Synthesis, characterization and drug release activity of poly(ε-caprolactone)/Fe₃O₄–alizarinred nanocomposites

Abstract

Applications of magnetic and fluorescent poly(ε-caprolactone) (PCL) is increased rapidly in the biomedical engineering field as a bioimaging probe. This urged us to do the present work. In the present investigation, the role of Fe₃O₄–AR dye nanohybrid on the ring opening polymerization (ROP) of CL was studied at different experimental conditions such as variation in [M] and [M/I]. Prior to ROP of CL, the nanohybrid was characterized by various spectral methods like FTIR spectroscopy, UV–visible spectroscopy, fluorescence spectroscopy, VSM and FESEM like analytical techniques. The Fe₃O₄–AR nanohybrid is used as an initiator for the ROP of CL. After the formation of PCL/Fe₃O₄–AR nanocomposite again the systems were characterized by FTIR spectroscopy, UV–visible spectroscopy, fluorescence spectroscopy, DSC, VSM and FESEM like methods. The interesting point noted in the present investigation is the size of Fe₃O₄ nanoparticle was found to be reduced after the nanohybrid and nanocomposite formation. The three functional groups of AR dye were involved in the ROP of CL. The binding constant and number of binding sites were determined.

Keywords:

• Nanohybrid
• ROP
• Binding constant
• Fe₃O₄
• Alizarin red
• HRTEM

Introduction

Poly(ε-caprolactone) (PCL) is an important biomedical plastic material that holds the potential to reduce the waste associated with the disposal of conventional polymers. The material scientists expect that synthesis of PCL by a bulk and single-step method without using any hazardous solvents and with luminescence and fluorescence properties since it is a biomedical candidate. This will increase the applications of PCL in the biomedical field. Various initiators are used for the synthesis of PCL. For example, in 2010 Ireland and co-workers¹ reported about the cationic organomagnesium-initiated ROP of ε-CL. The biomedical PCL can be synthesized using a functionalized initiators such as amine, hydroxyl, carboxyl, thiol, and amide.^2–10 The biomedically valuable PCL can be synthesized using isobutyl alumoxane.¹¹ Kinetics and mechanism of anionic ROP of ε-CL were explained in the literature.¹² Hydroxy butyl vinyl ether functionalized PCL was reported by Lipik et al.¹³ A novel Yttrium complex initiated ROP of ε-CL was done by Wu and research team.¹⁴ MWCNT-initiated PCL was synthesized and characterized by NMR and WAXS.¹⁵ Azemi et al.¹⁶ synthesized the solventless enzyme catalyzed ROP of ε-CL. Thiol functionalized,¹⁷ heteronuclear base functionalized,¹⁸ perfluoroalkane sulfonamide functionalized,¹⁹and phosphoester functionalized²⁰ PCL report is available in the literature. By thorough literature survey, we could not find any report based on the Alizarin red S (AR)-functionalized Fe₃O₄ initiated ROP of ε-CL and its drug release activity. The present methodology offers a solvent free ROP of ε-CL. Above all; PCL is an effective drug carrier material.

Magnetic nanoparticle particularly Fe₃O₄, has been effectively studied by various research team.^21–23 Unfortunately, the application of Fe₃O₄ was restricted due to the absence of the functional group on the Fe₃O₄ surface. Recently, our research team reported about the functionalization of Fe₃O₄ by different functionalities.²⁴ Synthesis of water soluble cyclic amide functionalized Fe₃O₄ was reported by Li and co-workers.²⁵ The literature survey reveals that AR-functionalized Fe₃O₄ report is not available. Recently, the polymer scientists are doing research with targeted drug delivery using Fe₃O₄, particularly in the photodynamic therapy (PDT) field. This motivated us to do the present investigation because, after the utilization, it is not necessary to remove the iron from the system since it is a biometal.

