Synthesis and characterization of multiwalled carbon nanotubes functionalized with chlorophyll-derivatives compounds extracted from Hibiscus tiliaceus

Diamond & Related Materials 89 (2018) 151–162

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Synthesis and characterization of multiwalled carbon nanotubes functionalized with chlorophyll-derivatives compounds extracted from Hibiscus tiliaceus

T

Miriam M. Tostado-Plascenciaa, Marciano Sanchez-Tizapaa,*, Adalberto Zamudio-Ojedab a

Departamento de Ciencias Naturales y Exactas, Centro Universitario de los Valles, Universidad de Guadalajara, Carretera Guadalajara-Ameca, Km 45.5, C.P. 46600, Ameca, Jalisco, México b Centro Universitario de Ciencias Exactas e Ingenierías, Universidad de Guadalajara, Blvd. Marcelino García Barragán No. 1421, esq. Calzada Olímpica, C.P. 44430, Guadalajara, Jalisco, México

A R T I C LE I N FO

A B S T R A C T

Keywords: Multiwalled carbon nanotubes Hibiscus tiliaceus Chlorophyll-derivatives Chlorophyllide Pheophorbide Chlorophyll

In this work, we are presenting the synthesis and characterization of compound materials based on multiwalled carbon nanotubes and chlorophyll-derivatives as chlorophyllide-a and pheophorbide-a, extracted from the plant Hibiscus tiliaceus. Three types of multiwalled carbon nanotubes were used: i) pristine carbon nanotubes (diameter < 10 nm); ii) acid-oxidized carbon nanotubes (diameter < 10 nm), and iii) nitrogen doped multiwalled carbon nanotubes (20 nm diameter). Materials were characterized by ultraviolet-visible spectroscopy, Raman spectroscopy, fluorescence spectroscopy, dynamic light scattering, and field effect scanning electron microscopy. The method we have used for the synthesis was straightforward and easy, and also, we have found evidence of interactions between carbon nanotubes and chlorophyll-derivatives. The interaction between both compounds was confirmed by the significant quenching of signals measured by optical techniques as ultraviolet-visible spectroscopy, Raman spectroscopy, and fluorescence spectroscopy. In addition, we found that interaction increased as the time went on. We also found evidence of the formation of covalent bonds because of the shifting of the absorbance peaks, showing the formation of J or H aggregates. Complementary information about the morphology, revealed a significant increase in the nanotubes thickness, in the range from 80 to 160% for nanotubes with diameters < 10 nm, while nitrogen doped nanotubes showed a lower increase (25–40%). In this way, we have synthesized and characterized compound materials based on multiwalled nanotubes and chlorophyll-derivatives compounds, extracted from natural sources. We synthesized compounds with different coatings that can be used in fields as gas sensing or water remediation.

1. Introduction Since the report about carbon nanotubes (CNT) synthesis, made by S. Iijima [1], these materials have been the source of diverse research about the properties of these materials. Carbon nanotubes by themselves have shown outstanding properties in diverse fields such as Physics, Mechanics, etc., and have been the base for several applications in these fields [2]. After intensive research about the properties of CNT, the investigations have been diversified to other fields, for example, i) synthesis and characterization of compound materials based on CNT and other materials, and ii) introduction of dopant elements in the graphene network, in order to synthesize materials with new properties. Regarding compound materials, carbon nanotubes have been mixed

*

with organic and inorganic compounds, as polymers [2-13], metal oxides [14-22], cellulose [23-25], gold [26], bio-polymers [27], polyurea [28], amino-acids [29], and other materials [30-34]. Concerning doped carbon nanotubes, there have been reports about doping with several materials [35-48], boron [47,49], tin-nitrogen [50], cobalt-nitrogen [44,51], silicon [52], metal-Co2P/nitrogen [53]. Among doped carbon nanotubes, nitrogen doping excels because several applications have been found for these materials, focusing most of them on catalysis [35,37,39,40,42-46,48], hydrogen peroxide detection [38], and glucose biosensing [41]. Applications in health deserve a special mention, there are advances in the applications of CNT-based novel delivery systems with an emphasis on findings that demonstrate their important roles in cancer imaging applications, demonstrating their potential as unique agents

Corresponding author. E-mail address: [email protected] (M. Sanchez-Tizapa).

https://doi.org/10.1016/j.diamond.2018.09.004 Received 23 April 2018; Received in revised form 22 July 2018; Accepted 5 September 2018 Available online 10 September 2018 0925-9635/ © 2018 Elsevier B.V. All rights reserved.

