2. Methods to Detect and Quantify Vitamin E within Materials
Vitamin E can be easily detected in different liquid media, such as in oils, serum, human milk, foods, etc., by different methods including HPLC, FTIR, RAMAN, UV-VIS, and spectrophotometric methods. Recently, some protocols were also developed for the analysis of vitamin E incorporated into cosmetics and food packaging and contained in food
[29][30][31][32][33][34].
The first method usually used to allow a simple and rapid quantitative determination of α-tocopherol is High-Performance Liquid Chromatography (HPLC). In fact, HPLC is one of the most powerful tools for the determination of fat-soluble vitamins and has been widely used for their separation and detection; different detectors can be used for vitamins such as UV-VIS, fluorescence, and mass spectrometry. In the case of vitamin E, typically the HPLC column is connected to an UV absorbance detector as the compound absorbs the ultraviolet light, particularly around 290 nm. This method is, in fact, used not only to analyse quantitatively the content of alpha-tocopherol in food or beverages, but also in cosmetics and in biological samples including human plasma and human milk
[31][34][35][36][37]. Another way to detect and quantify α-tocopherol is the Fourier Transform Infrared Spectroscopy (FTIR). Sandra et al. developed a quick procedure for the quantitative analysis of α-tocopherol in vegetable oils as an alternative to HPLC methods, using FTIR-ATR methodology. By analysing 13 vegetable oils, with a known content of vitamin E, a research team created a calibration curve which was then used to measure the alpha-tocopherol content of the vegetable oil concerned quantitatively
[38].
For qualitative measurements, FTIR was also used for detecting the functional groups of a hydrophobic film of vitamin E deposited on a copper substrate
[39]. Thanks to its clear absorbance peak at 290 nm, visible ultraviolet spectroscopy (UV-Vis) proved to be able to detect the presence of vitamin E even at low concentrations
[40][41].
Along with FTIR, RAMAN is a potential alternative method to have qualitative detection of the molecule. It is used to detect vitamin E in oil water emulsions and in biological samples
[42][43]. Surface-enhanced Raman spectroscopy (SERS) technology is of a high level of interest: it exploits the amplification of Raman diffusion by molecules adsorbed on a metal or on metallic nanoparticles
[44][45]. Typically, most SERS techniques use metal aqueous colloids as a substrate, which require that compounds to be analysed must be water soluble. For water insoluble analytes, such as vitamin E, the matter is more complicated. Given the disadvantage of using colloidal Ag nanoparticles to measure SERS of the analyte directly, Tiantian Cai et al. have successfully tried other methods to analyse vitamin E: after dissolving the compound in chloroform, the solution obtained is dripped onto the surface of a metal substrate with surface Raman activity. Another method could be to immerse the metal substrate in the sample solution containing vitamin E directly, to extract it after a certain time, and to measure it at RAMAN after the solvent has evaporated
[46].
Thanks to its antioxidant properties, vitamin E can also be analysed and quantified through all those methods that exploit chemical reactions, typically redox, to develop coloured compounds that are then measured spectrophotometrically. In general, spectrophotometric methods for vitamin E determination use oxidation of the aromatic ring of α-tocopherol, creating tocopherylquinone, by oxidizing agents that ultimately yield products with spectrophotometric staining. Among these methods, there is definitely the DPPH method, which uses a free radical of purple colour, which discolours when it reacts with vitamin E. Valeria M. et al. have used the DPPH method to compare the antioxidant power of drugs containing alpha-tocopherol. The problem of the DPPH method is its low reproducibility due to the low stability of the radical
[47].
Another such methodology is the Folin–Ciocâlteu (FC) reagent in an aqueous solution. In this case, however, given the insolubility of vitamin E in water, this methodology is not the optimal one. However, modifications have been made to the FC method to enable the measurement of lipophilic and hydrophilic antioxidants concentrations simultaneously
[48].
