INVESTIGATING IRREVERSIBLE SENSING OF OXYGEN INGRESS IN POLYMER FILMS CONTAINING LEUCO FORM OF INDIGO CARMINE

T. V. Sakhno, prof.
Poltava University of Economics and Trade;
N. N. Barashkov, prof.
R&D, Micro-Tracers, Inc, San Francisco, CA, United States;
Irina Irgibayeva,prof.
Anuar Aldongarov,PhD, Artur Mantel,PhD
Department of Chemistry, Eurasian National University, Astana, Kazakhstan


INVESTIGATING IRREVERSIBLE SENSING OF OXYGEN INGRESS IN POLYMER FILMS CONTAINING LEUCO FORM OF INDIGO CARMINE

Monitoring of oxygen concentration in various media is very important in medical , biomedical and environmental sciences, as well as  in industrial processing , where oxygen is often involved as reactant or product [1,2]. Technical and patent literature provide several examples of the optical sensors for oxygen [3,4]  the most common of which are sensors based on the quenching of luminescence of appropriate indicators [5]. For example, the study [6] reported that both the phosphorescence and fluorescence of surface-adsorbed dyes such as trypaflavin, benzoflavin, safranin, chlorophyll, porphyrins and others are sensitively quenched by molecular oxygen. Various methods for fluorescence sensing have been investigated [7,8]. Disadvantage of method based on fluorescent measurement is that the fluorescence intensity can vary due to changes of many parameters [9].
Several organic dyes change color from almost colorless or light yellow to blue after contact with oxygen (air) [4,10]. Relatively recently the authors of this study patented the tamper-evident closure indicator [11] based on observation of irreversible color change in polymer-based indicator installed in a special type of packaging to notify users when a seal has been broken. It requires that the packaging be either vacuum sealed or sealed in a modified atmosphere.
In this study we reported four types of polymer systems with different oxygen permeability, including poly(ethylene –co-vinyl alcohol) (EVOH), EVOH, modified with 7% of N,N’-bis(2,2,6,6-tetramethyl-4-piperidyl)-isophtalamide (Nylostab S-EED), EVOH, modified with 10% of Nylostab S-EED, and polyisoprene that have been investigated as carrier for leuco form of Indigo Carmine.  An irreversible and visible color change from yellowish to blue caused by a chromogenic reaction involving the oxidation of leuco dye by molecular oxygen has been registered The relationship between the oxygen permeability of polymer films and the rate of observed color changes (absorbance maximum at 610 nm) has been investigated. The possibility to use prepared materials as absorption-based opto-chemical sensors for oxygen has been discussed.
EXPERIMENTAL PART
Starting materials, like  poly(ethylene –co-vinyl alcohol), polyisoprene, tetrahydrofuran (THF), n-propanol and sodium dithionite have been purchased from Sigma-Aldrich and have been used without further purification.
N,N’-bis(2,2,6,6-tetramethyl-4-piperidyl)-isophtalamide (Nylostab S-EED) have been received from Clariant Corporation. Indigo Carmine Lake has been received from Colorcon Company
Film preparation:
20-80 micron thick films of  EVOH containing ~0.5% indigo carmine lake and 10%  Nylostab S-EED were prepared from dispersion containing 150 mg indigo carmine lake and 300 mg Nylostab S-EED in solution of  3.0 g EVOH (content of ethylene groups -44% mol) in 25 ml of  mixture n-propanol;water (1:1 v/v) (Trial I) or 35 ml of this mixture (Trial II). Films were spun on a clean, 145 mm disposable plastic petri dish with the edges removed. After placing a petri dish into the Spin-Coater (Chemat Technology, KW-4A) approximately 12-13 mL of dyed polymer solution was poured onto the center of the dish. A single spin cycle with speeds ranging from 500 to 2500 RPM has been used. Last, films were dried for 6 hours at 40oC and their thickness were measured.
The films with a thickness 50-60 microns prepared at 1000 RPM (Trial I) or 500 RPM (Trial II) were chosen for further tests which include the conversion of indigo carmine lake into its leuco form.
Films of polysoprene with an addition of  0.5% indigo carmine lake have been prepared in similar manner with using THF as solvent.
Freshly prepared 10 % aqueous solution of sodium dithionite has been used for transformation of indigo carmine lake into its leuco form. EVOH-based polymer films completely changed their color from blue to yellowish after being immersed in sodium dithionate solution for 1-2 min. Polyisoprene-based films needed treatment by this solution for 4-5 min in order to achieve the same result.
Absorbance spectra have been measured using SPECTRONIC Genesys 5 spectrophotometer.
RESULTS AND DISCUSSION
Fig. 1A shows the appearance of 50 micron thick film of EVOH containing 0.5% indigo carmine lake  immersed for 1 min into sodium dithionate solution, washed with DI water and partly dried into nitrogen atmosphere  (very top of film continued to be colored blue because it was not immersed into sodium dithionate solution).