An organic dye is an important key element in the PDT for the production of reactive oxygen species (ROS) towards the apoptosis of cancer cells. In 2009, Kayrak et al.²⁶ studied the optimization of synthetic method for the production of fluorescent and conductive wool. In 2014, Kohila and research team²⁷ studied the poly(congored) grafted silk fiber. On going through the literature, we could not find any report based on the AR-functionalized Fe₃O₄ nanohybrid initiated ROP of CL. In the present investigation, we chose AR dye because of hydroxyl and –SO₃Na like functional groups. The present investigation was done towards the synthesis and characterization of fluorescent dye functionalized magnetic nanohybrid. The novelty of the present methodology is an eco-friendly, biocompatible, economically cheaper and single-step preparation without any hazardous solvents.

Chemically conjugated drug delivery systems can be used in an effective way rather than the simply loaded drug delivery systems towards the sustainable release of drug and elimination of side effects in the pharmaceutical field. In this case, the rate of drug release is controlled by the rate of hydrolysis. In 2013, Khazaoi and co-workers²⁸ reported the drug release behavior of poly(styrene-alt-maleicanhydride) system. Sultana et al²⁹ studied the ciprofloxacin hydrochloride release from Povidone K30 matrix. Drug release activity of aspirin loaded PLGA-PEG-PLGA/montmorillonite microparticles was reported in the literature.³⁰ The other authors also reported about the drug release study.^31,32 By thorough literature survey, we could not find any report on the Rif. loaded drug release activity of fluorescent and magnetic PCL. Hence, the drug release study is the main aim of the present investigation. In order to avoid the photodegradation of dye molecule during the PDT process, the dye is chemically conjugated with the PCL (as an initiator). Moreover, during the PDT application, lesser amount of dye is injected into the system in order to avoid the side effects and cytotoxic effects. For this purpose, we have considered the AR dye-functionalized Fe₃O₄ nanohybrid as a chemical initiator for the ROP of ε-CL.

Experimental

Materials

ε-caprolactone (CL, Across chemicals, USA), stannous-2-ethyl hexanoate (Sn(Oct)₂, Across chemicals), FeCl₃, FeSO₄ and NaOH were freshly purchased from s.d fine chemicals, India and used as such. Alizarinred S (AR) was purchased from Spectrum chemicals, India. Diethylether, non-solvent, was purchased from Aldrich chemicals and used as received. CHCl₃ solvent was purchased from Aldrich chemicals, India. Rifambin (Rif, Ranbaxy Chemicals, India) was purchased in tablet form and used as such. Double distilled (DD) water was used for the preparation of reaction solutions.

Synthesis of AR-functionalized Fe₃O₄ nanohybrid system

The novelty of the present investigation is the synthesis of AR-functionalized Fe₃O₄ and the same is used as an initiator for the ROP of CL. The procedure in brief is mentioned here (Scheme 1). About 1 g AR dye was dissolved in 50 mL of DD water, taken in a 500 mL beaker under vigorous stirring conditions. About 2.5 g FeSO₄ and 5 g FeCl₃ were added inside the beaker under N₂ atmosphere at room temperature. Here, the Fe²⁺ to Fe³⁺ ratio was maintained at 1:2.³³ Further, 100 mL of DD water was added and mixed for 10 min. 10 g NaOH in 100 mL DD water was prepared separately. Thus, prepared NaOH solution was added to the reaction mixture under vigorous stirring condition till the content became dark black. Once the permanent dark black color was obtained, further addition of NaOH solution was stopped. The reaction was allowed further for two hours with stirring. The added NaOH nucleated the formation of Fe₃O₄. After 2 h of mixing the excess NaOH was removed by placing the beaker on a magnetic bar. Under the influence of magnetic field the AR decorated Fe₃O₄ molecules settled down, whereas the unreacted NaOH remained in the aqueous medium. The top aqueous layer was removed with the help of a pipette. The precipitate is washed with excess amount of DD water and placed on the magnetic bar. Under the influence of both gravitational and magnetic force, the AR-functionalized Fe₃O₄ settled down and the liquid was removed. This washing process was continued till the supernatant liquid attains the pH 7.0 and colorless. Finally, the precipitate was freeze dried, weighed, and stored in a zipper lock cover. The same procedure was followed for the synthesis of pure Fe₃O₄ in the absence of AR molecules. In the present investigation, AR contains two –OH groups and one –SO₃Na group. Hence, the AR physically adsorbed on the surface of the ferrite molecules. In 2013, Demin et al²⁹ explained the interaction between the ferrite and alkoxysilane compound. But in the present study, already there are lot of –OH groups present on the surface of ferrite due to NaOH medium, the –OH group of AR may not interact with the ferrite surface. Hence, the only possibility is the interaction of –SO₃Na with the metal surface.

