# Hybrid Mn-Porphyrin-Nanogold Nanomaterial Applied for the Spectrophotometric Detection ofβ-Carotene

Authors: Anca Lascu; Mihaela Birdeanu; Bogdan Taranu; Eugenia Fagadar-Cosma
Journal: Journal of Chemistry
 (2018)

---

## Abstract

A hybrid formed between Mn(III) tetratolyl-porphyrin chloride (MnTTPCl) and spherical gold colloid (n-Au), MnTTPCl/n-Au, was tested along with its component nanomaterials as promising candidates in the detection of β-carotene from ethanol solutions. Among the investigated nanomaterials, the largest β-carotene concentration interval detectable by UV-Vis spectrophotometry (9.80 × 10−6 M–1.15 × 10−4 M) was obtained when using the MnTTPCl/n-Au hybrid. This hybrid material gives rise to the widest absorption band, covering the range of 425 nm to 581 nm after treatment with β-carotene.


---

## Body

## 1. Introduction

The presumed antioxidant properties ofβ-carotene are well promoted [1], but previous studies show that the beneficial effect of supplementary β-carotene intake is not convincingly proven either for porphyria [2] or for diabetes prevention [3]. The average intake dosage of β-carotene as food additive is about 1-2 mg·person−1·day−1 [4].The protective effect ofβ-carotene against DNA mutations in the case of prolonged UVA (the longest of the three nonvisible electromagnetic wavelengths that come from the Sun, at 320–400 nanometers) exposure was documented [5]. Studies regarding the risk of age-related macular degeneration suggest that carotenoids [6, 7] have a beneficial preventing effect, but the authors agree that the results are controversial. Patients with rheumatoid arthritis present decreased levels of β-carotene, antioxidant vitamins, and enzymes in plasma and are subjected to oxidative stress [8].In order to minimize oxidative stress in patients with cystic fibrosis, a supplement of 1 mgbeta-carotene kg·BW−1·day−1 (maximally 50 mg β-carotene day−1) leads to the improvement of the quality of life [9].This vitamin A precursor is not synthesized by animal tissue and must be provided from diet or supplements. Nevertheless, the right amount of intake is not yet determined, an excess might lead to carotenosis, when the skin turns orange by the deposition of carotenoids in the outer layer of epidermis [10]. In some cases, such as heavy smokers, it was established that higher concentrations of β-carotene in the system can act as prooxidants, especially in the oxygenated tissues like the lungs, and can favor the incidence of cancer [11]. High doses of β-carotene can also determine the proliferation of prostate cancer cells [12]. Meanwhile, it was concluded that excessive amount of vitamin A in pregnant females is linked to birth defects like heart deformities and eye and lung diseases of the fetus [13].Numerous molecules present in natural or dietary products (carotenoids, polyphenol oligomers, and epicatechin) can act as scavengers and neutralizers of peroxynitrite, although theirin vivo peroxynitrite neutralizing activity is low [14].Taking into account this controversial benefic effect, the demand to accurately detectβ-carotene became compulsory.The water-insoluble carotenoids are transported by low-density lipoproteins through blood and can be detected in plasma or serum using high-pressure liquid chromatography (HPLC) [15]. Human blood plasma carotenoids were also comparatively detected after extraction by using HPLC [16], fluorimetric detection, UV detection, and electrochemical detectors. It was concluded that the electrochemical method is the most efficient one, using high applied voltage.Another HPLC method for the determination in plasma of vitamin C, vitamin E, andβ-carotene levels was associated with photodiode array detection [17]. In the particular case of β-carotene, measuring at the wavelength of 400 nm, the detection limit was 0.25 mg·mL−1.For the detection ofβ-carotene, from very small blood samples (200 μL) [18], an HPLC method using multiwavelength detection was also successfully applied.In the case of determining the content of carotenes in cooked food, a coupled HPLC method was employed [19] with UV-Vis spectrophotometric detection at the wavelength of 470 nm. Lycopene and β-carotene were detected in bakery products [20] by high-performance liquid chromatography with diode array and atmospheric pressure chemical ionization-mass spectrometry detection (HPLC-DAD-APCI-MS).For simultaneous determination of anthocyanoside andβ-carotene in pharmaceutical preparations [21], a third-derivative function of the ultraviolet spectrophotometry method using the zero-crossing technique was used. The technique could detect as little as 6.25 µg·mL−1 β-carotene without interferences.Raman spectroscopy was used for the detection ofβ-carotene and lycopene concentrations in living human skin, based on the changes of fingerprint carbon-carbon double bond stretch vibrations. The method allows rapid screening of carotenoid compositions in a noninvasive fashion and contributes to risk assessment in cutaneous diseases [22, 23].FT-IR and Raman spectroscopy were used for the rapid determination of bacterial carotenoids in soil [24]. Raman microspectrometry was used to detect β-carotene up to 0.25 mg·kg−1 in mixtures of polyaromatic hydrocarbons and usnic acid for the purpose of investigating Martian soil [25] with light equipment.Manganese porphyrins appear to be the most suitable for recognition of biologically active compounds (histamine, dopamine, and glucose) [26, 27] due to the high ability to form metal-ligand bonds and to alter the metal-centered redox potential, along with the distortion of planar structure.Manganese(III) complex of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin and gold nanoparticles deposited on fluorine tin oxide- (FTO-) coated glass electrodes were successfully used in the amperometric sensing of cysteine [28].The rich self-assembling possibilities of hybrid plasmonic couples due to structural plasticity of porphyrin molecules lead to achieving the required optical properties to be used in plasmonic sensing of vitamins and pharmaceutical compounds [29].Based on our previous experience concerning the sensing capacity of hybrid nanomaterials formed between porphyrins and nanosized noble metals [30–32], the present work focuses on obtaining and investigating the hybrid (MnTTPCl/n-Au) UV-Vis response toward β-carotene in ethanol solutions (Figure 1). A comparative study of the optical detecting capacity of this hybrid’s components including chloro[5,10,15,20-tetrakis-(4-methylphenyl)porphyrinato manganese(III)] (MnTTPCl) and bare gold nanoparticles (n-Au) was also performed.Figure 1
Structure of chloro[5,10,15,20-tetrakis-(4-methylphenyl)porphyrinato manganese(III)] (MnTTPCl) (a) and β-carotene (b).
(a)
(b)


