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. 2021 Oct 13;6(42):27833–27841. doi: 10.1021/acsomega.1c03459

Experimental and Computational Studies of Azo Dye-Modified Luminol Oligomers: Potential Application in Lithium Ion Sensing

Neetika Singh †, Elham S Aazam ‡, Ufana Riaz †,*
PMCID: PMC8552318  PMID: 34722983

Abstract

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With a view to design novel conjugated oligomers via a facile technique for its possible application in sensors, the present work reports oligomerization of Bismarck Brown (BB) dye with luminol. The structure was confirmed via IR studies, while the electronic transitions were confirmed by UV–visible studies. Morphological studies were carried out via SEM. Computational studies were carried out using the DFT method with a B3LYP 6-311G(d) basis set to investigate the optimized geometry, band gap, and vibrational and electronic transitions data. The HOMO–LUMO energies showed significant reduction in the band gap upon increasing the content of BB dye. The computational IR and UV spectra were noticed to be in close agreement with the experimental results. Spectrophotometric determination of Li ion was attempted using lithium chloride and a lithium carbonate drug commonly used in the treatment of bipolar disorder. The detection limit was noticed to be as low as 5.1 × 10–6 M, which could be used to design a Li ion sensor.

Introduction

One of the major challenges in the area of molecular electronics is the design of novel optoelectronic materials.1 Research based on conducting polymers (CPs) has become one of the most vital fields in functional materials due to their outstanding properties such as flexibility, redox switching, low cost, lightweight, etc.2,3 Significant progress has been made in designing CPs via chemical synthesis to achieve tunable electrochemical properties applicable in batteries,4 super capacitors,5 fuel cell electrodes,6 biosensors,7 desalination systems,8 drug release,9 ion-sensitive electrodes,10 and electro-chemo-mechanical actuators.11 However, obtaining polymers with controlled architecture is difficult because of the high molecular weight of these polymers due to the development of a cross-linked structure, which ultimately results in insolubility and intractability.12,13

In this regard, the design of low-molecular-weight oligomers/co-polymers of CPs has been extensively investigated as it helps in achieving controlled morphology besides ease of tuning of the electrochemical characteristics.14 Azo-based dyes exhibit multi-functionality and extended delocalization of the π-electronic clouds, and hence the tailoring of these dye moieties with CPs imparts multi-functionality to the polymeric architecture, which can help in attaining various kinds of redox states.14,15 The chemical synthesis of conjugated polymers in the presence of azo dyes has been investigated in the literature.16 Valtera et al.15 used dyes such as Acid Red 1, Orange G, and Sunset Yellow as soft templates for the synthesis of naphthyl phenyl diazene skeletons for the preparation of PPy in aqueous solution. Lei et al.16 explored the structural properties of indigo-based polythiophene (PTh), while Kaur et al.17 reported 4-dicyanomethylene-2-methyl-6-p-dimethyl aminostyryl-4H (DCM) pyran dye-doped thiophene-based conjugated polymers. Wei et al.18 synthesized polycarbazole (PCz)-conjugated polymers via the incorporation of intense light-absorbing organic dyes—dicyanovinyl group (DCN) and diethyl thiobarbituric acid (DTA) photovoltaic applications. Jung et al.19 synthesized CPs composed of isoindigo with 2-decyltetradecane (DT) and bithiophene with/without fluorination used in high-performance organic solar cells.

We have earlier reported the polymerization/co-polymerization of luminol with CPs such as poly(o-phenylenediamine) (POPD),20 poly(o-anisidine) (PoA),21 poly(α-naphthylamine) (PNA),22 and PCz.21

To the best of our knowledge, no work has been reported on the oligomerization of luminol with Bismarck Brown (BB) dye.23 BB dye has been mostly investigated as a dye pollutant and to date, we do not find any literature based on the utilization of the multi-functionality of BB dye for the design of CPs with enhanced fluorescence characteristics. Hence, the present work reports the synthesis and the design of BB dye oligomers using luminol in various molar ratios via a chemical polymerization method. The synthesized oligomers were characterized using FTIR, UV–visible, XRD, and TEM studies. Computational DFT studies were carried out using the DFT/B3LYP method with the 6-311G(d) basis set to compare and verify the experimental data. Lithium chloride was chosen as a model compound along with a lithium carbonate drug, which is widely used to treat mental disorders such as bipolar disorder.24 The drug has a narrow therapeutic index, and long-term effects can increase the chances of toxicity.24 Hence, a technique is required that can provide rapid measurement of lithium in an accurate manner to improve the compliance of lithium treatment with possible reduction in toxicity. We have used a spectrophotometric method for the detection of lithium in salt with the view of possibly fabricating a device for simple measurement of lithium doses.