Synthesis of PCL/Fe₃O₄–AR nanocomposites

A two-way necked 25 mL capacity round bottomed (RB) flask was taken and dried well. One neck acts as a sulfur free N₂ inlet, whereas the second one acts as a N₂ outlet. About 1 g CL was charged in the above said two way necked RB flask. With this 0.001 g Sn(Oct)₂ (catalyst) was charged. Here, the [M/C] = 1000. ROP of CL was carried out under different experimental conditions such as [M/I] and monomer variation. Required amount of AR-functionalized Fe₃O₄ was accurately weighed and mixed with the two-way necked RB flask. The contents were mixed for 10 min under N₂ purging at room temperature. After uniform mixing the two-way necked RB flask was kept in an oil bath at 160 °C. The ROP was allowed for 2 h under mild stirring condition.³⁴ At the end of the reaction the N₂ purging was stopped and the highly viscous liquid was dissolved in 25-mL CHCl₃ solvent. Thus, obtained polymer was purified by re-precipitation with the addition of diethylether. After the drying process for 6 h under fume hood, the Fe₃O₄/AR nanohybrid end capped PCL was obtained as a white crystalline powder. Thus, obtained white crystalline powder was weighed and stored in a zipper lock cover.

Characterization

Magnetic measurements were carried out with a superconducting quantum interference device magnetometer (Lakesore-7410-VSM, USA) with magnetic fields up to 7 T at 32 °C. FTIR spectra were recorded with the help of Shimadzu 8400 S, Japan model instrument by KBr pelletization method from 400 to 4000 cm⁻¹. Three milligram of polymer sample was ground with 200 mg of spectral grade KBr and made into a disk under the pressure of 7 tons. ¹H and ¹³C-NMR spectra of the polymer samples were recorded by using Bruker Biospin High Resolution Digital 300 MHz NMR spectrometer, USA. Dueterated chloroform (CDCl₃) was used as a solvent, and tetramethyl silane (TMS) served as an internal standard. UV–visible spectrum was measured by dissolving the sample in chloroform solvent. The instrument used here is Shimadzu UV-3600, Japan model instrument from 200 to 800 nm. The same sample solution was subjected to fluorescence emission measurement from 350 to 700 nm using Elico SL174 (India) instrument. The melting temperature (T_m) and the degradation temperature (T_d) of the polymer samples were determined by using Dupont Thermal Analyst 2000 Differential Scanning Calorimeter 910S, USA model instrument (simultaneous DSC and TGA analyzer). All the measurements were done under N₂ atmosphere in a temperature range of RT to 100 °C with 10 °C/min heating rate. Field emission scanning electron microscopy (FESEM) was used to examine the morphological behavior of the polymer with the help of FESEM – Hitachi S4800 Japan, instrument.(1)

where A₁₇₂₆ is corrected peak area of a peak at 1726 cm⁻¹ and A₇₃₀ is corrected peak area at 730 cm⁻¹. A Waters 2690 GPC instrument was used to determine the M_w of the polymer samples using THF as an eluent at room temperature at the flow rate of 1 mL min⁻¹ against polystyrene (PS) standards.