## 2. Materials and Methods

Chloro[5,10,15,20-tetrakis-(4-methylphenyl)porphyrinato manganese(III)] (MnTTPCl) was prepared from the corresponding porphyrin base, which was previously reported and fully characterized [33], by using classical metallation of the porphyrin-free ligand (in ethanol at porphyrin : manganese ratios of 1 : 20–1 : 30) [34]. The FT-IR, Raman, UV-Vis, and fluorescence spectroscopic data and AFM, SEM, TEM, and electrochemical characterizations of MnTTPCl were reported in already published papers [35] together with some specific characterizations for sensing applications of diclofenac, H2O2, and NO2 gas [32, 36]; β-carotene was purchased from Merck, and the gold colloid was synthesized according to recent literature data [37] and already characterized by UV-Vis, AFM, TEM, and STEM [31, 38]. The solvents used were purchased from Merck (THF) and Chemopar (ethanol).UV-visible spectra were registered on a JASCO model V-650 spectrometer in 1 cm quartz cuvettes. A Nanosurf®EasyScan 2 Advanced Research AFM (Switzerland) microscope was used for registering atomic force microscopy (AFM). The samples were deposited from solvent mixtures (THF/water/ethanol in different ratios) onto pure silica plates, and the surface imaging was performed at room temperature. AFM images were obtained in the noncontact mode.A Titan G2 80-200 TEM/STEM microscope (FEI Company, Netherlands) was used to record STEM and TEM images at 200 kV for samples prepared on 200 mesh carbon-coated copper grids. The TEM and STEM images were obtained using Digital Micrograph v. 2.12 and TEM Imaging & Analysis V. 4.7 software.


