Rheological properties of dextrin-riboflavin solutions

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Journal of Molecular Liquids 240 (2017) 597–603

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Rheological properties of dextrin-riboflavin solutions under thermal and UV radiation effects Barış Demirbay a,b, A. Ata Ayhan c, Nuran Cereyan d, Can Akaoğlu a, İlke Ulusaraç a, Neslihan Koyuncu e, F. Gülay Acar d,⁎ a

Physics Engineering Program, Institute of Science and Technology, İstanbul Technical University, 34469 Maslak, İstanbul, Turkey Electrical Engineering Department, Faculty of Electrical and Electronics Engineering, İstanbul Technical University, 34469 Maslak, İstanbul, Turkey Medicine Program, Faculty of Medicine, University of Szeged, Dom ter 9, 6720 Szeged, Hungary d Physics Engineering Department, Faculty of Science and Letters, İstanbul Technical University, 34469 Maslak, İstanbul, Turkey e Radiation Science and Technology Program, Energy Institute, İstanbul Technical University, 34469 Maslak, İstanbul, Turkey b c

a r t i c l e

i n f o

Article history: Received 10 February 2017 Received in revised form 3 May 2017 Accepted 16 May 2017 Available online 18 May 2017 Keywords: Biocompatible materials Polysaccharides UV sensitivity Arrhenius model Shear thickening behavior Dilatant fluids

a b s t r a c t The combination of saccharides and UV sensitive chemical molecules in the solution form has been in the interest of medicine for treating medical complications and imaging techniques due to their remarkable properties under UV radiation. Therefore, two samples of dextrin based riboflavin solutions containing 10 wt% of dextrin content and 0.1 wt% of UV sensitive riboflavin content were prepared at the room temperature. In order to understand how UV radiation affects the viscosity of prepared solutions at various temperatures, one of the prepared samples was protected from light while another sample was radiated to UV at a biologically harmless wavelength where the absorbance peaks are obtained by UV/visible spectroscopy. Dynamic viscosity measurements of UV-radiated and un-radiated solutions for different temperatures were performed with constant velocity where the most reliable torque is shown. It was understood that application of UV radiation increased the viscosity value of radiated sample at the room temperature, however; it did not alter the fluid types of test samples. Fluid type of all test solutions was found to be non-Newtonian dilatant fluids as the shear stress vs. shear rate curves of solutions reliably obeyed Ostwald-de-Waele equation. Flow type of all solutions was classified as shear thickening fluids (STF) since power law consistency coefficient (n) of Ostwald-de-Waele equation was found to be N1 (n N 1). Based on this experimental outcome, the effect of UV radiation on shear thickening behavior of the solutions which include low viscous biocompatible solutions was studied for the first time. Thermal behavior of all test solutions was mathematically modeled via Arrhenius viscosity equation. The degree of STF increased and viscosity of dextrin based riboflavin solutions was found to be slightly greater than other test samples at increasing temperature values after UV radiation. As expected, viscosity values of all test samples decreased at elevated temperatures. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Over the past decades, thanks to the development of technology, the extensive use of biological macromolecules in the form of fluids for the purpose of diverse medical implementations from drug delivery systems to tissue engineering has become a common interest in various multidisciplinary research fields [1–2]. Recently, the importance of the essential macromolecules such as carbohydrates and vitamins has drastically increased since these biocompatible molecules are biodegradable and can be derived from natural resources in various ways [3]. The human body uses carbohydrates, also called saccharides in biochemistry, as its main energy source which comes from the breakdown of glucose with glycolysis and oxidative phosphorylation. These processes ⁎ Corresponding author. E-mail address: [email protected] (F. Gülay Acar).

http://dx.doi.org/10.1016/j.molliq.2017.05.078 0167-7322/© 2017 Elsevier B.V. All rights reserved.