Drug release study

The ultimate aim of the present investigation is the drug release activity of PCL. The drug release study was carried out for the above synthesized materials and the procedure in brief is given below: 0.50 g PCL was taken in a disk form prepared under 7 tons of pressure. About 500 mL of gastric pH solution was prepared and the tablet suspended in the medium with stirring. ³¹ While stirring, the drug is slowly released and the same can be quantitatively measured at different intervals of time by pipetting 2 mL of aliquot by using UV–visible spectrophotometer. From the calibration curve, the % cumulative drug release (% CDR) was calculated as follows:(2)

Various possible models were dried and the best fit models are explained in the present investigation. The drug release models were done in a universal standard model.

Results and discussion

Characterization of Fe₃O₄/AR nanohybrid system

The FTIR spectrum of pure Fe₃O₄ is given in Figure 1a. The spectrum shows two peaks. A broad peak around 3466 cm⁻¹ is due to the –OH stretching of Fe₃O₄. During the Fe₃O₄ synthesis some amount of water molecules are intercalated. The metal oxide stretching appeared at 603 cm⁻¹.³⁵ Figure 1b represents the FTIR spectrum of AR decorated Fe₃O₄ nanohybrid. The –OH stretching appeared at 3444 cm⁻¹. The aromatic symmetric and anti-symmetric stretching was observed at 2889 and 2959 cm⁻¹, respectively. The bending vibration of –OH from AR appeared at 1637 cm⁻¹. The SO₂ stretching from AR appeared at 1216 cm⁻¹. The aromatic C–H stretching was observed at 870 and 920 cm⁻¹. The C–H out of plane bending vibration (OPBV) appeared at 719 cm⁻¹. The metal oxide stretching also appeared at 632 and 444 cm⁻¹. Hence, the appearance of new peaks such as –OH bending, SO₂ stretching, aromatic CH stretching, and C–H OPBV confirms the surface functionalization of Fe₃O₄ by AR dyes.

Figure 1 FTIR spectrum of a Fe₃O₄ and b Fe₃O₄–AR nanohybrid, UV–visible spectrum of c AR dye and d Fe₃O₄–AR nanohybrid, Fluorescence emission spectrum of e AR dye and f Fe₃O₄–AR nanohybrid, VSM of g Fe₃O₄ and h Fe₃O₄–AR nanohybrid, FESEM image of i Fe₃O₄ and j Fe₃O₄–AR nanohybrid system

The UV–visible spectrum of AR before and after functionalization reaction confirms the interaction between Fe₃O₄ and AR dye. A broad peak at 418.7 nm (Figure 1c) is responsible for the AR dye. After the functionalization reaction the same peak was red shifted to 431.9 nm (Figure 1d). The red shift in the peak after the functionalization reaction confirms the existence of chemical interaction between dye and Fe₃O₄. The SO₂ group of AR dye might interact with the Fe₃O₄ through the secondary forces of attraction. As a result, the peak was red shifted. The red shift in the peak after the nanohybrid formation was explained in our earlier communication.³⁴

The peculiar property of AR dye before and after surface functionalization reaction was tested by fluorescence spectroscopy. The fluorescence emission spectrum of pristine AR dye is shown in (Figure 1e) with the intensity of 806 cps. After the surface functionalization with Fe₃O₄ the FEI was suppressed to 606 cps (Figure 1f) without any shift in the peak. This confirms the quenching nature of Fe₃O₄.

