 13C chemical shift values are presented in Table S1, and the most interesting monomeric units found in the polymer matrix are presented in Figure 5.

Figure 4 Comparison between the 13C CP-MAS spectra recorded on the PDHBzy (red) and DHBzy (blue) samples – the amplitude of the latter was magnified by a factor of 3.
Figure 4

Comparison between the 13C CP-MAS spectra recorded on the PDHBzy (red) and DHBzy (blue) samples – the amplitude of the latter was magnified by a factor of 3.

Figure 5 The most probable structural units in PDHBzy are based on the assignment of the 13C CP-MAS spectra.
Figure 5

The most probable structural units in PDHBzy are based on the assignment of the 13C CP-MAS spectra.

For the assignment of the spectral lines to particular chemical groups containing 13C positions in the PDHBzy material, various assumptions were made about the multitude of possibilities in which the new monomer can oxidize in the polymerization process (Table S1). In the order of most deshielded C atoms, the peak at 193.2 ppm is specific for a terminal semiquinone/quinone (M3, M4, M12, M13, M14), which is related to a secondary C at 30 ppm of a terminal unit, similar to structure M3. The peaks at 169–163 ppm are specific for a COOH, COOR, or O═C–NH– group and can be found in units similar to M6, M7, M8, and M9. The interval at 140–131 ppm is the broadest, which contains C1, C3, C4, and C7 of the starting material in different combinations with other units common in all of M2–M14, specific for quaternary C atoms of the aromatic ring: C–OH, C–C, or C–N. The 117–115 ppm region is specific for tertiary aromatic C (M2–M14) in different arrangements and for C2, C5, and C6 of M1 (starting material). The peak 104 ppm is common for aromatic Ct in different combinations (M2–M14).

The most unexpected signals are the ones at 91, 83, and 54 ppm. The peak at 91 ppm can be an aromatic Cq specific for a quinone/semiquinone unit like M6 or for a Cq of an oxa-spiro ring also found in poly[3,4-dihydroxybenzhydrazide] (M11, M12) [26]. The peak at 83 ppm is also related to the oxa-spiro ring as the vicinity Ct–OH (M11) and also for Ct–OH (M4, M10) of a semiquinone. The most intense peak at 54 ppm is specific for a –CH–NH– bond as a result of the reduction of –CH═N– of the starting monomer or for a condensed intermediate after a Schiff base reaction between catechol and primary amine, Cs (M2, M7, M10, M12, M14) or Ct–OH (M10).

Summarizing the information retrieved from the analysis of the ss-NMR data, the structural units that are very likely to be formed in different ratios and combinations in PDHBzy are shown in Figure 5.

3.4 Fourier-transform infrared spectroscopy

The overlayed FT-IR spectra of DHBzy and PDHBzy reveal two major differences: one regarding the intensity and the other between the modification of the signals in the polymer versus monomer. For the monomer, the region of 600–800 cm−1 is specific for aromatic disubstituted and trisubstituted derivatives and ring deformation of phenyl. At ca. 1,000–1,250 cm−1, methyne skeletal C–C vibrations or tertiary amines (ν(C–N)) are commonly found, and the bands at 1,600–1,700 cm−1 denote a C═C–C aromatic ring stretch, open-chain imino (–C═N–), open-chain azo (–N═N–), aryl-substituted C═C, or conjugated C═C stretching vibrations. The final interval of 2,700–3,700 cm−1 is specific for aliphatic and aromatic C–H stretching, amino groups (ν(N−H)), H-bonded (OH stretch), and unbound hydroxy groups (OH stretch) or phenols (OH stretch). In the case of polymers, all signals become wider and less intense, which is visible here and confirms that the polymer formation was indeed successful. The most visible changes are the ones at 600–800, 1,000–1,250 and 2,800–3,800 cm−1. For the 600–800 cm−1 interval, for the polymer, the signal almost disappears due to the involvement of the aromatic ring in different combinations and its dearomatization from semiquinones or oxa-spiro rings. The transformation of the methyne skeletal C–C vibrations or tertiary amines in the polymerization process is observed by the disappearance of the signal corresponding to 1,100 cm−1. The involvement of the hydrazine moiety in different oligomeric units is also observed with the decrease of vibrational signals corresponding to 1,600–1,700 cm−1, which can also be attributed to the formation of different quinone/semiquinone components. The wider area at 2,700–3,600 cm−1 is common for these types of PDAs and their analogues and is a combination between aromatic and aliphatic C–H stretch, H-bonded (OH stretch), and unbound hydroxy groups (OH stretch), phenols (OH stretch) as well as aliphatic primary amines (NH stretch) (Figure 6) [27,28].