provide most of the energy need of the body for daily anabolic and catabolic activities [4]. The production of energy initially depends on the degradation of saccharides by hydrolysis with the utilization of different enzymes [5]. Different types of saccharides can form larger molecules to be stored up for further metabolic activities such as starch which includes most of the food sources [6]. Starch, as a polysaccharide, consists of dextrin or long chains of single sugar molecules such as glucose which are linked together by glycosidic bonds [7–8]. Dextrin is a polymer that consists of D-glucose units formed by the degradation of starch or glycogen by amylases that can be found in human saliva and gastrointestinal system and are produced mainly by the pancreas. Dextrin itself has to be broken down into glucose to be utilized in glycolysis and oxidative phosphorylation processes for the production of adenosine triphosphate (ATP) [9]. This process can be accomplished by different types of glycosyl hydrolase enzymes within the cell [10]. Dextrin can be utilized by most of the primary cells of the body when the required

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energy is more than the available energy for ATP production in glycolysis which does not require mitochondrial systems [11]. The usage of dextrin as an energy source within the cell increases their significance when they are used in drug delivery and targeting systems for fatal diseases such as cancer by combining them with different compatible substances or molecules [12–17]. Modern day approach to cancer treatments is not adequate due to non-cancer cells are being affected as much as cancer cells by the agents used for the treatment. Various treatment methods are becoming available every day, however, in the case of cancer, most of the effort and investment focused on targeted drug delivery systems. Recent studies show that uptake and activation of the cancer drug carriers within the cells are enabled by the increased need of the cancer cells to glucose components which indicates the importance of dextrin since it can be degraded and utilized through cellular enzymes. Degradation of dextrin releases the cancer drug which causes minimal damage to noncancerous tissue and maximum damage to the cancer cells due to their increased metabolic rate and high energy need [18]. The combination of dextrin and different substances are not solely limited to medication and treatment of diseases. In addition to their usage as a treatment option for cancer, they can also be used for the visualization and localization of the diseases through different radiologic and spectroscopy techniques. Particularly, the most recent optical imaging techniques of fluorescence and Raman spectroscopy have attracted an immense attention to monitor and to detect the biological materials as biomarkers in cancerous tissues and cells for early diagnosis [19–23]. In imaging studies, dextrin which is used as a medium can carry MRI contrast materials and reduces their toxic effects on kidneys during elimination [24]. Dextrin as a scaffold or medium is not confined only to the inside of the body, however; it can be used to hold different medications to hasten the healing of injuries on the outside as well. Different gels that contain dextrin can be applied to the skin when it is combined with various medications in burn victims which have high mortality rates due to severe hypovolemia and increased risk of infection [25]. In medicine, dextrin shows great promise and value to be used in diverse applications from imaging techniques to prevention and treatment of diseases as well as their combined use with vitamins. Vitamins are the essential components of a diverse set of reactions within the body and can be used as cofactors, coenzymes or as antioxidants which can reduce tissue and cell damage by inhibiting the formation of reactive oxygen species. They can be classified as water soluble or more lipid soluble. At this point, they can be separated by their properties of storage within the body which points out that more lipid soluble vitamins can be stored in the liver and other organs. Some special vitamin groups such as vitamin B2 which is known as UV sensitive riboflavin in the literature, required for various processes mainly in the form of a coenzyme to important enzymes that regulate different reactions in the body. Besides the use of riboflavin within the body, one of the unique properties of riboflavin derives from its affinity and reaction to ultraviolet light (UV). This is the main purpose of use of riboflavin in various studies that enables its second most important property of having flavin groups. Flavin molecules play a significant role in crosslinking when they are activated by the UV light [26]. The use of riboflavin as a photo cross-linker agent under UV radiation makes it invaluable in medical research of a disease called Keratoconus that can cause a reduction in tissue strength and weakening of the corneal layer due to the loss of collagen in the eye which results in a cone shape deformity and eventually leading to vision disturbances. The increase in the corneal integrity can be accomplished with the help of riboflavin solutions as a treatment method [27]. Another example of the strengthening properties of riboflavin in tissues is its usage in meniscus tears which are debilitating medical conditions that can have a long recovery and healing times because of its physiological low blood flow to the ligaments within the joints. The use of riboflavin within collagen increases tissue strength and reduces its recovery time [28]. Riboflavin, in addition to its use in tissues, can also be utilized in cancer therapy with the help