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with high-level ultrasonic emission, strong Raman scattering resonance, and interesting magnetic properties [54]. Regarding specifically to compound materials based on carbon nanotubes with natural extracts, or with compounds coming from live plants, there are still few reports. Natural rubber has been used for dispersing metallic and semiconductor multiwalled carbon nanotubes (MWNT) [55]. Carbon nanotubes have been used to influence the rheological behavior of rubber [56], to enhance the electrical conductivity [57,58], or to reinforce it [59]. There are some reports about composites with colorants, e.g., anthocyanin dyes extracted from frozen blackberries were mixed with carbon nanotubes for applications in solar cells [60]. Pyro-pheophorbide and riboflavin have been used to synthesize non-covalent compounds with CNT through π-π stacking interactions [61,62]. Barbinta et al. used chlorophyll-a extracted from spinach to synthesize liposome-CNT hybrid materials [63,64]. In this work, we are reporting the synthesis of compound materials based on carbon nanotubes and chlorophyll-derivatives, (chlorophyllide-a, Chlide-a, and pheophorbide-a, Ph-a), extracted from fresh Hibiscus tiliaceus leaves. The extracts were mixed with pristine and acid oxidized MWNT (10 nm diameter), and nitrogen doped multiwalled carbon nanotubes (20 nm diameter, CNxMWNT). The compound materials were characterized by ultraviolet-visible (UV–Vis), Raman, fluorescence, and infrared (Fourier Transform Infrared, FTIR) spectroscopies, as well as dynamic light scattering (DLS), and field effect scanning electron microscopy (FESEM). We found evidence about the formation of covalent bonds between chlorophyll-derivatives (Chl-d) and MWNT.

a)

b)

c)

d)

e)

f)

2. Materials and methods 2.1. Carbon nanotubes Pristine MWNT with diameter < 10 nm were obtained from Nanostructured & Amorphous Materials Inc. (5–15 μm length, 95% of purity, 40–600 m2/g) and were labeled as PMWNT. PMWNT were acidoxidized dissolving 5 mg of PMWNT in a solution of nitric and sulfuric acids (10 M:0.5 M), the mix was sonicated for 1 h (Branson 2510R-DTH ultrasonic cleaner, 40 kHz, 130 W), when the time was over, MWNT were filtered using Whatman filter paper No. 1 (pore diameter of 110 nm) and were washed until water resulting from filtrate had neutral pH. MWNT were dried in a stove at 60 °C and stored in closed vessels, acid-oxidized MWNT were labeled as FMWNT. CNxMWNT were synthesized as it is described elsewhere [65].

Fig. 1. Natural extracts, a) chlorophyll-a; b) carotenoids; c) chlorophyll-b; d) xanthophylls; e) chlorophyllide-a; f) pheophorbide-a. From a) to d) the samples were under UV irradiation. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

and Ph-a, 15 mL of the solution containing Cha was mixed with a solution of potassium hydroxide (KOH, 30%, Sigma-Aldrich), and stirred for 10 min (this step was carried out to cut the phytol tail of Cha in order to synthesize Chlide-a), the Chlide-a solution was divided into two halves. Acetic acid (CH3COOH, ACS reagent, ≥99.7%, Sigma-Aldrich) was added drop-wise to the one of the halves until the bright green color of the solution (Fig. 1e) changed to olive green color (Fig. 1f), this change of color indicated that the magnesium atom at the center of the Chlide-a had been dissolved, converting Chlide-a to pheophorbide-a, in this way we obtained Chlide-a and Ph-a compounds. This method was adapted from previous reports [66-68]. In order to observe the characteristic fluorescence of Chl-d, some of them were irradiated with UV light (Tecnolite, 120–127 V, 60 Hz, 20 W, 0.29 A, 370 nm) (Fig. 1a–d).