### 2.1. Experimental

#### 2.1.1. Formation of theMnTTPCl/n-Au Hybrid

A hybrid betweenMnTTPCl and gold colloid n-Au (MnTTPCl/n-Au) was prepared as previously published [32]: a solution of 0.5 mL MnTTPCl in THF (c = 1.1 × 10−6 M) (Figure 2) is added under stirring to 6 mL solution of gold colloid in water (c = 4.58 × 10−4 M).Figure 2
The superposed UV-Vis spectra of the nanomaterials (a). Detailed UV-Vis spectrum of theβ-carotene solution (c = 5 × 10−4 M) in ethanol (b).This ratio between components was chosen due to the fact that it gives rise to the broadest and most intense plasmonic band.The superposed UV-Vis spectra as presented in Figure2 showed the differences between the MnTTPCl spectrum, the plasmonic spectrum of gold colloid solution in water, and the wide plasmonic band of the MnTTPCl/n-Au hybrid. The detail represents the UV-Vis spectrum of the β-carotene solution in ethanol. The major peak of Mn-porphyrin is located at 477 nm, the highest intensity of the gold plasmon is positioned at 525 nm, and the plasmonic band of the MnTTPCl/n-Au hybrid is strongly bathochromically shifted to 590 nm, widening in the wavelength range of 480–750 nm and manifesting a hyperchromic effect. These three features of the hybrid nanomaterial recommend it for optoelectronic applications [39].


#### 2.1.2. Theβ-Carotene Detection

To 5 mL of each investigated solution of fixed concentration (MnTTPCl/n-Au or MnTTPCl or n-Au), 0.100 mL β-carotene solution in ethanol (c × 10−6 M; c = 1.99; 3.96; 5.92; 7.87; 9.80; 19.23; 28.30; 37.03; 45.45; 53.57; 61.40; 68.96; 76.27; 83.33; 90.16; 96.77; 103.31; 109.37; 115.38) is successively added. After 30 seconds of vigorous stirring at room temperature, the UV-Vis spectra are recorded for each step.


## 2.1. Experimental

### 2.1.1. Formation of theMnTTPCl/n-Au Hybrid

A hybrid betweenMnTTPCl and gold colloid n-Au (MnTTPCl/n-Au) was prepared as previously published [32]: a solution of 0.5 mL MnTTPCl in THF (c = 1.1 × 10−6 M) (Figure 2) is added under stirring to 6 mL solution of gold colloid in water (c = 4.58 × 10−4 M).Figure 2
The superposed UV-Vis spectra of the nanomaterials (a). Detailed UV-Vis spectrum of theβ-carotene solution (c = 5 × 10−4 M) in ethanol (b).This ratio between components was chosen due to the fact that it gives rise to the broadest and most intense plasmonic band.The superposed UV-Vis spectra as presented in Figure2 showed the differences between the MnTTPCl spectrum, the plasmonic spectrum of gold colloid solution in water, and the wide plasmonic band of the MnTTPCl/n-Au hybrid. The detail represents the UV-Vis spectrum of the β-carotene solution in ethanol. The major peak of Mn-porphyrin is located at 477 nm, the highest intensity of the gold plasmon is positioned at 525 nm, and the plasmonic band of the MnTTPCl/n-Au hybrid is strongly bathochromically shifted to 590 nm, widening in the wavelength range of 480–750 nm and manifesting a hyperchromic effect. These three features of the hybrid nanomaterial recommend it for optoelectronic applications [39].


### 2.1.2. Theβ-Carotene Detection

To 5 mL of each investigated solution of fixed concentration (MnTTPCl/n-Au or MnTTPCl or n-Au), 0.100 mL β-carotene solution in ethanol (c × 10−6 M; c = 1.99; 3.96; 5.92; 7.87; 9.80; 19.23; 28.30; 37.03; 45.45; 53.57; 61.40; 68.96; 76.27; 83.33; 90.16; 96.77; 103.31; 109.37; 115.38) is successively added. After 30 seconds of vigorous stirring at room temperature, the UV-Vis spectra are recorded for each step.


## 2.1.1. Formation of theMnTTPCl/n-Au Hybrid

A hybrid betweenMnTTPCl and gold colloid n-Au (MnTTPCl/n-Au) was prepared as previously published [32]: a solution of 0.5 mL MnTTPCl in THF (c = 1.1 × 10−6 M) (Figure 2) is added under stirring to 6 mL solution of gold colloid in water (c = 4.58 × 10−4 M).Figure 2
The superposed UV-Vis spectra of the nanomaterials (a). Detailed UV-Vis spectrum of theβ-carotene solution (c = 5 × 10−4 M) in ethanol (b).This ratio between components was chosen due to the fact that it gives rise to the broadest and most intense plasmonic band.The superposed UV-Vis spectra as presented in Figure2 showed the differences between the MnTTPCl spectrum, the plasmonic spectrum of gold colloid solution in water, and the wide plasmonic band of the MnTTPCl/n-Au hybrid. The detail represents the UV-Vis spectrum of the β-carotene solution in ethanol. The major peak of Mn-porphyrin is located at 477 nm, the highest intensity of the gold plasmon is positioned at 525 nm, and the plasmonic band of the MnTTPCl/n-Au hybrid is strongly bathochromically shifted to 590 nm, widening in the wavelength range of 480–750 nm and manifesting a hyperchromic effect. These three features of the hybrid nanomaterial recommend it for optoelectronic applications [39].