Albeit less used and dated, there are many other methods using different oxidizing reagents such as Fe(III)-bathophenanthroline, Cu(II)-neocuproine, or silver nitrate, but they require a rigid control of the conditions for precise results
[49]. Another method is the Emmerie and Engel colour reaction with ferric chloride: it is a precise and easy-to-perform reaction, and therefore the approach of choice for a routine clinical laboratory
[50]. Based on this work, more recently, Jameel G. Jargar et al. took advantage of different reagents such as 2,2′-bipyridyl, ferric chloride, and xylene to perform the colour reaction
[51].
Finally, although a very old and no longer used method, nitric acid combined with ethanol was used to oxidise α-tocopherol, forming the coloured red o-quinone which can be detected spectrophotometrically (
Figure 5)
[52].
Figure 5. Formation of Tocored with Nitric Acid.
Thanks to the vitamin E detection methods employed in various applications involving cosmetics, food packaging, and so on, it is possible to apply the above methods to the biomedical field for the detection of vitamin E when combined with different biomaterials.
Certainly, it is easy to find detection methods in the literature when vitamin E is combined with UHWMPE, as it is the most widely applied biomaterial coupled with vitamin E today.
For quantification methods, with HPLC analysis, it is convenient to quantify the vitamin E content within UHMWPE using a calibration curve produced from the areas of the HPLC peaks
[53]. Instead, Hufen Julia et al. developed an accurate method to detect α-tocopherol content in UHWMPE using HPLC analysis to separate it and determine its concentration by UV-Vis spectroscopy with a corresponding calibration curve
[54]. However, it is also possible to use only UV-VIS in absorbance mode combined with FTIR to quantify vitamin E within UHMWPE
[55]. Vitamin E blended with polyethylene induces yellowing of the sample; Martínez-Morlanes et al. exploited this characteristic using the colorimetric technique and reflectance spectroscopy to detect vitamin E embedded in polyethylene samples quantitatively
[56]. These types of methods, especially HPLC, are also used in drug delivery to calculate the drug encapsulation efficiency, resulting in the quantification of the vitamin E encapsulated within polymeric nanoparticles
[57][58][59]. With the same object, in tissue engineering, HPLC is used to quantify vitamin E content inside the matrices or scaffolds
[60][61].
For qualitative methods, since vitamin E is an extremely hydrophobic molecule, another important way to detect the presence of tocopherol on different substrates is definitely the measurement of the contact angle, the quickest test to evaluate a surface modification
[62]. Filippo Renò et al. used the contact angle measurement on PLA blended with Vitamin E, and they discovered that the blend enriched with vitamin E was more easily wetted
[63][64].
To get a more in-depth understanding of the chemical bonds between the substrate and the deposited molecule, the XPS technique is useful, as in the case of Elena Stoleru et al. who used XPS to have information about the stability of the chitosan/vitamin E coating deposited on a polyethylene substrate
[62].
Once the characteristic peaks of vitamin E are known, the FTIR analysis is helpful, not only to detect vitamin E
[65], but also to analyse the eventual shifts in wavenumber of the peaks that denote an interaction between tocopherol and the combined biomaterials. Ahmad Salawi et al. used the FTIR technique to analyse the interaction between a new copolymer called Soluplus and α-tocopherol for a wound-healing application
[66], and Joana T. Martins et al. studied the physiochemical effect of the incorporation of α-tocopherol in chitosan-based films through a different analysis including FTIR
[67].
In the biomedical field, the DPPH test is used to analyse the radical scavenging ability of vitamin E combined with biomaterials, as Elena Stoleru et al. did on a film electrosprayed with a chitosan/vitamin E formulation
[62]. DPPH was used also by Zhou Nier et al. to test the antioxidant activity of Au nanoparticles functionalized with Trolox (hydrophilic analogue of alpha-tocopherol)
[68][69]. The table (
Table 1) reports the characterization methods used to detect vitamin E when combined with different biomaterials.
Table 1. Method of Vitamin E detection when it is combined with biomaterials.