2.2. Chlorophyll-derivatives extraction For the extraction of Chl-d, Hibiscus tiliaceus leaves were taken from the gardens of our campus (no use of fertilizers or pesticides), veins of leaves were removed and leaves were washed and cut in small pieces. Subsequently, 500 mg of leaves were grounded, and mixed with 25 mL of acetone (Sigma-Aldrich, ACS reagent, ≥99.5%), and shook by hand for 10 min, the mixture acetone-leaves was filtered to separate the fiber. The remaining liquid labeled as Chl contained several compounds as chlorophyll-a, chlorophyll-b, carotenoids, and xanthophylls. Subsequently, 20 mL of the concentrated solution Chl was mixed with 30 mL of petroleum ether and 35 mL of deionized water, the solution was stirred at low speed for 10 min, this step was carried out twice to separate acetone and to keep Chl-d in the petroleum ether, the layer acetone-water was removed. Afterward, 25 mL of a solution water:methanol (92:8) (methanol from Sigma-Aldrich, anhydrous, 99.8%) was added. After adding the methanol, we observed two layers, the upper layer contained petroleum ether, chlorophyll-a (Cha) and carotenoids (Fig. 1a and b), and the bottom layer contained methanol, chlorophyll-b (Chb) and xanthophylls (Fig. 1c and d). As the volume of Cha was higher, we used this solution to extract chlorophyllide-a (Chlide-a) and pheophorbide-a (Ph-a). For the extraction of Chlide-a

2.3. Synthesis and characterization of compound materials The synthesis of compound materials was as follows: one milligram of MWNT (PMWNT, FMWNT or CNxMWNT) was mixed with 5 mL of Chl-d (Chlide-a or Ph-a) and sonicated for 15 min. The characterization of MWNT, Chl-d, and compound materials was done by UV-vis spectroscopy (Shimadzu 3600 unit), Raman spectroscopy (Raman DXR 152

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a)

Fig. 3. UV–Vis of concentrated chlorophyll, chlorophyll-a and chlorophyll-b extracted from fresh leaves of Hibiscus tiliaceus, Chl is concentrated chlorophyll (green line), CHa + K is chlorophyll-a plus carotenoids (red line), Chb + X is chlorophyll-b plus xanthophylls. The solvents used were: acetone (A), petroleum-ether (P.E.), and methanol (M). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3. Results and discussion 3.1. Characterization 3.1.1. Characterization of chlorophyll-derivatives by Raman and UV–Vis spectroscopies Fig. 2 shows the structural representation of Chlide-a and Ph-a molecules, the main difference between both molecules is the presence of magnesium atom at the center of the macro-cyclic ring in Chlide-a molecule, both molecules have some oxygenated groups that could work as anchoring points to establish bonds between Chl-d and MWNT. A solution of concentrated chlorophyll (Chl) is expected to absorb at two wavelength ranges: i) 400–450 nm, and ii) 650–700 nm. Cha absorbs at 400–450 nm and 680–700 nm, while Chb absorbs at 490–530 nm and 620–680 nm, however, slight changes could be expected depending on the solvent [69]. Fig. 3 shows the UV-vis spectra of Chl-d extracted from fresh leaves of Hibiscus tiliaceus, strong absorption was observed for the concentrated chlorophyll (sample Chl), as well as for the Cha sample, on the contrary, for the sample Chb, lower intensity absorptions were observed, which indicated lower concentration of this compound; for Cha and Chb samples we observed absorptions at 430 nm (Soret band) and 660 nm (satellite band), even more, the characteristic Q bands (3 bands) of porphyrin-like compounds were observed in the range from 500 to 650 nm. Fig. 4 shows the UV–Vis spectra of Chlide-a and Ph-a, it could be seen that the spectrum of Chlide-a was similar to the one of Cha (see Fig. 3), the difference was the redshift in the peak of maximum absorbance (430 → 438 nm), while that the spectrum of Ph-a showed a significant blue-shift (430 → 412 nm). In addition, there was a significant decrease (∼50%) in the absorbance, compared with the absorbance of Chl or Cha. Fig. 5 shows the Raman spectra of Chlide-a and Ph-a, the principal Raman signal in this spectrum was an intense (∼25,000 cps) and broadband in the range from 750 to 1750 cm−1. Several bands related to chlorophyll have been reported at wave-numbers (1387, 1328, 1283, 1196, 1157, 900–100 cm−1) near to the ones observed in this figure, as well as some bands related to carotenoids by Villar et al. and Jurado et al., who reported bands at 1530, 1155, and 1002 cm−1 [70,71].

b) Fig. 2. Structural formulas of chlorophyll-derivatives, a) chlorophyllide-a; b) pheophorbide-a. The difference between both molecules is the atom of magnesium at the center of macro-cyclic ring. Color code, white: hydrogen, gray: carbon, blue: nitrogen, red: oxygen, green: magnesium. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

microscope of Thermoscientific, laser of 633 nm), fluorescence spectroscopy (Varian Cary Eclipse fluorescence spectrophotometer, excitation wavelength of 402 nm), field emission scanning electron microscopy (FESEM) was carried out in a Hitachi S-5500 scanning electron microscope, DLS was done in a Malvern Zetasizer Nano SZ90 unit. Measurements of carbon nanotubes diameter were carried out using the software Gwyddion (Department of Nanometrology, Czech Metrology Institute), and the estimation of representative diameter was done measuring at least 100 nanotubes.