Results and Discussion

The solubility of the OBB/PLu oligomers is provided in the Supporting Information as Table S1. The oligomers were noticed to be soluble in DMSO, DMF, and NMP. The viscosity average molecular weight was calculated as per the method reported in our earlier studies.25,26 The intrinsic viscosity was measured as per the method reported in our previous studies.16,17 The value of intrinsic viscosity was calculated to be 0.78 for OBB/PLu-50/50, and the molecular weight was computed to be 6284, while for OBB/PLu-80/20, the intrinsic viscosity was found to be 0.61 and Mw = 5620. The intrinsic viscosity for OBB/PLu-20/80 was calculated to be 0.42, and the molecular weight was found to be 3348. The oligomers were obtained by a free radical polymerization mechanism as per our previously reported works.26,27 The monomers in the presence of ferric chloride undergo oxidation, which leads to the creation of a positive charge. The charged monomer then attacks either the same monomer or the co-monomer depending upon the reactivity ratios of the two monomers. The chemical polymerization of conducting polymers is usually carried out in an acidic medium to ensure doping of anions on the polymer chains. In our case, ferric chloride was used as an oxidant and is also reported to serve as a dopant in the presence of an acidic medium, and hence the chains obtained were those of Cl-doped oligomers as shown in Scheme 1.

Scheme 1. (I–III) Oligomerization of PLu with OBB Dye.

Scheme 1

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Optimization of the Geometries of Oligomers via DFT for Determination of Vibrational and Electronic Spectra and Analysis of Mulliken Charge and HOMO–LUMO Energies

The geometrical structures of the oligomers were optimized by taking one monomeric unit of BB and two units of luminol for OBB/PLu-20/80, one monomeric unit each of BB and luminol for OBB/PLu-50/50, and two monomeric units of BB with one monomeric unit of luminol for OBB/PLu-80/20. For pristine luminol, a trimer unit was taken for the geometry optimization (figures provided in the Supporting Information as Figure S1a–h). The optimized geometry of PLu oligomer was noticed to be non-planar, and the charge distribution was around the ring containing oxygen as well as amide groups. The optimized geometry of OBB/PLu-20/80 was noticed to be planar, and the charge distribution was mainly around the amino groups at the terminal aromatic ring of BB as well as on the luminol units. The optimized geometry of OBB/PLu-50/50 appeared to be twisted around the luminol units of the oligomer, and the charge was seen to be concentrated around the carbonyl and amide groups of luminol and also around the amino groups at the terminal aromatic ring of BB. Interestingly, the optimized geometry of OBB/PLu-80/20 was noticed to be highly planar among all oligomers and the charge was noticed to be highly dense at the amino linkages.

The highest occupied molecular orbitals of PLu (Figure 1a) were found to be evenly distributed over the backbone of the BB unit in the oligomer. The lowest unoccupied molecular orbitals appeared to be less symmetrical, which were also distributed along the BB unit. The HOMO–LUMO energy was computed to be 5.74 eV for PLu. In the oligomer OBB/PLu-20/80 (Figure 1b), the lowest unoccupied molecular orbitals were noticed to be distributed uniformly over the entire backbone of the oligomer, while the highest occupied molecular orbitals were noticed to be concentrated around the linkage of luminol and the BB unit. The HOMO–LUMO energy was calculated to be 1.65 eV in this case. However, in the case of OBB/PLu-50/50 (Figure 1c), the LUMO energy is found to be about 2 eV. The lowest unoccupied molecular orbitals appear to be symmetric over the most part of the backbone of the oligomer. OBB/PLu-80/20 in Figure 1d revealed a highly symmetric distribution of the lowest unoccupied molecular orbitals and the HOMO–LUMO energy was computed to be 1.29 eV.

Figure 1.

Figure 1

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HOMO–LUMO energies of (a) PLu, (b) OBB/PLu-20/80, (c) OBB/PLu-50/50, and (d) OBB/PLu-80/20.