Figure 6 Overlayed FTIR spectra of DHBzy and PDHBzy.
Figure 6

Overlayed FTIR spectra of DHBzy and PDHBzy.

3.5 Optical properties

The absorption properties of the hydrazine compound (DHBzy) and its related polymer (PDHBzy) were measured in methanol (a polar protic solvent, MeOH) and dimethyl sulfoxide (a polar aprotic solvent, DMSO). The UV–Vis spectrum of DHBzy (Figure 7) exhibited two absorption peaks at 288 and 310 nm in methanol solution and showed a slight bathochromic shift in DMSO solution (290 and 318 nm). These two absorption maxima can be attributed to the π–π* and n–π* transition of the catechol and hydrazine moieties [29,30]. The optical band gap (E gap) for the monomeric compound was calculated from the absorption spectrum edge using the absorption onset and was determined to be 3.60 eV in both solvents.

Figure 7 UV-Vis normalized absorption spectra of hydrazine monomer (DHBzy) (a) and its related polymer (PDHBzy) (b) in dimethyl sulfoxide (DMSO) and methanol (MeOH).
Figure 7

UV-Vis normalized absorption spectra of hydrazine monomer (DHBzy) (a) and its related polymer (PDHBzy) (b) in dimethyl sulfoxide (DMSO) and methanol (MeOH).

The UV-Vis absorption spectrum corresponding to the associated polymeric structure presented two absorption peaks in methanol; a strong band was observed around 290 nm, followed by a broad one at 340 nm. A bathochromic shift (12 nm for the first, and 25 nm for the second band) was obtained when the solvent was changed from MeOH to DMSO (Figure 7b). The UV-Vis spectra of PDHBzy recorded in methanol and DMSO appear a little different compared to the spectrum obtained from the kinetic studies (Figure 1), as was observed for other compounds in this class and can be explained by the solvent used for polymerization [13].

In order to investigate the fluorescence emission properties of the hydrazine monomer and its related polymer, several solvents were selected, such as MeOH, and DMSO, as well as mixtures of solvents in the study of physicochemical properties of organic compounds [31,32,33]. Figure 8 depicts the emission spectra of these two compounds recorded at different volume ratios of DMSO and water. A broad emission band at 382 nm was observed for DHBzy when a 0.1:2.9 volume of DMSO:water was used (Figure 8a). By increasing the DMSO quantity, a slight hypsochromic shift along with an increase in emission band intensity was observed until the volume of solvents was 1.5 mL (DMSO):1.5 mL (H2O). For this solvent ratio, two emission maxima were formed, which show a slight blue shift with no significant increase in intensity (Figure 8a). It is worth mentioning that all the solutions were measured at the same concentration, and the results obtained show us an enhancement of emission spectra intensity when measured in DMSO compared to DMSO/water. The PDHBzy showed an emission band at 470 nm when a 0.1:2.9 volume of DMSO/water was used (Figure 8b). Unlike for the monomer, a bathochromic shift (25 nm) together with a broadening and increase in the intensity of the bands were observed when increasing the ratio of DMSO to water.