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Fig. 4. UV–Vis spectra of freshly extracted chlorophyllide-a and pheophorbidea in aqueous methanol.

Fig. 6. UV–Vis spectra of freshly synthesized compounds FMWNT + Chlide-a and FMWNT + Ph-a.

a) Fig. 5. Raman spectra of freshly extracted chlorophyllide-a and pheophorbidea.

Parab et al. reported that some bands in the range 1100–1600 cm−1 are related to the stretching of CeC and CeN bonds, while bands in the range of 700–950 cm−1 are related to deformations outside of the tetrapyrrole [72]. Referring to Raman characterization of MWNT, it has already reported by Tostado-Plascencia et al. [65]. Concerning the effects of the method of functionalization on MWNT, the procedure developed in this work generates different functional groups (carboxylic, carbonyl, sulfate and hydroxylic) in the walls of MWNT [73], which in turn, enhances the chemical union of the different molecules that were used in this investigation. Furthermore, said functionalization allows the nanotubes to acquire a hydrophilic behavior, which helps to generate better dispersions of these, and more stable solutions, which in turn, helps the coating to be homogeneous throughout the sample.

b) Fig. 7. UV–Vis results of freshly synthesized compounds, a) Ph-a, PMWNT + Ph-a, and FMWNT-Ph-a; b) solution of Ph-a changes after 3 days from olive green color to a colorless solution and the formation of precipitates occurs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Ph-a, we will present results of these chlorophyll-derivatives and their respective compound materials. Specifically, we are interested in doing research on the properties of the chlorophyll-derivative without the magnesium atom (Ph-a), seeking the attachment of heavy metal atoms

3.1.2. Characterization of compound materials 3.1.2.1. UV–Vis spectroscopy. As this work was focused on Chlide-a and 154

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a) Fig. 9. Raman spectra of materials FMWNT + Chlide-a and FMWNT + Ph-a.

b)

c) Fig. 10. Raman spectroscopy of Ph-a and MWNT + Ph-a.

Fig. 8. Stability of MWNT + Ph-a materials, a) UV–Vis spectra of MWNT + Pha compounds after three days of having been synthesized; b) J aggregate, interaction between MWNT and Ph-a; c) H aggregate, interaction between Ph-a molecules grafted to the surface of MWNT.

analyze CNxMWNT + Ph-a as it was not easy to disperse this compound in methanol. The Soret band of Ph-a was at 410 nm, the position of the Soret band for the compound PMWNT+Ph-a was not clear, perhaps ∼400 nm, while for the compound FMWNT + Ph-a a small blue-shift (410 → 408 nm) was observed. It was observed that as the time passes, the color of the solution changes from the olive green color (Fig. 1f) to a colorless solution, and the formation of precipitates occurs after 3 days (Fig. 7b)). In order to gain better insight about the stability and the interaction between both compounds as the time passes, we did UV–Vis studies after 3 days of the extracting Ph-a and synthesizing composite materials, for this end, samples were sonicated previously in order to redissolve the compounds, the results of UV–Vis are shown in Fig. 8. The Soret band of Ph-a was almost at the same position as the one of the freshly synthesized Ph-a (410 → 412 nm). It was noteworthy the strong decrease in the Soret band for all the compound materials, as its

in water remediation applications. Fig. 6 shows the UV–Vis characterization of Chlide-a, Ph-a, and their compound materials with FMWNT. The compound with Chlide-a, showed practically no shifting (438 → 440), while the compound with Ph-a showed an important blueshift (412 → 397 nm). In addition to the shifting in the absorbance wavelengths, compound materials showed a strong decrease in the absorbance intensity, it has been well documented that a decrease in the absorbance, as well as a blue, or a redshift, are related with strong supra-molecular interactions between CNT and Chl-d [65,74,75]. As there were experimental issues with other compound materials, in Fig. 6 we have focused only on FMWNT mixed with Chlide-a and Ph-a. Fig. 7 shows the UV–Vis spectra of freshly synthesized compounds Ph-a, PMWNT + Ph-a, and FMWNT + Ph-a. It was not possible to 155

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Fig. 13. Fluorescence spectra of Ph-a and MWNT + Ph-a compounds in aqueous methanol. A.M. stands for aqueous methanol. In the inset the molecule of Ph-a is shown.