The magnetic property of Fe₃O₄ was studied by VSM technique and is shown in Figure 1g with the magnetization value 55 emu/g. But after the surface functionalization reaction the VSM value of Fe₃O₄ was suppressed to 49.9 emu/g (Figure 1h). The decrease in VSM value confirms the existence of chemical interaction between the Fe₃O₄ and AR dye. In the case of AR dye, three types of functional groups such as –OH, –SO₃Na, and C=O were observed and the –SO₃Na group is found to be more active towards the metal oxide surface rather than the –OH and C=O groups. The –SO₂ group of AR dye might interact with the metal surface through the weak secondary forces.³⁶

The surface morphology, size, and shape of Fe₃O₄ nanoparticles before and after surface functionalization reactions were analyzed by FESEM technique. Before the surface functionalization reaction, a nanospherical, and nanorod shape was observed (Figure 1i). The size of the sphere was calculated as 85 nm whereas the size of the nanorod was calculated as 300–500 nm. But after the surface functionalization reaction (Figure 1j), the nanorod shape of Fe₃O₄ disappeared and one can see a nanosphere-like morphology of Fe₃O₄ with the size of less than 50 nm. This indicates that after the surface functionalization reaction both the size and shape were altered. Due to the mild thermal force and mechanical force the size and shape of Fe₃O₄ were altered. Anbarasan et al³⁷ reported about the conversion of Au nanosphere into nanorod during the insitu polymerization N-isopropylacrylamide. In the present investigation, the observation is just reversed.

Characterization of PCL/Fe₃O₄–AR nanocomposite system

The surface catalytic effect of nanohybrid towards the ROP of CL was tested at different [M/I]. While increasing the [M/I] from 10–400 the corrected peak area of carbonyl stretching of PCL was increased. The FTIR spectrum of PCL synthesized at various [M/I] is given in Figure 2a–e. The important peaks are characterized below: a broad peak around 3500 cm⁻¹ explains the –OH stretching of PCL. The aliphatic C–H symmetric and anti-symmetric stretching was noted at 2860 and 2942 cm⁻¹, respectively. The C=O stretching of PCL was noted at 1717 cm⁻¹.³⁴ The ester C–O–C linkage of PCL was identified at 1185 cm⁻¹. The C–H OPBV could be seen at 729 cm⁻¹. The M–O stretching was noted at 448 cm⁻¹ as a small hump. From the FTIR spectrum it was found that, while increasing the [M/I], the ratio of FTIR-RI[C=O/C–H OPBV] was linearly increased. This confirms the ROP of CL by the nanohybrid. In order to find out the efficiency of the nanohybrid towards the ROP of CL a log–log plot was made between [M/I] and RI_(C=O/CH) (Figure 2f). This plot shows two regions. It means initially the ROP of CL was very fast and later it was found to be slow. While increasing the [M/I] the number of initiating sites were reduced. As a result the rate of ROP of CL was lowered. At the same time the molecular weight of the polymer was increased. From the plot, the slope value was determined as 0.25 which confirmed the 0.25 order of ROP CL with respect to the nanohybrid. (i.e.) R_(ROP) α [nanohybrid]^0.25. This indicates that 0.25 mol of initiator is required to form 1 mol of PCL. The order of the reaction proved the initiating efficiency of nanohybrid towards the ROP of CL. The MWCNT-NIR dye system exhibited 0.50 order of reaction towards the ROP of CL.³⁸ When compared to the literature; the present system exhibited a good result.

Figure 2 FTIR spectrum of PCL/Fe₃O₄–AR nanocomposites synthesized at [M/I] of a 10, b 50, c 100, d 200, e 400 and f plot of log(RI_[C=O/C–H]) vs. log[M/I]