## 2.1.2. Theβ-Carotene Detection

To 5 mL of each investigated solution of fixed concentration (MnTTPCl/n-Au or MnTTPCl or n-Au), 0.100 mL β-carotene solution in ethanol (c × 10−6 M; c = 1.99; 3.96; 5.92; 7.87; 9.80; 19.23; 28.30; 37.03; 45.45; 53.57; 61.40; 68.96; 76.27; 83.33; 90.16; 96.77; 103.31; 109.37; 115.38) is successively added. After 30 seconds of vigorous stirring at room temperature, the UV-Vis spectra are recorded for each step.


## 3. Results and Discussions

### 3.1. UV-Vis Analysis regarding the Influence ofβ-Carotene on the Sensitive Materials: MnTTPCl/n-Au or MnTTPCl or n-Au

#### 3.1.1.MnTTPCl/n-Au Hybrid Solution Treated with β-Carotene

Analyzing the overlapped spectra (Figure3), it can be concluded that the increase in β-carotene concentration that is added to the MnTTPCl/n-Au hybrid solution leads to the decrease in intensity of the plasmonic band. One clear isosbestic point that is observed at 685 nm (Figure 3(b)) proves that the phenomenon is due to some equilibrium processes leading to the formation of chemical intermediates between β-carotene and the MnTTPCl/n-Au hybrid. The dependence between the absorption intensity of the plasmon and the β-carotene concentration is linear (Figure 4) with an excellent correlation coefficient of 99.4%. The β-carotene concentration domain for which the MnTTPCl/n-Au hybrid is able to detect the antioxidant molecule is quite large, ranging two orders of magnitude. The lowest concentration of detected β-carotene is 9.804 × 10−6 M, and the highest is 1.1538 × 10−4 M.Figure 3
Overlapped UV-Vis spectra of the successive adding ofβ-carotene solution to the MnTTPCl/n-Au hybrid (a). Details of the isosbestic point at 685 nm (b).Figure 4
The dependence between the intensity of absorption of the hybrid plasmon and theβ-carotene concentration.


#### 3.1.2.Nano-Au Treated with β-Carotene

As can be seen in Figures5(a) and 5(b), the plasmon intensity decreases with the increase in β-carotene concentration and discretely shifts to lower wavelengths (523 nm for the β-carotene concentration of 9.804 × 10−6 M and 518 nm for the concentration of β-carotene of 6.1403 × 10−5 M). Two isosbestic points are detectable, at 515 nm (Figure 5(b)) and 556 nm, respectively, indicating the formation of a certain complex between β-carotene and gold nanoparticles. As a conclusion, the gold nanoparticles investigated in this shape and size cannot detect β-carotene in this range of concentrations with accurate sensitivity.Figure 5
Overlapped UV-Vis spectra for addingβ-carotene solution to gold colloid solution (a). Details of the isosbestic point at 515 nm (b).


#### 3.1.3.MnTTPCl Treated with β-Carotene

It can be observed that the increase inβ-carotene concentration leads to the increase in the intensity of the absorption of the Soret band (Figure 6). Nevertheless, after a certain concentration in β-carotene (2.2 × 10−5 M), the solution registers a twist and the intensity of absorption remains constant or even decreases beyond. An isosbestic point is detectable at 645 nm on the last Q band of the porphyrin (Figure 6(b)).Figure 6
Overlapped UV-Vis spectra for the detection ofβ-carotene solution in ethanol by MnTTPCl solution in THF (a). Details of the isosbestic point at 645 nm (b).The maximumβ-carotene concentration detectable in a linear fashion by the MnTTPCl solution is 1.92 × 10−5 M (Figure 7). The detected β-carotene concentration is in the range of 6.58 × 10−6 M–1.92 × 10−5 M. This domain is narrower than the one detected by the MnTTPCl/n-Au hybrid solution.Figure 7
Dependence between the Soret band intensity of theMnTTPCl solution and β-carotene concentration.