405 nm), and also for the compound with CNxMWNT (412 → 408 nm). It is important to mention that it was possible to analyze the compound CNxMWNT + Ph-a. It has been reported that a redshift is related with the formation of supra-molecular configurations called J aggregates [76], this type of aggregates is synthesized because of strong interactions between Ph-a and MWNT, i.e. the establishment of covalent bonds between both compounds (Fig. 8b). On the other side, blue-shift is related to the formation of H aggregates as the one shown in Fig. 8c, i.e., the union of two J aggregates through the formation of bonds molecules of Ph-a. It could be concluded that the decrease of the intensity of the bands, combined with the higher shifting of bands, revealed that the interaction between MWNT and Ph-a get strengthened as the time passes.

Fig. 11. Infrared spectra of Ph-a, FMWNT, and FMWNT + Ph-a materials. Table 1 Particle size and zeta potential of MWNT and compound materials. In order to make this analysis, 50 μL of the compound material was taken and mixed in a solution of methanol (92%) and water (8%). Material

Particle size (nm)

Zeta potential ζ (mV)

PMWNT FMWNT CNxMWNT PMWNT + Ph-a FMWNT + Ph-a CNxMWNT + Ph-a

1970.0 1682.0 – 580.0 300.0 440.0

−12.0 −10.0 – −135.0 −155.0 −85.0

3.1.2.2. Raman spectroscopy. Fig. 9 shows the Raman spectra of the materials FMWNT + Chlide-a and FMWNT + Ph-a. It was noticeable that intensities decreased from ∼25,000 cps for Chl-d (Chlide-a and Pha, Fig. 5) to ∼200 cps for compound materials, this decrease represented a reduction of 99%. This result also indicated a strong interaction between Chl-d and MWNT. The explanation for this phenomenon could be that as Chl-d are rich in electrons capable of participating in fluorescence, or Raman responses, when Chl-d are mixed with MWNT, the electrons from Chl-d are combined with the holes of MWNT, in this way the amount of free electrons it is reduced, and as a consequence the Raman intensity decreases; these results are in accordance with the UV–Vis analysis. For this analysis, only FMWNT were chosen, as the compounds containing this material were the ones that showed the highest decrease in Raman intensity. Fig. 10 shows the Raman characterization of the compound materials MWNT + Ph-a. All the spectra showed two bands, with the higher one at ∼1000 cm−1. The stronger decrease observed for Raman intensity (−28% for CNxMWNT, −64% for PMWNT, and −86% for FMWNT), confirmed the significant interaction between MWNT and Pha.

Fig. 12. Fluorescence spectra of Chlide-a and MWNT + Chlide-a compounds in aqueous methanol. A.M. stands for aqueous methanol. In the inset the molecule of Chlide-a is shown.

3.1.2.3. Infrared spectroscopy. Fig. 11 shows the infrared spectra of Pha, FMWNT, and FMWNT + Ph-a. The spectra of FMWNT were similar to the ones of PMWNT and CNxMWNT, therefore only FMWNT results are presented. The spectrum of Ph-a resembles the ones of tetraphenylporphyrin [77] and protoporphyrin IX [65]. Bands in the range from 3000 to 2800 cm−1 are related to the stretching of the bonds CeH of CH2 and CH3, these groups are present in Ph-a and FMWNT [78]. The band at 1644 cm−1 could be related to double bonds

intensity was barely measurable, moreover, there was a significant shifting for all the cases, redshift for the compound with PMWNT (412 → 419 nm), blue-shift for the compound with FMWNT (412 →

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Fig. 14. FESEM of PMWNT and PMWNT+Chl-d materials, a) PMWNT; b) PMWNT + Chlide-a; c) PMWNT + Ph-a.

3.1.2.4. Dynamic light scattering. Table 1 shows particle size and zeta (ζ) potential of MWNT and their compounds. Particle size gives information about how MWNT are dispersed, and it has an inverse relationship with the ζ potential. It is noteworthy the higher increase of the ζ potential as MWNT were mixed with Chl-d. The smallest particle size of this compound, coupled with its highest ζ potential indicate that the functionalization of PMWNT helps to anchor Ph-a on FMWNT surface, which in turn, produces a stronger interaction between FMWNT and Ph-a, in line with UV–Vis results (Figs. 7 and 8), and Raman spectroscopy (Fig. 10). As it was said before, there were experimental issues with CNxMWNT, therefore, it was not possible to characterize this material by DLS.