The chemical structure of PCL was confirmed by NMR spectroscopy. Figure 3a represents the ¹H-NMR spectrum of PCL/Fe₃O₄–AR nanocomposite system. Peaks corresponding to standard TMS and solvent appeared at 0 and 7.3 ppm, respectively. A signal at 4.2 ppm is corresponding to the –OCH₂ proton. This is in accordance with our earlier publication.³⁹ The –CO₂–CH₂ proton signal appeared at 2.4 ppm. The remaining methylene protons of PCL appeared between 1 and 2 ppm. The peak integration value confirmed the chemical structure and composition of PCL. Peaks corresponding to AR dye are not appeared due to non-solvation effect or lesser amount of AR dye used as an initiator (M/I = 100). The structure of PCL was further confirmed by ¹³C-NMR spectroscopy (Figure 3b). Solvent peak appeared at 78 ppm. A signal due to the carbonyl carbon appeared at 173 ppm.²⁸ Here, the aromatic and aliphatic carbonyl carbons are merged with each other and appeared as a single peak. Moreover, the quantity of AR taken was low (M/I = 100). The –OCH₂ and –CO₂–CH₂ signals appeared at 62 and 34 ppm, respectively. The methylene carbon signals appeared between 20 and 30 ppm. Thus, the NMR spectra confirmed the chemical structure of PCL.

Figure 3 A ¹H-NMR and B ¹³C-NMR spectrum of PCL/Fe₃O₄–AR nanocomposite synthesized at [M/I] = 100

The UV–visible spectrum was recorded for PCL, synthesized at various [M] is shown in Figure 4a–e. The spectrum shows a peak at 446.4 nm. It was found that while increasing the [M], the absorbance at 446.4 nm was found to be reduced. This can be explained as follows: While increasing the [M] the number of monomers were increased at constant initiator concentration. The number of initiator shared by the monomer was reduced. As a result, the absorbance at 446.4 nm was reduced. Using the UV–visible spectroscopy one can find out the binding constant by plotting 1/[CL] vs. A₀/A–A₀. Figure 4f indicates the same with a linear plot. The binding constant was determined from the [I/S] as 0.28 × 10⁵ M⁻¹. The binding constant value again confirms the existence of strong interaction between AR dye and PCL units. In comparison with the literature,³⁴ we found that while changing the size and shape of the nanohybrid system, automatically the binding constant was changed.

Figure 4 UV–visible spectrum of PCL/Fe₃O₄–AR nanocomposites synthesized at [M] of a 0.50 g, b 1.5 g, c 2.0 g, d 2.5 g, e 3.0 g and f plot of 1/[CL] vs. A₀/[A−A₀]

While using a dye it is very important to find out the fluorescence property. Fluorescence emission spectrum was recorded for PCL samples synthesized at various [M]. It was found that while increasing [M], the FEI was decreased and the spectrum is shown in Figure 5a–e. This is due to the increase in [M], ultimately quenches the FEI of AR dye. In order to find out the number of binding sites involved in the ROP of CL and binding constant, the plot of log(weight of monomer) vs. log[I₀−I/I] (Figure 5f) was made and the slope value was determined as 1.06 and the intercept value was noted as 2.64. The slope value is the binding constant, whereas intercept value indicates the number of binding sites. This is in accordance with our earlier publication.³⁴ In the case of AR dye; 2–OH and 1–SO₃Na groups are available. During the ROP of CL, the –OH group was actively involved in the ROP of CL.³⁴ The –SO₃Na group was binded with ferrite through the physical forces. This physical force is very weak and it is cleaved when it is heated to higher temperature. Now the –SO₃Na group is attached with the PCL chain via secondary forces of attraction. The involvement of these 3 functional groups towards the ROP of CL was confirmed by FE spectroscopy.

Figure 5 Fluorescence emission spectrum of PCL/Fe₃O₄–AR nanocomposites synthesized at [M] of a 0.50 g, b 1.5 g, c 2.0 g, d 2.5 g, e 3.0 g and f plot of log(weight of monomer) vs. log[I₀−I/I]