Figure 8 (a) Emission spectra of the hydrazine compound (λ exc = 310 nm) and (b) polymeric structure (λ exc = 340 nm), recorded in different compositions of the DMSO/water mixture.
Figure 8

(a) Emission spectra of the hydrazine compound (λ exc = 310 nm) and (b) polymeric structure (λ exc = 340 nm), recorded in different compositions of the DMSO/water mixture.

The excitation spectrum recorded for DHBzy in a mixture of 0.1 mL DMSO and 2.9 mL water shows two maxima at 278 and 308 nm (Figure S3a). Once the ratio of DMSO/water decreases, a slight bathochromic shift can be observed, an unequal increase in the intensity of the two excitation bands, followed by a drastic decrease of the intensity when only DMSO was used as a solvent. The excitation spectrum in pure DMSO appropriately reproduces the absorption profile obtained for this compound (Figure 7a). These phenomena occur because, in water/DMSO and water/DMF mixtures, water molecules tend to form a strong network among themselves instead of forming hydrogen bonds with organic molecules from the solution [32,34].

In order to study the effects of the solvents on the emission spectra of hydrazine compounds and their corresponding polymer, dimethyl sulfoxide (DMSO), a mixture of water/3% DMSO and methanol was used as solvents for comparison. A bathochromic shift of 90 nm (Figure 9a) for the emission spectra of the hydrazine compound was observed when the solvent was changed from DMSO to water/DMSO mixture to methanol. To the best of our knowledge, a bathochromic shift of the emission spectra in methanol compared to DMSO has not been reported in the literature so far. Unlike the monomer, a hypsochromic shift of 50 nm (Figure 9b) in the emission spectra was observed for the polymer when the solvent was changed from DMSO to water/DMSO mixture to methanol (Figure 9b). These shifts can be assigned to intermolecular hydrogen bonding between the polymer and the DMSO, resulting in the reduction of the HOMO-LUMO gap, as reported for other diazo compounds [35].

Figure 9 Solvatochromic fluorescence properties of the monomer (a) (λ exc = 310 nm) and polymer (b) (λ exc = 340 nm) in DMSO (black), H2O/DMSO (red line), and methanol (blue line).
Figure 9

Solvatochromic fluorescence properties of the monomer (a) (λ exc = 310 nm) and polymer (b) (λ exc = 340 nm) in DMSO (black), H2O/DMSO (red line), and methanol (blue line).

The QY of the new compound DHBzy and its related polymer PDHBzy, measured at λ exc = 310 nm and 340 nm, respectively, using quinine sulfate (0.05 M) as a fluorescent standard [36,37], was found to be 1.13, and 0.68%, respectively. This compound presents low fluorescence compared to others from this class [38], which might be due to the disordered state and diversity of the polymer units. Further investigations should be done in order to improve it by reaction parameters or the addition of other fluorophores, monomers, or other fluorescence polymers in the polymerization process.

It is worth mentioning here that TiO2 nanoparticles coated with PDHBzy were analyzed using a confocal system. The samples were simply placed on a microscope glass and analyzed as such. Various excitation lines at 405, 445, and 488 nm were employed, and the fluorescence was investigated in all four available channels. Figure S2 presents the optical image overlapped with the fluorescence one (left), as well as only the fluorescence image, showing red emission coming from some of the aggregates. This indicates that in the presence of TiO2, the fluorescence of PDHBzy shifts towards the red spectral range. Further investigations of the fluorescence properties of the PDHBzy are needed; however, the fact that the fluorescence is not quenched completely in the presence of TiO2 nanoparticles encourages us to further research its properties for applications such as biotechnology and nanomedicine.

3.6 Electrochemical properties

The electrochemical behavior of a 1 mM solution of DHBzy and PDHBzy in pH 7.4 phosphate buffer was investigated using CV and DPV with a platinum working electrode. In the case of the monomer, the CV shows a pair of well-defined redox couples (Figure 10a) with an oxidation peak at +0.35 V vs Ag/AgCl and the corresponding reduction peak at +0.14 V vs Ag/AgCl, while in the DPV curve, an oxidation peak at +0.3 V is observed (Figure 10b). This behavior suggests that the hydroxyl groups (1) are oxidized to the corresponding ortho-benzoquinone (2) (Scheme 4) [39,40].