C]C [79]. As Ph-a was dissolved in aqueous methanol it was observed the strong band, characteristic of eOH group, near to ∼3357 cm−1. The bands at 2983 and 2844 cm−1 could be related to the stretching mode of the bonds CeH (υCeH), [80]. In the range from 1700 to 1000 cm−1, there were several bands of porphyrins [77], some of them are: 1431 cm−1 (stretching of C]N, υC]N), 1398 cm−1 (stretching of pyrrole, υC]H) [77]. Concerning the spectrum of FMWNT, there were some absorptions in the region of eOH groups (∼3600 cm−1) and in the region of carbonyl groups (1640, 1660 cm1). The main point in this figure is that the compound FMWNT + Ph-a keeps several characteristics of Ph-a, even more, some of the characteristics are enhanced, such enhancement is expressed as stronger absorptions (∼1500, ∼3700 cm−1). The preservation of the characteristics of Ph-a in the compound is an additional evidence of the good interaction between both materials.

3.1.2.5. Fluorescence spectroscopy. Fluorescence is a mechanism by which a molecule is excited by an electromagnetic radiation, and when the stimulus is turned off, the molecule and its electrons go back 157

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Fig. 15. FESEM of FMWNT and compound materials, a) FMWNT; b) FMWNT + Chlide-a; c) FMWNT + Ph-a.

CNxMWNT > FMWNT > PMWNT, resulting in a maximum decrease of −95 % (Fig. 12). It was noteworthy that for PMWNT and FMWNT there was a redshift, from 672 nm to 669 nm, while for CNxMWNT there was no shifting, however, the reduction in the fluorescence was the highest. For the compounds with Ph-a (Fig. 13), the fluorescence of Ph-a alone was a little higher compared with Chlide-a. The fluorescence decrease as a function of the type of MWNT was CNxMWNT > PMWNT > FMWNT, reaching a maximum decrease of 96%. In this case, the reduction was almost the same for PMWNT and FMWNT, and there was no shifting. It was interesting that in all the cases there was a significant decrease in fluorescence, these results confirm that there was good interaction between MWNT and Chl-d. Results of fluorescence technique

to the basal state. When electrons go back, release energy as light, it is well known that porphyrins and chlorophyll-derivatives have this property. When these materials have no electrons available for this process, e.g. because electrons have been transferred to another compound attached to the fluorescent one, there is a decrease in the fluorescence, i.e., the fluorescence decrease is an indicator of the level of interaction between both molecules. Compounds containing chlorophyll-derivatives were excited at wavelength = 402 nm (a wavelength value that was taken from absorbance measurements). Fig. 12 shows the emission spectra of Chlide-a and its compound materials, while Fig. 13 shows the spectroscopy of Ph-a and its respective compounds, the maximums for Chlide-a and Pha were at ∼672 nm. For Chlide-a compounds the fluorescence decrease as a function of the type of nanotubes followed the order 158

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Fig. 16. FESEM of CNxMWNT and compound materials, a) CNxMWNT; b) CNxMWNT + Chlide-a; c) CNxMWNT + Ph-a.

be that some electrons coming from CNxMWNT are occupying energy states that produce fluorescence.

Table 2 Increment in the thickness of MWNT after mixing with chlorophyll-derivatives. Type of MWNT

Chlide-a (%)

Ph-a (%)

PMWNT FMWNT CNxMWNT

160 80 25

80 120 40

3.1.2.6. Field effect scanning electron microscopy. Figs. 14–16 show FESEM images of compounds materials MWNT + Chl-d and Table 2 shows the increase in the thickness of MWNT after mixing with Chlide-a or Ph-a. In the case of PMWNT (Fig. 14), it was observed a higher increase in the thickness, especially when PMWNT were mixed with Chlide-a. Another difference was the change in the surface roughness of MWNT, while PMWNT showed a rougher surface, nanotubes in the compound PMWNT + Chlide-a showed a less rugged, and flat surface. On the other hand, the compound PMWNT + Ph-a showed a rougher surface with an increased diameter. Regarding FMWNT and their compound materials, it could be seen that after the chemical oxidation the structure of MWNT was damaged

are in some way different from the ones of Raman or UV–Vis spectroscopies, as the degree of the decrease for CNxMWNT was the highest. It has been reported that CNxMWNT shows a great number of defects along its structure, this fact increases the reactivity of nanotube walls [81]. That property could allow a better interaction between the carbon atoms of the nanotube with the nitrogen atoms of the pyrrole rings of chlorophyll-derivatives. Another reason for the higher decrease could 159