### 3.2. FT-IR Analysis

In the FT-IR spectrum ofβ-carotene (Figure 8, line 1), the alkenyl C=C streching bonds can be identified at 1620–1680 cm−1 [24], and the peaks above 3000 cm−1 are indicative of unsaturated chains. In the case of MnTTPCl treated with β-carotene (Figure 8, line 2), the FT-IR spectrum contains the corresponding bands for C-H aliphatic bonds at 2861 cm−1 and 2967 cm−1, as well as the absorption band at 906 cm−1 characteristic for C-H vinyl-substituted bonds. In the spectrum of the gold colloid treated with β-carotene (Figure 8, line 3), the weak absorption band at 1043 cm−1 could be attributed to the CH3 bond rocking from β-carotene. In the spectra of the materials treated with β-carotene (Figure 8, lines 2, 3, and 5), there are common features such as the enlargement of the O-H bands from 3300 to 3400 cm−1 (Figure 8, lines 3, 5) as well as the characteristic band around 1060 cm−1 that can be attributed to manganese-porphyrin δ C-H bonds [35] from pyrrole and from phenyl (Figure 8, curves 2 and 5).Figure 8
Overlapped IR spectra:β-carotene (line 1); MnTTPCl treated with β-carotene (line 2); n-Au treated with β-carotene (line 3); MnTTPCl (line 4); MnTTPCl/n-Au treated with β-carotene (line 5).


### 3.3. AFM Analysis

The AFM images of theMnTTPCl/n-Au hybrid show a significant change of the aggregation process after being exposed to β-carotene. In the case of the bare hybrid (Figures 9(a) and 9(b)), triangule-shaped structures having average dimensions of 278.2 nm are further aggregated by both J-type and H-type processes to form ordered oriented rows. Finally, an organized multilayer composed of triangular bricks is formed.Figure 9
AFM images for theMnTTPCl/n-Au hybrid before (a, b) and after treatment with β-carotene (c, d).In the case of theMnTTPCl/n-Au hybrid exposed to β-carotene (Figures 9(c) and 9(d)), the dimension of the aggregates slightly increased to 318.2 nm (Figure 9(c)) and the number of assembled molecules diminished. It can be concluded that both J- and H-type aggregation processes are scarcer than those in the case of the bare nanomaterial. In the map in shadows for 4 nm (Figure 9(d)), it can be observed that the aggregates of the hybrid treated with β-carotene form pyramid-like or conical superstructures with an average height distribution of 21 nm–50 nm. Besides, some kvataron-shaped structures [40] are visible (Figure 9(c)).Anisometric nanoparticles of gold (Figures10(a) and 10(b)) have generated significant interest since they can display various properties as a function of orientation [41]. Under the influence of β-carotene, the gold nanoparticles (n-Au) (Figures 10(c) and 10(d)) aggregate into triangles varying in dimensions between 192.4 nm and 268.6 nm. These triangles further agglomerate into linked structures having an average height distribution of 12.9 nm–22 nm.Figure 10
AFM images forn-Au before (a, b) and after treatment with β-carotene (c, d).Under the influence ofβ-carotene, the MnTTPCl generates inhomogeneous layers (Figures 11(c) and 11(d)), manifesting the same behavior as the MnTTPCl/n-Au hybrid and n-Au.Figure 11
AFM images forMnTTPCl before (a, b) and after exposure to β-carotene (c, d).


### 3.4. STEM Images

STEM images recorded at small magnifications for the complexMnTTPCl/n-Au exposed to β-carotene display large ovoidal structures with lacey edges (Figure 12). The same organization was previously reported for gold nanomaterials [42].Figure 12
STEM images forMnTTPCl/n-Au hybrid treated with β-carotene.
(a)
(b)In comparison, STEM images forMnTTPCl after exposure to β-carotene (Figure 13) show smaller but well-shaped stellar-type structures that could further organize into more complex chain-like aggregates.Figure 13
STEM images recorded forMnTTPCl treated with β-carotene.
(a)
(b)


### 3.5. TEM Images

HR-TEM images obtained forMnTTPCl/n-Au hybrid treated with β-carotene (Figure 14(a)) as well as for MnTTPCl treated with β-carotene (Figure 14(c)) evidenced areas with specific crystalline organization. This phenomenon might be attributed to a deep change in structure and morphology of the metalloporphyrin due to axial ligation of the analyte. Measurements performed on the FTT corresponding to one such area indicated a spacing between crystal planes of 2.2 Å (Figure 14(c)) and two families of crystal planes that intersect at angles less than 90 degrees.Figure 14
HR-TEM images recorded for theMnTTPCl/n-Au hybrid treated with β-carotene (a), the n-Au treated with β-carotene (b), and MnTTPCl treated with β-carotene (c).HR-TEM images recorded for the gold colloid treated withβ-carotene revealed spherical and quasispherical gold nanoparticles embedded in an amorphous layer, probably consisting of β-carotene (Figure 14(b)).