Figure 10 Cyclic and differential pulse voltammograms recorded for 1 mM (a and b) DHBzy and (c and d) PDHBzy in pH 7.4 phosphate buffer using a platinum working electrode.
Figure 10

Cyclic and differential pulse voltammograms recorded for 1 mM (a and b) DHBzy and (c and d) PDHBzy in pH 7.4 phosphate buffer using a platinum working electrode.

Scheme 4 Electrochemical oxidation of DHBzy to the ortho-benzoquinone analogue.
Scheme 4

Electrochemical oxidation of DHBzy to the ortho-benzoquinone analogue.

For the polymer, two pairs of well-defined peaks are observed in the CV: the oxidation peaks are located at +0.54 V and +0.72 V vs Ag/AgCl, while the reduction peaks are at +0.49 V and +0.65 V vs Ag/AgCl (Figure 10c). Also, in the DPV, two oxidation peaks are detected at +0.51 V and +0.67 V vs Ag/AgCl (Figure 10d). These results indicate the existence of two types of electrochemically reactive species in the polymer composition.

The electrochemical behavior of both monomers and polymers makes them promising materials for the design of electrochemically active composite electrodes. Therefore, they can be adapted to a wide range of applications in electrochemical sensing.

4 Conclusions

In the present work, we introduced a complex study related to the structure of poly-3,4-dihydroxybenzylidenehydrazine obtained in the presence of NaIO4 as an oxidizing reagent in a mixture of methanol/water to obtain both the bulk polymer as well as coated TiO2 particles and glass. From a structural point of view, by ss-NMR and FTIR, the polymerization mechanism provides an unconventional system of structural units after the oxidation of the monomer with oxa rings, hydrazone units, or aliphatic amines, species that reduce the expected fluorescence for azo chromophore like species [41]. The coating morphology of PDHBzy@TiO 2 and PDHBzy@Glass shows a smooth polymer layer, especially by AFM, with a thickness of ∼5 nm at 20 h deposition time and predominant hydroxy groups for PDHBzy@TiO 2 in zeta potential variation with pH. The optical properties of the monomer and polymer in methanol and DMSO as solvents provide fluorescence properties, measured at l exc = 310 and 340 nm, respectively, with a specific QY of 1.13% for DHBzy and 0.68% for PDHBzy. The results obtained by CV and DPV show an electrochemical response for both monomer and polymer with both oxidation and reduction potential. Based on the physico-chemical analysis and opto-electronical properties, the present 3,4-dihydroxybenzylidenhydrazine and poly-3,4-dihydroxybenzylidenhydrazine are good platforms for different applications like biomedicine, catalysis, water depollution, or sensoristic applications.

Acknowledgments

The Elta 90 Medical Research team is acknowledged for access to the Nikon confocal system.

  1. Funding information: This work was funded by UEFISCDI through the project PN-III-P1-1.1-TE-2021-0048 and Programme 1 – Development of the National Research and Development System, Subprogramme 1.2 – Institutional Performance – Funding Projects for Excellence in RDI, Contract No. 37PFE/30.12.2021.

  2. Author contributions: N. T.: methodology, writing; formal analysis, and investigation: N.T. A.F., D.B., C.F., A.V., and A.P.: conceptualization, methodology, supervision, funding acquisition, and writing – original draft preparation; A.F., D.B., C.F., and A.V.: resources. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  3. Conflict of interest: The authors state no conflict of interest.

