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DOI: 10.1039/C5RA05508C (Paper) RSC Adv., 2015, 5, 59903-59911

Development of smart leathers: incorporating scent through infusion of encapsulated lemongrass oil†

Punitha Velmurugana, Nishad Fathima Nishtera, Geetha Baskarb, Aruna Dhathathreyanc and Jonnalagadda Raghava Rao*a
aCouncil of Scientific and Industrial Research – Central Leather Research Institute, Chemical Laboratory, Adyar, Chennai 600 020, India. E-mail: jrrao@clri.res.in; Fax: +91 44 24911589; Tel: +91 44 24411630
bCouncil of Scientific and Industrial Research – Central Leather Research Institute, Industrial Chemistry Laboratory, Adyar, Chennai 600 020, India
cCouncil of Scientific and Industrial Research – Central Leather Research Institute, Biophysics Laboratory, Adyar, Chennai 600 020, India

Received 28th March 2015 , Accepted 22nd June 2015

First published on 22nd June 2015

Abstract

Fragrant nanospheres were formed by an emulsion polymerization technique using chitosan and acrylic acid as a wall material. Several parameters such as the ratios of lemongrass oil[thin space (1/6-em)]:[thin space (1/6-em)]surfactant and chitosan[thin space (1/6-em)]:[thin space (1/6-em)]acrylic acid, and relative lemongrass oil content in the nanospheres were analysed. Subsequently, the physico-chemical characteristics of nanospheres were characterized using FT-IR, dynamic light scattering, and SEM. The results showed that nanospheres were formed using ratios of lemongrass oil[thin space (1/6-em)]:[thin space (1/6-em)]Triton X-100 at 1[thin space (1/6-em)]:[thin space (1/6-em)]1, and of chitosan[thin space (1/6-em)]:[thin space (1/6-em)]acrylic acid at 1[thin space (1/6-em)]:[thin space (1/6-em)]2. The nanospheres showed high oil loading (∼300%) and encapsulation efficiency (∼33%) and had an average size of 117 ± 11 nm. In addition to that, the antimicrobial activity of the nanospheres was tested against bacteria (Bacillus subtilis, Bacillus cereus) and fungi (Rhizoctonia solani, Macrophomina phaseolina, Aspergillus fumigatus). Further, the application of nanospheres loaded with lemongrass oil in leather processing was optimized (stage of addition and percentage offer). Physico-chemical characteristics such as physical strength, morphology, washability, perception evaluation, water diffusion and organoleptic properties were investigated. The results revealed that the nanospheres were embedded in the leather matrix and that their fragrance persisted after washing with water and solvent. These nanospheres act as good delivery vehicles for the manufacture of fragranced leather and will add economic value to the leather.

Introduction

Leather, when used as a material for apparel and upholstery, has an edge over synthetic materials owing to its uniqueness. In today’s world, people expect materials to exhibit more than one property. In order to meet these changing demands, attempts are being made to develop “smart leather”.1,2 Smart leather is a leather that responds to an external stimuli as well as performing its regular function. In this present work, encapsulated oils have been incorporated into leathers such that controlled release of scent oils is achieved in order to make the leather scented. With the new trend of imparting pleasant smells, one can hope to satisfy psychological nodes of perception, feel and smell, in leather garments, leatherwear, upholstery and fancy leather etc., as a new means of adding value. This scent infused leather will increase the economic value of the product.

The essential oil extracted from plant sources consists of terpenes, sesquiterpenes and terpenoid hydrocarbons. These have potential applications in various fields, such as fragrances in cosmetic industries, aromas in the food industry and as antimicrobial agents in medicinal fields to name a few.3,4 The essential oils in these scent oils are highly volatile in nature due to the aromatic groups present in their hydrocarbon chain.3 The application of scent oil in leather as such, is impossible as they would evaporate rapidly due to their high volatility. In order to control their release, scent oils are microencapsulated using a biopolymer, such that they can be retained for a prolonged time. Lemongrass oil, which is one of the essential oils, is extracted from the plant Cymbopogon citratus. This oil has excellent antimicrobial activity against bacillus species,5 and therefore is widely used as a preservative and for medicinal applications. Lemongrass oil has antimicrobial activity against Candida albicans, and therefore can be used to treat the infections caused by Candida albicans.6