## 3.1. UV-Vis Analysis regarding the Influence ofβ-Carotene on the Sensitive Materials: MnTTPCl/n-Au or MnTTPCl or n-Au

### 3.1.1.MnTTPCl/n-Au Hybrid Solution Treated with β-Carotene

Analyzing the overlapped spectra (Figure3), it can be concluded that the increase in β-carotene concentration that is added to the MnTTPCl/n-Au hybrid solution leads to the decrease in intensity of the plasmonic band. One clear isosbestic point that is observed at 685 nm (Figure 3(b)) proves that the phenomenon is due to some equilibrium processes leading to the formation of chemical intermediates between β-carotene and the MnTTPCl/n-Au hybrid. The dependence between the absorption intensity of the plasmon and the β-carotene concentration is linear (Figure 4) with an excellent correlation coefficient of 99.4%. The β-carotene concentration domain for which the MnTTPCl/n-Au hybrid is able to detect the antioxidant molecule is quite large, ranging two orders of magnitude. The lowest concentration of detected β-carotene is 9.804 × 10−6 M, and the highest is 1.1538 × 10−4 M.Figure 3
Overlapped UV-Vis spectra of the successive adding ofβ-carotene solution to the MnTTPCl/n-Au hybrid (a). Details of the isosbestic point at 685 nm (b).Figure 4
The dependence between the intensity of absorption of the hybrid plasmon and theβ-carotene concentration.


### 3.1.2.Nano-Au Treated with β-Carotene

As can be seen in Figures5(a) and 5(b), the plasmon intensity decreases with the increase in β-carotene concentration and discretely shifts to lower wavelengths (523 nm for the β-carotene concentration of 9.804 × 10−6 M and 518 nm for the concentration of β-carotene of 6.1403 × 10−5 M). Two isosbestic points are detectable, at 515 nm (Figure 5(b)) and 556 nm, respectively, indicating the formation of a certain complex between β-carotene and gold nanoparticles. As a conclusion, the gold nanoparticles investigated in this shape and size cannot detect β-carotene in this range of concentrations with accurate sensitivity.Figure 5
Overlapped UV-Vis spectra for addingβ-carotene solution to gold colloid solution (a). Details of the isosbestic point at 515 nm (b).


### 3.1.3.MnTTPCl Treated with β-Carotene

It can be observed that the increase inβ-carotene concentration leads to the increase in the intensity of the absorption of the Soret band (Figure 6). Nevertheless, after a certain concentration in β-carotene (2.2 × 10−5 M), the solution registers a twist and the intensity of absorption remains constant or even decreases beyond. An isosbestic point is detectable at 645 nm on the last Q band of the porphyrin (Figure 6(b)).Figure 6
Overlapped UV-Vis spectra for the detection ofβ-carotene solution in ethanol by MnTTPCl solution in THF (a). Details of the isosbestic point at 645 nm (b).The maximumβ-carotene concentration detectable in a linear fashion by the MnTTPCl solution is 1.92 × 10−5 M (Figure 7). The detected β-carotene concentration is in the range of 6.58 × 10−6 M–1.92 × 10−5 M. This domain is narrower than the one detected by the MnTTPCl/n-Au hybrid solution.Figure 7
Dependence between the Soret band intensity of theMnTTPCl solution and β-carotene concentration.