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Received: 2024-01-10
Revised: 2024-03-27
Accepted: 2024-04-12
Published Online: 2024-05-06

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  177. Applications of nanomedicine-integrated phototherapeutic agents in cancer theranostics: A comprehensive review of the current state of research
  178. Smart bionanomaterials for treatment and diagnosis of inflammatory bowel disease
  179. Beyond conventional therapy: Synthesis of multifunctional nanoparticles for rheumatoid arthritis therapy
https://doi.org/10.1515/ntrev-2024-0026
Keywords for this article
polydopamine analogues; fluorescence; surface coating; structural analysis; cyclic voltammetry
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  144. Electrochemically prepared ultrathin two-dimensional graphitic nanosheets as cathodes for advanced Zn-based energy storage devices
  145. Enhanced catalytic degradation of amoxicillin by phyto-mediated synthesised ZnO NPs and ZnO-rGO hybrid nanocomposite: Assessment of antioxidant activity, adsorption, and thermodynamic analysis
  146. Incorporating GO in PI matrix to advance nanocomposite coating: An enhancing strategy to prevent corrosion
  147. Synthesis, characterization, thermal stability, and application of microporous hyper cross-linked polyphosphazenes with naphthylamine group for CO2 uptake
  148. Engineering in ceramic albite morphology by the addition of additives: Carbon nanotubes and graphene oxide for energy applications
  149. Nanoscale synergy: Optimizing energy storage with SnO2 quantum dots on ZnO hexagonal prisms for advanced supercapacitors
  150. Aging assessment of silicone rubber materials under corona discharge accompanied by humidity and UV radiation
  151. Tuning structural and electrical properties of Co-precipitated and Cu-incorporated nickel ferrite for energy applications
  152. Sodium alginate-supported AgSr nanoparticles for catalytic degradation of malachite green and methyl orange in aqueous medium
  153. An environmentally greener and reusability approach for bioenergy production using Mallotus philippensis (Kamala) seed oil feedstock via phytonanotechnology
  154. Micro-/nano-alumina trihydrate and -magnesium hydroxide fillers in RTV-SR composites under electrical and environmental stresses
  155. Mechanism exploration of ion-implanted epoxy on surface trap distribution: An approach to augment the vacuum flashover voltages
  156. Nanoscale engineering of semiconductor photocatalysts boosting charge separation for solar-driven H2 production: Recent advances and future perspective
  157. Excellent catalytic performance over reduced graphene-boosted novel nanoparticles for oxidative desulfurization of fuel oil
  158. Special Issue on Advances in Nanotechnology for Agriculture
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  160. Nanomaterials: Cross-disciplinary applications in ornamental plants
  161. Special Issue on Catechol Based Nano and Microstructures
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  163. In vitro anticancer activity of melanin-like nanoparticles for multimodal therapy of glioblastoma
  164. Poly-3,4-dihydroxybenzylidenhydrazine, a different analogue of polydopamine
  165. Chirality and self-assembly of structures derived from optically active 1,2-diaminocyclohexane and catecholamines
  166. Advancing resource sustainability with green photothermal materials: Insights from organic waste-derived and bioderived sources
  167. Bioinspired neuromelanin-like Pt(iv) polymeric nanoparticles for cancer treatment
  168. Special Issue on Implementing Nanotechnology for Smart Healthcare System
  169. Intelligent explainable optical sensing on Internet of nanorobots for disease detection
  170. Special Issue on Green Mono, Bi and Tri Metallic Nanoparticles for Biological and Environmental Applications
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  173. Biogenic fabrication and multifunctional therapeutic applications of silver nanoparticles synthesized from rose petal extract
  174. Metal oxides on the frontlines: Antimicrobial activity in plant-derived biometallic nanoparticles
  175. Controlling pore size during the synthesis of hydroxyapatite nanoparticles using CTAB by the sol–gel hydrothermal method and their biological activities
  176. Special Issue on State-of-Art Advanced Nanotechnology for Healthcare
  177. Applications of nanomedicine-integrated phototherapeutic agents in cancer theranostics: A comprehensive review of the current state of research
  178. Smart bionanomaterials for treatment and diagnosis of inflammatory bowel disease
  179. Beyond conventional therapy: Synthesis of multifunctional nanoparticles for rheumatoid arthritis therapy
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