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(compare Fig. 14a) with Fig. 15a), as a reduction in the diameter (7 → 6 nm), and some shortening was observed. There were remarkable differences on the compound materials, compared with the compound materials containing PMWNT. For the former, there was an increase in the diameter, however, PMWNT were kept dispersed despite the mixing with Chl-d, on the contrary, for FMWNT there was formation of agglomerates after mixing FMWNT with Chl-d, it seemed to be that the grafting of oxygenated groups on the surface of MWNT enhanced the formation of covalent bonds between both materials. Even more, the compound FMWNT + Ph-a exhibited a higher increase in thickness (Fig. 15c and Table 2). Concerning to compounds containing CNxMWNT, it was observed a lower increase in the thickness, perhaps due to the metallic character of CNxMWNT, however, the lower increase could be misleading, as CNxMWNT have the larger initial diameter, there is a large surface area to coat. In spite of the lower increase (∼40%), it was higher, compared with previous reports using protoporphyrin X, [65]. CNxMWNT alone tended to be bundled because of Van der Waals interactions, however, after mixing with Chl-d, CNxMWNT were dispersed. It was easy to verify the formation of H aggregates, i.e. the union of two or more MWNT through the formation of covalent bonds between Chl-d molecules attached to the surface of the MWNT (see Fig. 16c), in line with all the information presented above, specially the results from UV–Vis (Fig. 8). Referring to the length of nanotubes, is of the order of microns and usually, they are not completely straight, they are slightly undulated. When MWNT are subjected to chemical treatments, their ends, having a hemispherical shape, are the first to be affected, and after treatments, the length may be shorter than the initial one [81]. However, it is difficult to know how much they shortened because, as mentioned, they are wavy and the shortening can be of the order of measurement error. In the same way, the possible final increase, when putting them with the molecules that were embedded in the surface, is the order of a few nanometers and it is difficult to have a specific conclusion. Is not easy to establish correlations between the morphology observed in FESEM and the properties, as a slight random behavior was observed throughout the characterizations, however, PMWNT showed a strong compatibility with chlorophyll-derivatives. In general, compound materials containing Chlide-a presented compact and flat surface, while compounds containing Ph-a showed a rougher surface. Compounds with Chlide-a showed cluster and agglomerated morphologies, while compounds with Ph-a were more dispersed, the agglomerated morphology could be the reason behind the slightly higher quenching of the fluorescence with these compounds. After analyzing the results of the characterizations, it is possible to conclude some facts, we have developed for the first time a method of synthesis of compound materials based on MWNT and chlorophyll derivatives with some advantages, i) the procedure for synthesizing compound materials is easy and straightforward; ii) the procedure of extraction of chlorophyll-derivatives is quite simple; iii) chlorophyllderivatives can be extracted almost from any plant (abundant sources); v) the procedure of extraction of chlorophyll-derivatives is cheap; vi) a wide variety of coatings can be obtained with this procedure; vii) the procedure developed allows to synthesize MWNT with thicker coatings than previous reports, and viii) using multiwalled carbon nanotubes we have synthesized coatings as thicker as the ones reported for single walled carbon nanotubes, which usually are more compatible to porphyrin-like compounds. The compound materials developed in this research could find application in gas sensing, e.g. with gases that show chemisorption in MWNT as ammonia, the barrier of porphyrin-like compounds could act avoiding this phenomenon. Another application could be in phytoremediation for heavy metals adsorption.