of photodynamic therapy by activating riboflavin molecules [29]. Biocompatibility of riboflavin allows it to be used in most of the solutions that do not contain only collagen, but saccharides as well. It has been showed that dextrin and the group of various saccharides can have interactions with riboflavin [30]. Polysaccharides in drug delivery systems have been investigated and demonstrated that when they are combined with a crosslinking agent such as riboflavin, it resulted in more stable drug carriers that can have long half-lives within the body and enhance their effect [31]. Dissolution time and the stability of the carrier molecules in biological systems are strongly dependent on the viscosity and flow characteristics. Before biomedical implementations, determination of viscosity of these materials in the form of solutions, as well as compatibility of their viscosity with interior liquids within the body, are quite significant for the provision of homeostasis in human physiology. Therefore, by taking these remarkable studies and medical information into consideration, in our present research, the effects of UV and thermal radiation on rheological properties of dextrin-riboflavin solutions at different temperatures were studied. Fluid types of all test samples were classified via investigation of shear stress versus shear rate and viscosity versus shear rate curves and corresponding mathematical models were obtained respectively. The change in dynamic viscosity of the samples was analyzed by considering viscosity versus temperature curves before and after UV radiation at different temperatures in the range between 18 °C and 45 °C. 2. Materials and methods 2.1. Chemicals Dextrin (corn starch, purchased from Sigma Aldrich, Germany, ≥99% purity) and riboflavin (purchased from Sigma Aldrich, Germany, ≥99% purity) were used in the powder form during the process of preparation of solutions. 2.2. Experimental process All experimental procedures were done at the room temperature about 21 ± 2 °C. 2.2.1. Mathematical abbreviations of test samples Prior to experiments, all test samples were encoded via special abbreviations to describe which concentrations the solutions have and to clarify either the test sample is UV radiated or not. Test samples and their formulations were given in Table 1. 2.2.2. Preparation of dextrin-riboflavin solutions The proportion of 10 wt% dextrin is widely used both in food science and biological studies according to literature review [32–33]. Besides of this, the proportion of 0.1 wt% of riboflavin (RF) solutions has still been using both in biological studies and medical treatments as well [34–38]. Therefore, these concentrations were taken as reference in this study before preparation process of the solutions. For this purpose, dextrin powder was dissolved in distilled water (dH2O) considering the concentration of 10 wt% on magnetic stirrer equipment for 40 min to solve dextrin molecules at 30 °C with the speed of 1200 rpm. Then, RF powder was dissolved in dH2O considering the concentration of 0.1 wt% for 40 min at 30 °C with the speed of 1200 rpm within different

Table 1 Mathematical abbreviations of the prepared solutions. Test Sample

Dextrin content D (wt%)

10D 10 10D0.1RF-a 10 10D0.1RF-b 10

Riboflavin content RF (wt%)

UV-Radiation state

– 0.1 0.1

– Radiated Un-radiated

B. Demirbay et al. / Journal of Molecular Liquids 240 (2017) 597–603

glass beaker. At the second stage, two samples of 18 ml of dextrin-riboflavin solutions which are made up of 80% of pure dextrin solution and 20% of pure RF solution were mixed in different glass beakers at 30 °C for 40 min. Finally, one sample of 18 ml of pure dextrin solution and 18 ml of two samples of dextrin-based RF solutions were prepared respectively. At this point, it should be noted that glass beakers of solutions which contain RF content were coated by an aluminum foil to protect the solutions from light exposure during the experimental process in our laboratory where the windows are coated with UV protective thin films. Because of UV-sensitive nature of RF, light exposure can trigger the flavin molecules within RF to initiate the onset of photo-crosslinking [39]. 2.2.3. UV radiation process and UV/visible absorbance & fluorescence measurements Absorbance measurements were performed by using UV/visible absorbance spectrophotometer (Shimadzu UV-150-02) at the room temperature. The peaks at absorbance graphs provide us the wavelength values where the most UV absorption comes out within riboflavin molecules. After obtaining the peak values which are 372 nm and 444 nm, the fluorescence measurements were performed to understand how the sample of 10D0.1RF emits back the excitation energy by using fluorescence spectrophotometer (Perkin Elmer LS 50) at 372 nm which corresponds to the biologically harmless excitation wavelength. Because, particularly in medical applications, the first and third absorbance peaks of pure RF sample which corresponds to 278 nm and 444 nm cause distortion in DNA and chemical burns respectively. Therefore, excitation wavelength that is 372 nm was chosen. As a result of photocrosslinking (CXL) ability of riboflavin at 372 nm, its mechanical properties can be enhanced in medical treatments under UV radiation [40]. In order to benefit crosslinking ability of riboflavin and to get how UV radiation affects the dynamic viscosity and flow behavior of the solutions, one of the prepared samples of 10D0.1RF was radiated to 6 mW/cm2 UV to excite each RF molecules within dextrin-riboflavin solutions by using UV light source at 372 nm for 8 min as it was described in our previous study paper [39].