The phase transition of PCL during heating was analyzed by DSC. Figure 6a–e indicates the DSC thermogram of PCL synthesized at various [M/I]. While increasing the [M/I], the T_m of PCL is also proportionally increased. The T_m value was varied between 60.6 and 63.9 °C. The increase in T_m is due to the increase in molecular weight of PCL.³⁴ Another one important point noted here is while increasing [M/I] the area of the melting peak is also increased. This confirms the increase in molecular weight of PCL while increasing the [M/I] variation. The molecular weight of PCL is discussed in the forth coming session. The thermal stability of PCL/Fe₃O₄–AR nanocomposite systems was studied by TGA at the heating rate of 10 °C/min under nitrogen atmosphere (Figure 6f–j). Generally, the thermogram exhibited a two-step degradation process. The first minor weight loss below 200 °C is associated with the removal of moisture and cleavage of dye units from the PCL backbone. The second major weight loss around 310 °C is ascribed to the degradation of PCL backbone. The interesting point noted here is after the major degradation, the % weight residue remained above 400 °C is increased with the decrease of [M/I] ratio (i.e.) increase of Fe₃O₄–AR nanohybrid concentration. This confirmed that the nanohybrid was effectively initiated the ROP of CL under nitrogen atmosphere at 160 °C. Thus, both DSC and TGA explained the thermal properties of the PCL/Fe₃O₄–AR nanocomposite.

Figure 6 DSC thermogram a–e and TGA thermogram f–j of PCL/Fe₃O₄–AR nanocomposites synthesized at [M/I] of a f 10, b,g 50, c,h 100, d,i 200 and e,j 400

In order to confirm the ROP of CL in the presence of Fe₃O₄–AR nanohybrid system the GPC analysis was used. Figure 7a and b indicates the GPC images of PCL synthesized at the [M/I] of 10 and 400, respectively. When the [M/I] was maintained at 10, the M_w, M_n, and polydispersity (PD) values were determined as 7535 g/mol, 3717 and 2.02, respectively. The PCL synthesized at the [M/I] value of 400, the M_w, M_n, and PD values were determined as 16412 g/mol, 9067 and 1.81, respectively. This indicates that while increasing the [M/I] ratio, the M_w of the PCL was increased. This is due to the decrease in initiator concentration (i.e.) the number of initiating centers were reduced. The PD inferred that at higher [M/I] ratio, the polymer yielded a narrow molecular weight without any branching or cross linking. Hence, the GPC results proved that the Fe₃O₄–AR nanohybrid is an effective initiator towards the ROP of CL. This is in accordance with the literature report.³⁴

Figure 7 GPC traces of PCL/Fe₃O₄–AR nanocomposite systems synthesized at a [M/I] = 10 and b [M/I] = 400

Figure 8 shows the VSM loop of PCL/Fe₃O₄–AR nanocomposite system synthesized at [M/I] = 100. The VSM value was determined as 24.57emu/g. The VSM value of pristine Fe₃O₄ and surface-functionalized Fe₃O₄ were greater than that of the present system. The decrease in VSM value is due to the encapsulation of Fe₃O₄ by PCL chains. In our earlier publication.³⁴ we explained the decrease in VSM after the insitu polymerization by four factors. The same four factors are suitable for the present investigation too.

Figure 8 VSM loop of PCL/Fe₃O₄–AR nanocomposite synthesized at [M/I] = 100

Figure 9a shows the FESEM image of PCL/Fe₃O₄–AR nanocomposite synthesized at [M/I] = 100. The broken stone like morphology confirms the presence of PCL. The nanosized Fe₃O₄ was dispersed on the PCL backbone. The size of Fe₃O₄ nanoparticles was determined as approximately 25 nm. This is definitely smaller than the size of pristine Fe₃O₄ and nanohybrid system. The decrease in particle size in the present system is associated with the effective utilization of high thermal energy. Hence, this shows that thermal treatment is one of the ways to produce nanosized materials. The HRTEM image of PCL/Fe₃O₄–AR nanocomposite system is given in Figure 9b. The image indicates the presence of both agglomerated and non-agglomerated Fe₃O₄ nanoparticles. The size of Fe₃O₄ is varied between 10 and 40 nm. The darkened area confirmed the agglomerated Fe₃O₄ nanoparticle. Thus, the HRTEM image confirmed the dispersion of Fe₃O₄ nanoparticle on the surface of PCL.³⁴