## 3.1.1.MnTTPCl/n-Au Hybrid Solution Treated with β-Carotene

Analyzing the overlapped spectra (Figure3), it can be concluded that the increase in β-carotene concentration that is added to the MnTTPCl/n-Au hybrid solution leads to the decrease in intensity of the plasmonic band. One clear isosbestic point that is observed at 685 nm (Figure 3(b)) proves that the phenomenon is due to some equilibrium processes leading to the formation of chemical intermediates between β-carotene and the MnTTPCl/n-Au hybrid. The dependence between the absorption intensity of the plasmon and the β-carotene concentration is linear (Figure 4) with an excellent correlation coefficient of 99.4%. The β-carotene concentration domain for which the MnTTPCl/n-Au hybrid is able to detect the antioxidant molecule is quite large, ranging two orders of magnitude. The lowest concentration of detected β-carotene is 9.804 × 10−6 M, and the highest is 1.1538 × 10−4 M.Figure 3
Overlapped UV-Vis spectra of the successive adding ofβ-carotene solution to the MnTTPCl/n-Au hybrid (a). Details of the isosbestic point at 685 nm (b).Figure 4
The dependence between the intensity of absorption of the hybrid plasmon and theβ-carotene concentration.


## 3.1.2.Nano-Au Treated with β-Carotene

As can be seen in Figures5(a) and 5(b), the plasmon intensity decreases with the increase in β-carotene concentration and discretely shifts to lower wavelengths (523 nm for the β-carotene concentration of 9.804 × 10−6 M and 518 nm for the concentration of β-carotene of 6.1403 × 10−5 M). Two isosbestic points are detectable, at 515 nm (Figure 5(b)) and 556 nm, respectively, indicating the formation of a certain complex between β-carotene and gold nanoparticles. As a conclusion, the gold nanoparticles investigated in this shape and size cannot detect β-carotene in this range of concentrations with accurate sensitivity.Figure 5
Overlapped UV-Vis spectra for addingβ-carotene solution to gold colloid solution (a). Details of the isosbestic point at 515 nm (b).


## 3.1.3.MnTTPCl Treated with β-Carotene

It can be observed that the increase inβ-carotene concentration leads to the increase in the intensity of the absorption of the Soret band (Figure 6). Nevertheless, after a certain concentration in β-carotene (2.2 × 10−5 M), the solution registers a twist and the intensity of absorption remains constant or even decreases beyond. An isosbestic point is detectable at 645 nm on the last Q band of the porphyrin (Figure 6(b)).Figure 6
Overlapped UV-Vis spectra for the detection ofβ-carotene solution in ethanol by MnTTPCl solution in THF (a). Details of the isosbestic point at 645 nm (b).The maximumβ-carotene concentration detectable in a linear fashion by the MnTTPCl solution is 1.92 × 10−5 M (Figure 7). The detected β-carotene concentration is in the range of 6.58 × 10−6 M–1.92 × 10−5 M. This domain is narrower than the one detected by the MnTTPCl/n-Au hybrid solution.Figure 7
Dependence between the Soret band intensity of theMnTTPCl solution and β-carotene concentration.


## 3.2. FT-IR Analysis

In the FT-IR spectrum ofβ-carotene (Figure 8, line 1), the alkenyl C=C streching bonds can be identified at 1620–1680 cm−1 [24], and the peaks above 3000 cm−1 are indicative of unsaturated chains. In the case of MnTTPCl treated with β-carotene (Figure 8, line 2), the FT-IR spectrum contains the corresponding bands for C-H aliphatic bonds at 2861 cm−1 and 2967 cm−1, as well as the absorption band at 906 cm−1 characteristic for C-H vinyl-substituted bonds. In the spectrum of the gold colloid treated with β-carotene (Figure 8, line 3), the weak absorption band at 1043 cm−1 could be attributed to the CH3 bond rocking from β-carotene. In the spectra of the materials treated with β-carotene (Figure 8, lines 2, 3, and 5), there are common features such as the enlargement of the O-H bands from 3300 to 3400 cm−1 (Figure 8, lines 3, 5) as well as the characteristic band around 1060 cm−1 that can be attributed to manganese-porphyrin δ C-H bonds [35] from pyrrole and from phenyl (Figure 8, curves 2 and 5).Figure 8
Overlapped IR spectra:β-carotene (line 1); MnTTPCl treated with β-carotene (line 2); n-Au treated with β-carotene (line 3); MnTTPCl (line 4); MnTTPCl/n-Au treated with β-carotene (line 5).