Experimental and Theoretical IR Analysis

The IR spectrum of pristine PLu (Table 1 and Supporting Information, Figure S2a–d) revealed that the NH stretching vibration peak at 3355 cm–1 was due to the presence of a secondary amine, while the theoretical spectrum revealed the NH stretching vibration peak at 3350 cm–1. The peaks associated with benzenoid and quinonoid rings appeared at 1587, 1492, and 1326 cm–1, while the theoretical spectrum exhibited the same peaks at 1590, 1480, 1485, 1350, and 1320 cm–1. The CN stretching vibration was observed at 1234 cm–1. The C–H bending vibration peak was found at 1110 cm–1, while the peaks at 995 and 793 cm–1 were correlated to the presence of a substituted benzene ring and fused aniline ring of luminol.20−22 The peaks of PLu were similar to the values reported in our previous studies20,22 and therefore confirmed the polymerization of luminol.

Table 1. IR Spectral Data of PLu and OBB/PLu Oligomers.

samples functional group wavenumber (cm–1) wavenumber (cm–1) (theoretical)
PLu N–H stretching 3355 3350
imine stretching 1610 1620
C=C stretching (quinonoid) 1587 and 1492 1590, 1580, and 1485
C=C stretching (benzenoid) 1326 1350 and 1320
C–N stretching 1234 1230
C–H bending 1110 1120 and 1110
substituted benzene ring 995 990 and 930
fused aniline ring of luminol 793 795
OBB/PLu-20/80 N–H stretching 3310 3300
imine stretching 1624 1628
C=C stretching (quinonoid) 1587 and 1492 1589 and 1488
C=C stretching (benzenoid) 1336 1378 and 1330
C–N stretching 1234 1230
C–H bending 1109 1090
substituted benzene ring 887 992
fused aniline ring of luminol 790 793
OBB/PLu-50/50 N–H stretching 3334 and 3197 3300 and 3093
imine stretching 1624 1630
C=C stretching (quinonoid) 1473 1486
C=C stretching (benzenoid) 1371 and 1324 1369 and 1347
C–N Stretching 1220 1224
C–H bending 1120 1090
substituted benzene ring 887 990
fused aniline ring of luminol 790 790
OBB/PLu-80/20 N–H stretching 3130 3110
imine stretching 1610 1639 and 1618
C=C stretching (quinonoid) 1587 and 1490 1590 and 1486
C=C stretching (benzenoid) 1332 and 1319 1347 and 1323
C–N stretching 1224 1224
C–H bending 1051 1043
substituted benzene ring 925 920
aniline ring of luminol 786 790
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The oligomer OBB/PLu-20/80 also revealed an NH stretching vibration peak at 3310 cm–1, and the theoretical spectrum revealed the same peak at 3300 cm–1. The imine stretching peak was noticed at 1624 and 1628 cm–1 in the experimental and theoretical spectrum, respectively. The quinonoid and benzenoid ring stretching vibration peaks were noticed at 1587, 1492, and 1336 cm–1. The theoretical spectrum revealed the same peaks at 1589, 1488, 1378—, and 1330 cm–1. The CN stretching vibration peak appeared at 1234 cm–1 in the experimental spectrum, and the same vibration was seen at 1230 cm–1 in the theoretical spectrum. The substituted and fused aniline ring peaks were found to be in close agreement in the theoretical and experimental spectra. The oligomer OBB/PLu-50/50 revealed a shift of 34 cm–1 in the experimental spectrum, while the imine stretching peak showed a minor shift of 6 cm–1. The quinonoid and benzenoid peaks showed a shift of 10 cm–1, while the CN peak revealed a small shift of 4 cm–1. The aromatic peaks, however, were found to be in close agreement. Similarly, the experimental IR spectrum of OBB/PLu-80/20 revealed peaks that were found to be in good agreement with the theoretical spectrum. The presence of the above peaks established the structure of the oligomers as shown in Scheme 1. Significant changes were noticed in the NH region of the oligomers due to the synergistic interaction between the two moieties.