А B C D E    F
Fig. 1. A- Appearance of  50 micron thick film of EVOH containing ~0.5% indigo carmine lake  immersed into sodium dithionate solution and partly dried into nitrogen atmosphere; B-F- the same film exposed to the air for 5 sec (B), 10 sec (C), 15 sec (D), 20 sec (E) and 30 sec (F).
Fig. 2 shows schematic of oxygen permeation in polymer films containing the dispersed leuco form of Indigo Carmine and rearrangement in the chemical structure of the dye responsible for changing in the film coloration.

Fig. 2. Schematic of oxygen permeation in polymer films containing the dispersed leuco form of Indigo Carmine.
The measurements of the absorbance spectra of colorless (leuco lake dye/ EVOH) film (Fig. 1A) requires the constant purging of nitrogen into the measuring unit of spectrophotometer Spectronic Genesis 5. The result of this measurement is shown in Fig.3 (spectrum 1). The film is only semi-transparent which leads to the noticeable absorbance within 450-800 nm spectral region. The spectrum  2 in Fig. 3 shows the strong absorbance band with maximum around 610 nm which appears after 10-30 sec of exposure colorless film to the air.

Fig. 3. Absorbance spectra of 50 micron thick film of EVOH containing 0.1% indigo carmine lake and 10% Nylostab S-EED measured in nitrogen atmosphere (1); the same film  exposed to the air for 30 sec (shown in Fig. 1F).
Table 1 summarizes data on the compositions of the investigated polymer films, and the exposure times required for complete oxidation.
Table 1. Compositions of polymer films used in experiments with the leuco form of Indigo Carmine.
*) Time period for increasing the absorbance value at wavelength 610 nm by a value of 0.5 or higher
CONCLUSIONS
1.Four types of polymer systems with different oxygen permeability, including poly(ethylene –co-vinyl alcohol) (EVOH), EVOH, modified with 7% of N,N’-bis(2,2,6,6-tetramethyl-4-piperidyl)-isophtalamide (Nylostab S-EED), EVOH, modified with 10% of Nylostab S-EED, and polyisoprene that have been investigated as carrier for leuco form of Indigo Carmine.
2 Kinetics of a chromogenic reaction involving the oxidation of leuco dye by molecular oxygen has been evaluated by a spectrophotometric monitoring of increasing absorbance at wavelength of 610 nm.
3. The relationship between the oxygen permeability of polymer films and the rate of observed color changes has been investigated and recommendations for choice of polymer matrix suitable for making  absorption-based opto-chemical sensors for oxygen have been formulated.
References
1. J.R. Bacon, J.N. Demas, Determination of oxygen concentrations by luminescence
quenching of a polymer-immobilized transition-metal complex, Anal. Chem. 59 (1987) 2780–2785.
2. M.F. Choi, P. Hawkins, A fibre-optic oxygen sensor based on contact charge transfer
absorption, Sens. Actuators A 30 (1996) 167–171.
3. O.S. Wolfbeis, Fiber-optic chemical sensors and biosensors, Anal. Chem. 80, (2008) 4269–4283.
4. S.Wilhelm, O.S.Wolbeis, Irreversible sensing of oxygen ingress, Sensors and Actuators B, 153, (2011) 199-204.
5. I.Klimant, O.S. Wolfbeis, Oxygen-sensitive luminescence materials based on silicone-soluble ruthenium diimine complexes, Anal. Chem. 67 (1995) 3160–3166.
6. Y.Amao(Ed.), Probes and polymers for optical sensing of oxygen, Microchim. Acta 143 (2003) 1–12.
7. G. Orellana, M.C. Moreno-Bondi (Eds.), Frontiers in Chemical Sensors: Novel Principles and Techniques, Springer, Berlin, 2005.
8. C. McDonagh, C.S. Burke, B.D. MacCraith, Optical chemical sensors, Chem. Rev.108 (2008) 400–422.
9. M.Y. Berezin, S. Achilefu, Fluorescence lifetime measurements and biological imaging, Chem. Rev. 110 (2010) 2641–2684.C
10. G.Lewis “Tamper evident packaging”, US Pat. Application 20040050740.
11. N.Barashkov, T.Sakhno “Tamper-evident closure for food packaging”, US Provisional Patent Application #62/495,496 of 09/15/2016.
12. Zs.Peter, Cs.Kenyo, K.Ranner, et al, “Decreased oxygen permeability of EVOH through molecular interactions”, eXPRESS Polymer Letters, 2014, v. 8(10), 756-766.
13. “Permeability coefficient of common polymers” (http://www.faybutler.com/pdf_files/HowHoseMaterials AffectGas3.pdf ).