## 3.3. AFM Analysis

The AFM images of theMnTTPCl/n-Au hybrid show a significant change of the aggregation process after being exposed to β-carotene. In the case of the bare hybrid (Figures 9(a) and 9(b)), triangule-shaped structures having average dimensions of 278.2 nm are further aggregated by both J-type and H-type processes to form ordered oriented rows. Finally, an organized multilayer composed of triangular bricks is formed.Figure 9
AFM images for theMnTTPCl/n-Au hybrid before (a, b) and after treatment with β-carotene (c, d).In the case of theMnTTPCl/n-Au hybrid exposed to β-carotene (Figures 9(c) and 9(d)), the dimension of the aggregates slightly increased to 318.2 nm (Figure 9(c)) and the number of assembled molecules diminished. It can be concluded that both J- and H-type aggregation processes are scarcer than those in the case of the bare nanomaterial. In the map in shadows for 4 nm (Figure 9(d)), it can be observed that the aggregates of the hybrid treated with β-carotene form pyramid-like or conical superstructures with an average height distribution of 21 nm–50 nm. Besides, some kvataron-shaped structures [40] are visible (Figure 9(c)).Anisometric nanoparticles of gold (Figures10(a) and 10(b)) have generated significant interest since they can display various properties as a function of orientation [41]. Under the influence of β-carotene, the gold nanoparticles (n-Au) (Figures 10(c) and 10(d)) aggregate into triangles varying in dimensions between 192.4 nm and 268.6 nm. These triangles further agglomerate into linked structures having an average height distribution of 12.9 nm–22 nm.Figure 10
AFM images forn-Au before (a, b) and after treatment with β-carotene (c, d).Under the influence ofβ-carotene, the MnTTPCl generates inhomogeneous layers (Figures 11(c) and 11(d)), manifesting the same behavior as the MnTTPCl/n-Au hybrid and n-Au.Figure 11
AFM images forMnTTPCl before (a, b) and after exposure to β-carotene (c, d).


## 3.4. STEM Images

STEM images recorded at small magnifications for the complexMnTTPCl/n-Au exposed to β-carotene display large ovoidal structures with lacey edges (Figure 12). The same organization was previously reported for gold nanomaterials [42].Figure 12
STEM images forMnTTPCl/n-Au hybrid treated with β-carotene.
(a)
(b)In comparison, STEM images forMnTTPCl after exposure to β-carotene (Figure 13) show smaller but well-shaped stellar-type structures that could further organize into more complex chain-like aggregates.Figure 13
STEM images recorded forMnTTPCl treated with β-carotene.
(a)
(b)


## 3.5. TEM Images

HR-TEM images obtained forMnTTPCl/n-Au hybrid treated with β-carotene (Figure 14(a)) as well as for MnTTPCl treated with β-carotene (Figure 14(c)) evidenced areas with specific crystalline organization. This phenomenon might be attributed to a deep change in structure and morphology of the metalloporphyrin due to axial ligation of the analyte. Measurements performed on the FTT corresponding to one such area indicated a spacing between crystal planes of 2.2 Å (Figure 14(c)) and two families of crystal planes that intersect at angles less than 90 degrees.Figure 14
HR-TEM images recorded for theMnTTPCl/n-Au hybrid treated with β-carotene (a), the n-Au treated with β-carotene (b), and MnTTPCl treated with β-carotene (c).HR-TEM images recorded for the gold colloid treated withβ-carotene revealed spherical and quasispherical gold nanoparticles embedded in an amorphous layer, probably consisting of β-carotene (Figure 14(b)).


## 4. Conclusions

Vitamin A precursor (β-carotene) has to be provided from diet or supplements, and the right amount of intake should be monitored. Encouraged by the multitude of analytical applications of manganese porphyrins for the detection of active biological molecules, we investigated a metallated porphyrin, chloro[5,10,15,20-tetrakis-(4-methylphenyl)porphyrinato manganese(III)], both alone and in complex with gold colloid for the detection of β-carotene.It can be stated that the Mn-porphyrin and the hybrid gold nanomaterial are adequate and sensitive candidates for the optical detection of minute quantities ofβ-carotene, showing linear dependences in UV-Vis spectroscopy between the intensity of absorption and the β-carotene concentration. The best results are obtained using the MnTTPCl/n-Au hybrid. This material gives rise to the widest absorption band and is able to detect the largest β-carotene concentration interval: 9.80 × 10−6–1.15 × 10−4 M. Electron transmission microscopy (HR-TEM) images obtained after treatment with β-carotene for both the MnTTPCl and MnTTPCl/n-Au hybrid evidenced areas with specific crystalline organization, proving that a deep change in structure due to axial ligation of the analyte is taking place.



---

*Source: 5323561-2018-05-30.xml*