Fluorescence and Confocal Analysis

The emission spectra of PLu and its co-oligomers with OBB are shown in Figure 2. The emission spectrum of PLu upon excitation at 350 nm revealed a peak at 420 nm, which has been reported in our previous studies and was correlated to the S1–So transition.20−22 However, upon loading of OBB (up to 20%), the emission spectrum revealed a redshift of 80 nm and was noticed at 500 nm. Similarly, the emission spectra of OBB/PLu-50/50 and OBB/PLu-80/20 revealed an intense but broad peak at 500 nm. The intensity of the peak was noticed to increase with the increase in the loading of OBB and was therefore associated with the emission peak of OBB. Quantum yield (φ) was calculated, taking rhodamine B (RhB) as a reference material.20−22 The calculated φ values were obtained as 0.0010, 0.039, 0.048, and 0.081, revealing that the quantum yield increased with the increase in the OBB content in the oligomer (Table 2). These results displayed that the emission in the UV/visible range could be obtained by choosing the appropriate composition of the monomers in the oligomer.

Figure 2.

Figure 2

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Fluorescence spectra of PLu and OBB/PLu oligomers.

Table 2. Quantum Yield of PLu and Its Oligomers with BB Dye.

sample λmax (nm) integrated area quantum yield (φ)
PLu 400 1.20 × 103 0.0010
OBB/PLu-20/80 500 2.15 × 103 0.039
OBB/PLu-50/50 500 2.86 × 104 0.048
OBB/PLu-80/20 500 3.22 × 104 0.081
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The confocal image of PLu in Figure 3a exhibited blue emission, which has been previously observed for PLu and its co-polymers.20−22 The confocal image of OBB/PLu-20/80 and OBB/PLu-50/50 in Figure 3b,c showed scattered particles exhibiting red emission, while in the case of OBB/PLu-80/20 in Figure 3d, the intensity as well as the brightness of the red emission was noticed to be higher, thereby corroborating the results of the fluorescence studies that the increase in the emission intensity was attributed to the increase in the loading of OBB in the oligomer. The studies therefore confirmed that the fluorescence emission could be tuned to show red emission by controlling the OBB dye content in the oligomer.

Figure 3.

Figure 3

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Confocal images of (a) PLu, (b) OBB/PLu-20/80, (c) OBB/PLu-50/50, and (d) OBB/PLu-80/20.

Experimental and Theoretical UV–visible Studies

The UV–visible spectrum of PLu in Figure 4a revealed peaks at 290 and 420 nm. The 290 nm peak was associated with the π–π* transition, while the 420 nm peak was correlated to the n–π* transition as reported in our previous studies.22−25 The same peaks appeared at 300 and 405 nm, respectively, in the theoretically computed spectrum. The experimental UV–visible spectrum of OBB/PLu-20/80 (Figure 4b) revealed peaks at 350 and 430 nm, whereas the theoretical spectrum revealed peaks at 350 and 450 nm. The peak around 430 nm in the experimental spectrum and 450 nm in the theoretical spectrum was attributed to the presence of OBB dye. The experimental UV spectrum of OBB/PLu-50/50 and OBB/PLu-80/20 (Figure 4c,d) also showed peaks at 350 and 450 nm, and the intensity of the peaks in the theoretical spectrum were noticed to be higher. Interestingly, the peak intensities corresponding to the 450 nm peak were noticed to increase with the increase in the content of OBB in the oligomer. The experimental oscillator strength values were calculated to be 0.17, 1.1, 0.9, and 1.6 for PLu, OBB/PLu-20/80, OBB/PLu-50/50, and OBB/PLu-80/20, respectively, which were in close agreement with the theoretical values in Table 3.

Figure 4.

Figure 4

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UV–visible spectra of (a) PLu, (b) OBB/PLu-20/80, (c) OBB/PLu-50/50, and (d) OBB/PLu-80/20.

Table 3. Oscillator Strength of PLu and OBB/PLu Oligomers.

polymer/co-polymer λmax, experimental (theoretical) oscillator strength, experimental (theoretical)
PLu 420 (405) 0.17 (0.18)
OBB/PLu-20/80 430 (450) 1.1 (1.2)
OBB/PLu-50/50 450 (450) 0.9 (1.1)
PBB/PLu-80/20 450 (450) 1.6 (1.8)
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Morphological Studies

The SEM micrograph of PLu in Figure 5a showed a granular morphology forming agglomerates of various sizes. The SEM micrograph of OBB/PLu-20/80 in Figure 5b also revealed a granular structure predominantly resembling pristine PLu, while the SEM micrograph of OBB/PLu-50/50 in Figure 5c exhibited a layered morphology, revealing bright agglomerates. The morphology of OBB/PLu-80/20 in Figure 5d showed formation of an intense granular structure with tiny grains stacked together. The morphology was noticed to undergo a major transformation upon increasing the loading of OBB in PLu. The results revealed that the content of PLu/OBB had a significant impact on the morphological features as well besides the electronic properties, and the loading of the either moieties could be easily identified via the drastic change from dense bright cluster formation to dark granular agglomerate formation.