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excitation wavelength both for fluorescence and UV radiation process. According to studies in the literature, the absorbance peaks of pure riboflavin solutions were shown at wavelengths of 278 nm, 371 nm and 444 nm respectively [41–42]. As it is clearly shown in Fig. 1a, there is only 1 nm shift in absorbance wavelength of 10D0.1RF when pure RF solution is mixed with dextrin solution. It means that addition of dextrin into RF solution almost did not change the original UV spectrum of pure RF solution. Excitation of RF molecules within solution at 372 nm which is appropriate for biomedical applications, lead to maximum energy absorption in the solution. Therefore, one of the prepared 10D0.1RF samples was radiated to UV for 8 min to initiate the onset of photocrosslinking via excitation of molecules by using UV source. Besides of absorbance measurements, fluorescence measurements of 10D0.1RF were taken at 372 nm to obtain in which wavelength the highest energy emission has occurred. As it is seen in Fig. 1b that fluorescence intensity was found about 336 a.u. and the highest peak in fluorescence measurements was observed at 537 nm which corresponds to the wavelength range of green color in the visible spectrum. It means that the maximum emission energy which is absorbed by the sample of 10D0.1RF emits back at 537 nm with lower energy. 3.2. Viscosity measurements Hysteresis rheogram of shear stress-shear rate was given in Fig. 2 for the description of the effect of UV sensitive riboflavin on fluid types of

2.2.4. Dynamic viscosity measurements Dynamic viscosity measurements of the samples 10D, 10D0.1RF-a and 10D0.1RF-b were performed at different velocities through using rotational viscometer (Fungi-Lab premium series) attached to temperature control unit that is laboratory heat bath. In order to determine the fluid types, the charts of shear stress vs. shear rate and viscosity vs. shear rate were obtained at the room temperature for each test sample. Afterward, to understand the effect of temperature on dynamic viscosity of each test sample, the measurements were performed in the temperature range between 18 °C and 45 °C at fixed rpm value where the torque of viscometer gives the most reliable results. 18 ml of prepared test samples were added to the jacket of viscometer for measurements and LCP spindle which is appropriate for low viscous fluids was used in viscometer during the experiments. 3. Results and discussion 3.1. UV/visible absorbance and fluorescence measurements In order to determine the excitation wavelength of 10D0.1RF for UV radiation, absorbance measurements were taken at the wavelength range between 300 nm and 500 nm at the room temperature since the wavelength spectrum below 300 nm can cause undesirable effects in biomedical applications. As a result of absorbance measurements, one of the prominent peaks of 10D0.1RF was found at 372 nm which is almost similar to the wavelength of second absorbance peak of pure riboflavin solutions. Moreover, the second absorbance peak of 10D0.1RF was found at 444 nm. As it is described, the applications of UV in biomedical implementations at 444 nm can lead to hazardous effects on biological systems. Therefore, 372 nm was considered as an

Fig. 1. a UV Absorbance spectra in the UV spectrum range between 300 nm to 500 nm. b Fluorescence spectroscopy results of 10D0.1RF under excitation at 372 nm.