materials based on multiwalled carbon nanotubes and natural chlorophyll-derivatives extracted from the plant Hibiscus tiliaceus. Using optical and structural techniques it has been possible to analyze the interactions between both materials. When multiwalled carbon nanotubes and chlorophyll-derivatives were mixed, there was the formation of covalently linked compound materials. It was demonstrated that chlorophyll-derivatives have an active role in dispersing, trapping and covering carbon nanotubes. It was also demonstrated that pristine carbon nanotubes have a more active surface, in which chlorophyllderivatives can be grafted easily. The compound materials developed in this research could find application in gas sensing, solar energy or for remediation of heavy metals. Acknowledgments We are grateful to Consejo Nacional de Ciencia y Tecnología (CONACyT) México, for the scholarship for the Ph-D studies of Miriam Marcela Tostado-Plascencia (226169/211589). We are grateful to Dr. José Campos Álvarez from the Instituto de Energías RenovablesUniversidad Nacional Autónoma de México for FESEM analysis. We acknowledge the support from the Centro de Investigación en Nanociencias y Nanotecnología of the Centro Universitario de los Valles at Universidad de Guadalajara, Jalisco México. We are grateful to Dr. Celso Velázquez Ordóñez, Dra. María Luisa Ojeda Martínez, and Dr. Miguel Ojeda Martínez for fluorescence measurements, and we acknowledge also the support of the infrastructure obtained for the Laboratorio de Desarrollo de Materiales of the Centro Universitario de los Valles of the Universidad de Guadalajara through the project SEPCONACyT Ciencia Básica 2011, 167833, México. References [1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [2] M.F.L. De Volder, S.H. Tawfick, R.H. Baughman, A.J. Hart, Carbon nanotubes: present and future commercial applications, Science 339 (2013) 535–539. [3] S.N. Habisreutinger, T. Leijtens, G.E. Eperon, S.D. Stranks, R.J. Nicholas, H.J. Snaith, Carbon nanotube/polymer composites as a highly stable hole collection layer in perovskite solar cells, Nano Lett. 14 (2014) 5561–5568. [4] K. Liew, Z. Lei, L. Zhang, Mechanical analysis of functionally graded carbon nanotube reinforced composites: a review, Compos. Struct. 120 (2015) 90–97. [5] J. Gu, P. Xiao, J. Chen, J. Zhang, Y. Huang, T. Chen, Janus polymer/carbon nanotube hybrid membranes for oil/water separation, ACS Appl. Mater. Interfaces 6 (2014) 16204–16209. [6] C. Kingston, R. Zepp, A. Andrady, D. Boverhof, R. Fehir, D. Hawkins, J. Roberts, P. Sayre, B. Shelton, Y. Sultan, V. Vejins, W. Wohlleben, Release characteristics of selected carbon nanotube polymer composites, Carbon 68 (2014) 33–57. [7] Y. Liu, S. Kumar, Polymer/carbon nanotube nano composite fibers—a review, ACS Appl. Mater. Interfaces 6 (2014) 6069–6087. [8] X. Cao, X. Wei, G. Li, C. Hu, K. Dai, J. Guo, G. Zheng, C. Liu, C. Shen, Z. Guo, Strain sensing behaviors of epoxy nanocomposites with carbon nanotubes under cyclic deformation, Polymer 112 (2017) 1–9. [9] J. Abraham, P.M. Arif, P. Xavier, S. Bose, S.C. George, N. Kalarikkal, S. Thomas, Investigation into dielectric behaviour and electromagnetic interference shielding effectiveness of conducting styrene butadiene rubber composites containing ionic liquid modified MWCNT, Polymer 112 (2017) 102–115. [10] J. Abraham, P.M. Arif, L. Kailas, N. Kalarikkal, S.C. George, S. Thomas, Developing highly conducting and mechanically durable styrene butadiene rubber composites with tailored microstructural properties by a green approach using ionic liquid modified MWCNTs, RSC Adv. 6 (2016) 32493–32504. [11] M. Sharma, S. Sharma, J. Abraham, S. Thomas, G. Madras, S. Bose, Flexible EMI shielding materials derived by melt blending PVDF and ionic liquid modified MWNTs, Mater. Res. Express 1 (2014) 035003. [12] J. Sebastian, N. Schehl, M. Bouchard, M. Boehle, L. Li, A. Lagounov, K. Lafdi, Health monitoring of structural composites with embedded carbon nanotube coated glass fiber sensors, Carbon 66 (2014) 191–200. [13] L. Vertuccio, V. Vittoria, L. Guadagno, F. De Santis, Strain and damage monitoring in carbon-nanotube-based composite under cyclic strain, Compos. Part A 71 (2015) 9–16. [14] S. Mallakpour, E. Khadem, Carbon nanotube-metal oxide nanocomposites: fabrication, properties and applications, Chem. Eng. J. 302 (2016) 344–367. [15] B. Dalkıran, P.E. Erden, E. Kılıç, Amperometric biosensors based on carboxylated multiwalled carbon nanotubes-metal oxide nanoparticles-7, 7, 8, 8-tetracyanoquinodimethane composite for the determination of xanthine, Talanta 167 (2017) 286–295. [16] C. Mingdong, Y. Huangzhong, J. Xiaohua, L. Yigang, Optimization on microwave absorbing properties of carbon nanotubes and magnetic oxide composite materials,

4. Conclusions In this work, we have synthesized and characterized new compound 160

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