Technique |
Combined Material |
Molecule Detected |
Method |
Information |
Ref. |
HPLC |
UHWMPE |
α-tocopherol |
HPLC connected to UV/Vis diode array detector at 297 nm, construction of calibration curve of HPLC peak area. |
Quantitative |
[53] |
UHWMPE |
α-tocopherol |
HPLC connected to UV/Vis diode array detector, construction of calibration curve of absorbance peak area at 290 nm |
Quantitative |
[54] |
Collagen mesh |
α-tocopherol |
HPLC connected to a fluorescence detector, detection at excitation wavelength of 290 nm and emission wavelength of 330 nm |
Quantitative |
[61] |
Alginate and hyaluronate film |
α-tocopherol acetate |
HPLC connected to UV/Vis diode array detector, construction of calibration curve of absorbance peak area at 285 nm |
Quantitative |
[60] |
Hyaluronic-acid-based β-cyclodextrin copolymer |
α-tocopherol |
HPLC connected to UV/Vis diode array detector |
Quantitative |
[58] |
PNIPAM-b-PCL-b-PNIPAM copolymer |
α-tocopherol |
HPLC equipped with a differential refraction index detector |
Quantitative |
[59] |
UV-VIS |
UHWMPE |
α-tocopherol |
Construction of calibration curve of absorbance peak area at 290 nm |
Quantitative |
[55] |
UHWMPE |
α-tocopherol |
Analysis of reflectance spectra which presents a minimum around 290 nm and a decrease of reflectance at 400–500 nm. |
Detection |
[56] |
Hyaluronic acid |
α-tocopherol succinate |
Construction of calibration curve of absorbance peak area at 285 nm |
Quantitative |
[70] |
PLA+PCL |
α-tocopherol acetate |
Construction of calibration curve of absorbance peak area at 284 nm |
Quantitative |
[65] |
Colorimetric Assay |
UHWMPE |
α-tocopherol |
The yellowing of the sample was analysed through three parameters (a,b,L) of CIELAB colour space, and a calibration curve of colour distances was constructed. |
Quantitative |
[56] |
FTIR-ATR |
UHWMPE |
α-tocopherol |
Analysis of peaks. For quantitative analysis, calibration curve of these peaks is needed. |
Analysis of Vitamin E transformation products in polymer samples prior to extraction and quantitative. |
[53] |
Collagen |
α-tocopherol |
Analysis of main peaks |
Characterization of film |
[71] |
FTIR-ATR |
Magnetite |
α -tocopheryl succinate |
Analysis of main peaks |
Characterization of chemical modification of nanoparticles |
[72] |
Chitosan |
α-tocopherol |
Analysis of peaks |
Physical bonds and chemical interactions are reflected by changes in characteristic spectral peaks. |
[67] |
Chitosan |
α-tocopherol |
Analysis of peaks |
Characterization of nanoparticles |
[73] |
PCL/PLA |
α-tocopherol acetate |
Analysis of peaks |
Characterization of membranes |
[65] |
Soluplus |
α-tocopherol |
Analysis of peaks |
Analysis of bonding between Soluplus/vitamin E |
[66] |
Polyethylene |
α-tocopherol |
Analysis of peaks from 600–4000 cm −1 |
Analysis of interaction between vitamin E and chitosan |
[62] |
XPS |
Polyethylene |
α-tocopherol |
All binding energies were referenced to the C1s peak at 285 eV. |
Analysis of covalent bonding |
[62] |
DPPH |
Polyethylene |
α-tocopherol |
The scavenging activity was estimated RSA (%) = (1 − (A sample/Acontrol)) × 100, measuring the adsorption at 515 nm after 30 min in dark condition. |
Radical scavenging activity evaluation |
[62] |
Chitosan |
α-tocopherol |
The scavenging activity was estimated RSA (%) = (1 − (A sample/Acontrol)) × 100, measuring the adsorption at 517 nm after 30 min in dark condition. |
Radical scavenging activity evaluation |
[67] |
Collagen/chitosan |
α-tocopherol |
DPPH were measured by the adsorption at 517 nm after 30 min in dark condition. DPPH loss which is a concentration of DPPH radicals reacted with antioxidants. |
Antioxidant activity |
[61] |
Contact Angle |
Polyethylene |
α-tocopherol |
Contact angle titrations were performed by measuring sets of contact angles at each pH value. |
Analysis of hydrophobic behaviour as pH increases |
[51] |
PLA |
α-tocopherol |
Static contact angle |
Analysis of material wettability change |
[63][64] |