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B. Demirbay et al. / Journal of Molecular Liquids 240 (2017) 597–603 Table 2 Fitting constants of Ostwald-de-Waele viscous flow model.

Fig. 2. Shear stress – shear rate rheogram of the samples of 10D, 10D0.1RF before and after UV radiation.

prepared solutions. As it is clearly seen in Fig. 2 that flow type of each test sample was found to be non-Newtonian Dilatant fluids at the room temperature due to there was no linear change between shear rate and corresponding viscosity values. It is also shown that there is no energy loss transformed into heat since there are no gaps between loading and unloading curves of the hysteresis rheogram. The addition of riboflavin content into dextrin solutions did not alter the fluid type before and after UV radiation, however; viscosity of the dextrin-riboflavin solution increased after UV radiation. For rotational viscometer, the measurement range of viscosity differs depending upon concentrations of solution, application of UV radiation and torque range of spindle which is attached to viscometer. The equipment can run a dynamic viscosity test up to 99% N.m. of torque value, otherwise, the mechanism cannot make an appropriate measurement. In torque range between 1% N.m. and 99% N.m., while the LCP spindle of viscometer took a measurement for the samples of 10D and 10D0.1RF-b with the maximum speed of 220 rpm, the viscosity of 10D0.1RF-a was measured with the maximum speed of 250 rpm at maximum torque value. It means that the sample of 10D0.1RF-a had shown resistance behavior to flow direction of a fluid with higher velocity and under the higher quantity of shear stress since the number of bonds between dextrin and riboflavin molecules increased as a result of photo-crosslinking (CXL). Therefore, the viscosity value of 10D0.1RF-a increased after UV radiation. Viscous flow model of test samples obeyed Ostwald-de-Waele model which is known in literature by Power law for non-Newtonian dilatant fluids as follows: τ ¼ kp γ_

Test Sample

kp (10−4 Pa·sn)

n

Adj. R2

10D 10D0.1RF-a 10D0.1RF-b

6.94 ± 0.92 8.47 ± 1.73 9.12 ± 0.86

1.62 1.61 1.56

0.99 0.98 0.99

critical shear rate value [47]. In that case, it is possible that aggregation of particles in test samples acts as formed particle clusters at increasing shear rate values up to critical shear rate [48]. Above critical shear rate, inter-molecular and inter-particulate interactions between molecules in test samples can lead to the formation of strong bonds which have greater resistance to breakage along shear [49]. During viscosity measurements, the increment in the rotation speed of LCP spindle caused the increase in shear rate as well. Therefore, as we consider that the molecules began to act as solid-like particles and formed particle clusters became enlarged structures at higher shear rates. So that viscosity of samples and energy dissipation increased [50–52]. As it is also clearly seen in Fig. 3, critical shear rate ðγ_ c Þ values which correspond to the onset of shear thickening behavior are shown for 10D at point b, for 10D0.1RF-a at point a and for 10D0.1RF-b at point c where point a is 97.72 s−1, point b is 110.72 s−1 and point c is 134.47 respectively. Critical shear rate value of dextrin-riboflavin solution shifted (from c to a) as a result of the increase in viscosity after UV radiation. Riboflavin molecule consists of ribitol and flavin group which are bonded to each other by carbon-nitrogen (C\\N) bond. Flavin group has a tendency to be involved in oxidation and reduction reactions when it is stimulated by UV light. The effect of photosensitization leads to the production of reactive oxygen species from riboflavin which includes singlet oxygen (O2), • super-oxides (O− 2 ), hydroxyl radicals ( OH) and hydrogen peroxide (H2O2) [53–59]. As a phenomenon, the oxidation state of the flavin molecules can be followed up by considering the color of the resultant aqueous solutions. Super-oxidized flavins take a specific color of yelloworange while fully-oxidized flavins are shown as light yellow [60–61]. The change in oxidation state of the dextrin-riboflavin solution after UV radiation can be clearly seen in the given Fig. 4 and it is understood that flavin molecules within our test sample are fully-oxidized after UV radiation. Another way of monitoring the oxidation state of flavin molecules is to take UV–visible absorption and fluorescence spectra. According to the literature review, oxidized flavins have the highest absorbance peak at 450 nm and fluorescence at about 515 nm and