Figure 9 a FESEM and b HRTEM images of PCL/Fe₃O₄–AR nanocomposite synthesized at [M/I] = 100

Figure 10 shows the size of Fe₃O₄ nanoparticles, I indicate the size of pristine Fe₃O₄, II indicates the size of Fe₃O₄ in nanohybrid and III indicates the size of Fe₃O₄ in PCL/Fe₃O₄–AR nanocomposite system. The nanocomposite preparation not only reduced the size but also controlled the shape of Fe₃O₄ nanoparticles.³⁷

Figure 10 Plot of System vs. Size of Fe₃O₄

The drug release activity of PCL nanocomposite was studied in the presence of Rif. as a model drug. The drug release study was carried out at gastric pH and the same was confirmed by UV–visible spectroscopy. It was found that while increasing the time the λ_max value at 262.7 nm was increased (Figure 11a–h). Various drug release models like zero-order reaction, first-order reaction, Higuchi model, Korsmeyer and Peppas model and Hixson model were tried among which the first-order reaction exhibited the maximum R² value of 0.91. Figure 11i indicates the plot of log (%CDR retained) vs. Time. This indicates that the drug was slowly released into the medium via dissolution process in the gastric pH condition. The drug release mechanism was determined by using Korsmeyer and Peppas model (Figure 11j). From the plot, the slope value was calculated as 0.492 which is less than n (i.e.) 0.89. This confirms the non-Fickian transport mechanism. In our previous communication, we reported that the folic acid conjugated PCL indicated the Fickian-type mechanism,³⁸ but in the present system, the drug is not chemically conjugated with PCL.

Figure 11 a–h UV–visible spectrum of Rif. released at different interval of time and i plot of (Time) vs. (log(% CDR retained) and i plot of log(Time) vs. log(%CDR)

For the sake of comparison, the drug release mechanism of PCL nanocomposite was carried out by considering AR dye as a model drug. Here, also various drug models were tried. The UV–visible spectrum was recorded for different interval of time (Figure 12a–l). Among the models, the Korsmeyer and Peppas model exhibited the maximum R² value of 0.97. Figure 12m indicates the plot of log (%CDR) vs. log (Time). The slope value of the Korsmeyer and Peppas model was determined as 0.29, which is less than 0.5. Again this confirms the non-Fickian transport mechanism of drug release. This study indicates that the drug release followed the non-Fickian transport mechanism while using PCL as a drug carrier and the drug is chemically conjugated or not.

Figure 12 a–l UV–visible spectrum of model drug released at different time interval and m plot of log(Time) vs. log(% CDR)

Scheme 1 Synthesis of PCL/ Fe₃O₄ –AR nanocomposite

Conclusions

From the above kinetic study the niche points are presented here as conclusion. The initiating capability of nanohybrid was tested by FTIR-RI method and further co-supported by 0.25 order of reaction with respect to [M/I]. The binding constant was determined from UV–visible spectroscopy as 0.28 × 10⁵ M⁻¹. The number of binding sites was determined as 2.64 from FE spectroscopy. The T_m value of PCL was increased with the increase of [M/I]. The VSM value was found to be reduced after the nanohybrid and nanocomposite formation through the surface functionalization and encapsulation reactions, respectively. The size of Fe₃O₄ nanoparticle was also reduced after the nanohybrid and nanocomposite formation. The drug release followed the non-Fickian transport mechanism. The present investigation inferred that one can reduce the size of the material via thermal and mechanical treatment. In overall comparison, the present investigation yielded somewhat better results when compared with the literature particularly with the GPC results.

Disclosure statement

No potential conflict of interest was reported by the authors.

Acknowledgment

Mrs. G. Vijaylakshmi, Assistant Professor, Department of English is gratefully acknowledged for her valuable help during this manuscript preparation work.