Figure 5.

Figure 5

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SEM micrographs of (a) PLu (mag: 50,000×), (b) OBB/PLu-20/80 (mag: 100,000×), (c) OBB/PLu-50/50 (mag: 50,000×), and (d) OBB/PLu-80/20 (mag: 50,000×).

Sensing Studies of Li+ Ions

Recently, in the field of metal sensing, polymers have become more and more important owing to some advantages like easy fabrication and signal amplification compared to a simple organic compound. We have carried out spectrophotometric determination of Li+ ions using LiCl salt and a Li2CO3 drug, which is highly used in psychiatric treatment. Dilute solutions of oligomers OBB/PLu-20/80 and OBB/PLu-80/20 (5.0 × 10–6 M) were taken, and LiCl/Li2CO3 drug solution (2.5 × 10–3 M) was added to the polymer solution to record the change in the peak intensities of the oligomers. Upon progressive addition of Li+ ion solution to the oligomers, the absorption intensities were noticed to decrease with the increase in the concentration of the Li+ ions (figures provided in the Supporting Information as Figure S3a–f). The absorbance change (Ao – A420)/Ao at 420 nm showed a gradual decrease with the increase in the Li+ ion concentration. The fitting curve showed an excellent change in absorbance with concentration, thereby depicting linear detection with an R2 value of 0.9978 (figures provided in the Supporting Information as Figure S4a–c). The detection limit (LOD) of OBB and its oligomers with PLu was calculated and is shown in Figure 6. The LOD values were computed to be 5.1 × 10–6 M, 5.6 × 10–6 M, and 7 × 10–6 M for OBB, OBB/PLu-20/80, and OBB/PLu-80/20, respectively. The values were found to be acceptable for designing these oligomers as probes in the detection of Li.

Figure 6.

Figure 6

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LOD values of OBB and its oligomers.

Conclusions

Oligomers of PLu were synthesized using different loadings of the dye and luminol with a view to design conjugated oligomers for their application in sensing devices. The loading of BB dye as well as luminol was established via experimental and theoretical IR as well as UV studies. The surface morphology of oligomers was found to be dependent on the loading of luminol as well as BB. The quantum yield (φ) values were found to increase with the increase in the loading of the BB dye. The HOMO–LUMO energy was found to decrease with the increase in the loading of dye. Spectrophotometric determination of a Li2CO3 drug confirmed the lowest limit of detection to be 5.1 × 10–6 M, which was good enough to design sensing probes for Li+ ion detection in real samples. Studies on the detection of lithium in blood samples are underway and shall be published soon.

Experimental Section

Bismarck Brown dye (SD Fine Chem Ltd., India), luminol (Sigma Aldrich, USA), ferric chloride (Merck, India), hydrochloric acid (Merck, India), N-N-dimethyl pyrrolidone (NMP) (Merck, India), dimethyl formamide (DMF), and distilled water were used without further purification.

Polymerization of Luminol with Bismarck Brown Dye

Bismarck Brown dye (BB) (1 g, 2.3 × 10–3 mol) was dissolved in an aqueous solution of 2 M HCl (25 mL), and luminol (Lu) (1 g, 5.6 × 10–3 mol) dissolved in DMF (5 mL) was added to a 250 mL Erlenmeyer flask. The solution was stirred at 25 °C for 20 min. Ferric chloride (1 g, 6.0 × 10–3 mol) dissolved in distilled water (25 mL) was added to the reaction mixture and further stirred on a magnetic stirrer for 2 h at −5 °C. The obtained product was kept in a deep freezer for 24 h. The removal of unreacted ferric chloride was ensured by washing several times with distilled water and testing the filtrate for the presence of Fe ions using potassium ferrocyanide as per the reported method.27 The obtained product was isolated via Soxhlet extraction and then dried in a vacuum oven for 72 h at 70 °C to ensure complete removal of water and impurities. The obtained oligomer was designated as OBB/PLu-20/80 based on the molar ratio taken for synthesis. A similar procedure was adopted for the oligomerization of OBB/PLu-80/20, OBB/PLu-50/50, and pure luminol, which was designated as PLu.