n

where τ is shear stress, γ_ is shear rate, kp is Power law consistency coefficient (Pa.sn) and n is flow behavior index [43]. By taking the rheogram of shear stress–shear rate into consideration, the mathematical parameters of Ostwald-de-Waele model equation were calculated and the results were given in Table 2. In the case of n N 1 for Ostwald-de-Waele flow model, the fluids are considered as shear thickening fluids (STF) which are defined as a sharp increase in viscosity when shear rate reaches its critical value [44–46]. According to Table 2, it is clearly shown that flow behavior index values (n) of test samples are all N1, thus the solutions can be classified as STF since it is also shown in Fig. 3 which corresponds to the chart of viscosity versus shear rate. The bonds which formed among molecules are not strong enough either the dextrin solution contains riboflavin or not at low shear rates. Thus, structural breakdown comes out which results in shear thinning and viscosity decreased until it reaches

Fig. 3. Viscosity– shear rate rheogram of the samples of 10D, 10D0.1RF before and after UV radiation. Critical shear rate values are shown for 10D at b, for 10D0.1RF-a at point a and for 10D0.1RF-b at point c.

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520 nm, respectively [60]. In our experiments, the highest absorption and fluorescence of dextrin-riboflavin were investigated at 444 nm and 537 nm with a very small shift, respectively due to the existence of dextrin within riboflavin solution. It also proves the existence of oxidized flavin molecules within our test sample after UV radiation. Oxygen species which arise from oxidized flavins are quite reactive and have a tendency to bond with carbon-based (organic) molecules such as dextrin. Therefore, it can be said that, when flavin moiety of riboflavin received the UV light, the existence of produced reactive species in the solution enhance the probability of bond formation among riboflavin and dextrin molecules. Based on UV radiation, the formation of the bonds between dextrin and riboflavin molecules altered the chemical conformation of formed clustered structures. Larger conformations consisting of longer chains and groups of molecules were formed due to UV radiation on dextrin-riboflavin solutions. As we consider that in consequence of the formation of volume fractions on the backbone of the entire structure and increase in molecular chain length can promote the degree of shear thickening behavior thanks to photo-CXL [50,62– 63]. Formed volume fractions lead to restricting the movement of entire molecule so that viscosity of the sample 10D0.1RF increased after UV radiation. Besides of the measurements at the room temperature, dynamic viscosity experiments of the test samples were performed at different temperatures as well. Measurements were taken at fixed rotational velocity values which are 160 rpm for 10D, 150 rpm for 10D0.1RF-a and 170 rpm for 10D0.1RF-b where the torque of spindle gives the most reliable test results. Depending upon the experimental results, mathematical models of fit curves which are shown in Fig. 5 including viscosity change vs. 1/temperature graph, reliably obeyed the Arrhenius model. Another name for a power-law fluid like studied dextrin - riboflavin solutions with the exponential dependence of viscosity on temperature is a first-order fluid. The equation of Arrhenius model in dynamic viscosity can be given in the form of Ea

η ¼ η0 eRT where η is viscosity, η0 is Arrhenius entropic factor (also known as preexponential factor) which theoretically corresponds to viscosity at infinite temperature η∞, Ea is Arrhenius activation energy, R is universal gas constant and T is temperature [64–65]. Fitting constants based on Arrhenius viscosity equations of each curve were given in Table 3 with approximate errors. As it is clearly seen in Table 3, owing to enthalpy change (or Arrhenius activation energy) between the products and the reactants was

601

Fig. 5. Viscosity change vs. 1/Temperature graph of 10D, 10D0.1RF before and after UV radiation.

found to be positive, the reaction in the solution is endothermic. It means that the products of the reaction have greater enthalpy than reactants since the heat is absorbed by the system. At the reference temperature, the Arrhenius entropic factor ηo of 10D0.1RF-a was found to be a little bit higher in consequence of polymerization after UV radiation even if the sample has only 0.1% of riboflavin content. When the heat is given to test samples, the volume of molecules within test samples increases as the particles gain velocity and they tend to spread out. Formation of a momentum transfer takes place from lower momentum to higher momentum. Therefore, surface tension, buoyancy, and density of the solutions diminished [39]. In the direction of positive velocity gradient, the shear stress decreased. In that case, an act of molecules begins to disrupt the effect of the unbalanced forces on the surface of prepared solutions. It both weakens their sheet-like barrier of weakly bound molecules and lowers the surface tension as well. Because of low surface tension, the penetration of molecules gets easier. Depending upon given heat, the dynamic viscosity of the test samples can be affected by slipping up and acting apart of accelerated particles on each other in random direction. The increase in velocity let particles move and pass each other in an easier way so that the viscosity of test samples decreased. On the other hand, the test samples gain more energy and much faster vibrations at elevated temperature levels. As the temperature increases, the bonds among molecules and chemical interactions between molecules become weaker. As a result, the resistance of test samples decreases on account of the formation of free spaces between molecules tend to increase. Therefore, the viscosity of all test samples decreased at elevated temperature levels. When the heat is given to the system, the viscosity of pure dextrin solution which is 10D was found to be less than dextrin-riboflavin solutions before and after UV radiation procedure. It is also seen in Table 3 that the sample of 10D0.1RF-a has a greater activation energy than other test samples. In Arrhenius flow model, activation energy can be defined as a minimum energy which is required for molecules in the solution to get rid of the interactions and influence between neighboring molecules [66–67]. For the breakage of bonding and interactions between connected molecules, larger amounts of activation energy are Table 3 Fitting constants of Arrhenius viscosity equation.

Fig. 4. Color change of flavin groups within riboflavin-dextrin solution before (a) and after (b) UV radiation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Test Samples

η0 (cP)

Ea (kJ/mol)

Adj . R2

10D 10D0.1RF-a 10D0.1RF-b

0.02 0.04 0.02

11.10 ± 0.38 11.27 ± 0.70 9.55 ± 0.40

0.99 0.98 0.99

602

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required. Therefore, as we think that the reason why 10D0.1RF-a had higher activation energy is the formation of cross-linked bonds between dextrin and RF molecules due to the occurrence of the photo-CXL phenomenon. Even if a very low amount of RF concentration that is 0.1 wt% was used as a reinforcing agent in dextrin solution, activation energy and the degree of shear thickening behavior of dextrin-riboflavin solution increased after UV radiation. However, the viscosity of 10D0.1RF-a was slightly increased after UV radiation when it is compared to sample of 10D0.1RF-b.

4. Conclusion In this research paper, UV radiation and thermal effects on dynamic viscosity and rheological properties of UV sensitive dextrin-riboflavin solutions were reported. It can be concluded that addition of riboflavin into dextrin solutions did not alter flow type of the test solutions before and after UV radiation process, however; it changed the viscosity values and activation energies of the test samples. Flow type of each test solution was found to be non-Newtonian shear thickening fluids. The effect of UV radiation on shear thickening behavior of the solutions which include UV sensitive biocompatible riboflavin was studied for the first time. Implementation of UV radiation which results in the production of reactive oxygen species from riboflavin leads to photo cross-linking and increased the viscosity and the degree of shear thickening behavior of dextrin-RF solutions at the room temperature. When the heat is given to the system, the activation energy of dextrin-riboflavin solutions was found to be greater after UV radiation which concludes into bond formation between dextrin and riboflavin molecules based on the reactivity of produced oxygen species. UV/visible absorption and fluorescence spectra results were consistent with literature reports which indicate the existence of reactive oxygen species within our test samples. Even though the adverse effects of reactive oxygen species can limit the application of riboflavin in vivo studies, cells have specific enzymes called superoxide dismutases to prevent and to reduce their undesired effects. Therefore, UV radiated riboflavin in conjunction with dextrin molecules can be used as carriers for drugs in carrier fluids of the body such as blood since they have a long dissolution time because of increase in viscosity after UV radiation.

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