Characterization

Spectral Studies

IR measurements were carried out on a Fourier transform infrared (FTIR) spectrophotometer model Shimadzu IRA Affinity–1 in the form of KBr pellets. UV–visible spectra were taken on a Perkin-Elmer-Lamda-Ez-25 by dissolving the oligomers in NMP. Fluorescence spectra of the sample were taken in solution form (NMP solvent) and recorded on a fluorescence spectrophotometer model Horiba Fluorolog @ 3-11.

Morphological Studies

SEM images were taken on a field emission-scanning electron microscope (FE-SEM, Leo Supra 50VP, Carl Zeiss, Germany) equipped with an energy-dispersive X-ray system. Fluorescence images were obtained using a laser confocal microscope with a fluorescence correlation spectroscope (Olympus FluoView FV1000) equipped with a He–Ne laser and oil immersion objective.

Theoretical Studies

DFT and TD-DFT calculations were performed using GAUSSIAN 09 software, and the optimized geometries were obtained using the DFT/B3LYP method with the 6-311G(d) basis set.21,25 The oscillator strength, HOMO–LUMO energies, and band gap were determined using the optimized geometries with the same basis set. The same optimized structures were used in the vibrational frequency calculations. The UV–vis spectra of geometry-optimized structures were simulated at TD-DFT/B3LYP using the 6-311G(d) basis set.

Sensing Studies of Li+ Ions in Solution

We employed DMF solutions to perform UV titration experiments. The stock solutions (100 μL) comprised deionized water containing LiCl and a Li2CO3 drug. The oligomer solution (5 × 10–6 M) was prepared in DMF. The LiCl and Li2CO3 stock solutions (2.5 × 10–5 M) were prepared by dissolving an appropriate amount of salt in DMF (0.00035 g, 5 × 10–6 M). The salt solution was then added to the oligomer solutions (200 μL) in various concentrations ranging from 10 to 100 μL, and the UV–visible spectra were recorded in a wavelength range of 300–800 nm.

The detection limit for Li+ ions by the oligomer was calculated based on IUPAC 3σ criteria using a UV/vis spectrophotometer with a 1 cm path-length cuvette. A blank solution of oligomer (5 × 10–6 M) in DMF was scanned at least five times to determine the standard deviation of the blank. The same solution was then titrated with Li+ ions (2.5 × 10–3 M). After each addition, the solution was shaken and then incubated for 3 min before scanning at room temperature. A linear calibration curve was established by plotting the absorbance against the concentration of Li+ ions. The LOD was calculated using the equations LOD = 3σ and σ = SD/s, where SD is the standard deviation of the blank oligomer solution and s is the slope of the calibration curve.

Acknowledgments

The authors wish to acknowledge the DST-PURSE, Jamia Millia Islamia (JMI), New Delhi-110025, India, for providing the instrumentation facility. N.S. wishes to acknowledge the RGNF-SRF, UGC, India, for providing funding support to conduct this research work.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c03459.

  • (Table S1) Solubility test of polymers and co-polymers; (Figure S1) optimized geometry of PLu, Mulliken charge distribution in PLu, optimized geometry of OBB/PLu-20/80, Mulliken charge distribution in OBB/PLu-20/80, optimized geometry of OBB/PLu-50/50, Mulliken charge distribution in OBB/PLu-50/50, optimized geometry of OBB/PLu-80/20, and Mulliken charge distribution in OBB/PLu-80/20; (Figure S2) FTIR spectra of PLu, OBB/PLU-20/80, OBB/PLu-50/50, and OBB/PLU-60/20; (Figure S3) UV spectra depicting change in absorbance of OBB and its oligomers with PLu upon addition of lithium chloride and lithium carbonate; (Figure S4) relationship between absorbance change and concentrations of Li+ ions in lithium chloride and lithium carbonate using OBB, OBB/PLu-20/80, and OBB/PLu-80/20 (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. U.R. conceptualized the work and analyzed the results while N.S. carried out the experimental studies, and E.S.A. contributed in analyzing and interpreting morphological studies.

The authors declare no competing financial interest.

Supplementary Material

ao1c03459_si_001.pdf (1.8MB, pdf)

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