Microencapsulates are tiny particles surrounded by a coating to give a small capsule. The material inside is referred to as core or active material and the material outside is the wall, shell or membrane. Microcapsules are defined as particles, spherical or irregular, in the size range of about 50 nm to 2000 μm or larger, and are composed of an excipient polymer matrix (shell or wall) and an incipient active component (core substance), and can be used for controlled release of active components under specific conditions.7 Microcapsular materials encompass a wide variety of products including paper coating, pressure sensitive adhesives, detergent components, detoxifying agents, implantable hormonal cells, sprayable agricultural aids, magnetic contrast agents, and controlled release medication.8 Chitosan, a copolymer of n-glucosamine and N-acetyl glucosamine, has one primary amino and one free hydroxyl group in its carbon chain. This composition induces a cationic charge in the chitosan. This makes chitosan a suitable biopolymer for interaction with anionically charged surfaces. With the above parameters, the degree of deacetylation and molecular weight range determines chitosan’s application for the controlled release of the encapsulated materials or solid forms.9,10

This study aims for the incorporation of the lemongrass oil into the leather and through microencapsulation using chitosan, enabling controlled release of the lemongrass oil. The chitosan and collagen interaction establishes electrostatic and hydrogen bonding, which retains the encapsulated scent oils by these strongly adhering to the collagen. In addition, this improves the mechanical properties of the leather by imparting softness to the leather.11 Chitosan has potent applications in leather processing as a retanning agent12 and an antimicrobial coating,13 and has the ability to act as an efficient shell material for the encapsulation of the scent oils. These are the reasons for it to be chosen in this study.

Materials and methods

Materials

Lemongrass oil was purchased from Aromax trading corporation, Chennai, India. The chemicals such as chitosan, non-ionic surfactant, acrylic acid, and potassium persulphate were purchased from Sigma. Except the leather chemicals, all other chemicals used were of analytical grade. Leather chemicals were purchased from BASF, Colourtex, Clarient and Stahl Pvt. Ltd, India.

Methods

Microencapsulation of lemongrass oil. Chitosan was suspended in acrylic acid in various ratios 1[thin space (1/6-em)]:[thin space (1/6-em)]0.5, 1[thin space (1/6-em)]:[thin space (1/6-em)]1, 1[thin space (1/6-em)]:[thin space (1/6-em)]2, and vortexed in order to dissolve it. The optimized ratio in which chitosan is completely dissolved in acrylic acid was taken for the encapsulation of the lemongrass oil (Table S1 ESI †). Lemongrass oil was dispersed as fine droplets in water under constant stirring. The lemongrass oil to non-ionic surfactant Triton X-100 ratio was varied from 10 to 1 for the formation of stabilized emulsions. The light barrier properties, or light transmittance, of the emulsions were measured by exposing the emulsions to light at a wavelength of 600 nm using a spectrophotometer (Shimadzu). Emulsions were transferred to a glass cuvette with a 1 cm path length and the transmittance of the emulsions was measured relative to that of a reference cell containing distilled water at 25 °C and measured at different time intervals.14 To avoid the effect of differences in the initial transmittance of samples, the results are presented as the ratio between values measured at different times with the initial transmittance (normalized values).

In order to encapsulate the emulsion, the optimized 1[thin space (1/6-em)]:[thin space (1/6-em)]2 chitosan–acrylic acid mixture was added to the emulsion (oil[thin space (1/6-em)]:[thin space (1/6-em)]surfactant 1[thin space (1/6-em)]:[thin space (1/6-em)]1) with ammonium persulphate added to it to initiate the polymerization process. The entire reaction was carried out at 55 ± 1 °C under continuous stirring for 8 hours in order to achieve the effective encapsulation of lemongrass oil. After the completion of reaction, the internal temperature of the round bottom flask was lowered to 25 ± 1 °C and the encapsulated product was transferred to falcon tubes, and further freeze dried at −40 °C for 24 hours after which lyophilisation was done and the product was stored at 4 °C for further analysis.

Characterization of emulsion and encapsulated lemongrass oil.
Determination of the oil load, oil content and encapsulation efficiency. A known amount of lyophilized sample was crushed and dissolved in 1% Triton X-100 in distilled water. The solution was stirred and vortexed to ensure complete extraction of oil in Triton X-100 solution. Percentage oil load, oil content and encapsulation efficiency was determined using a calibration curve through extrapolating the absorbance values of the unknown concentration of the product. The preparation of the calibration curve for lemongrass oil is described below: known and sequential concentrations of lemongrass oil were dissolved in 1% Triton X-100 in distilled water and scanned in the ultraviolet range using UV-Vis spectrophotometer. The calibration curve was plotted by absorbance versus concentration at the λmax = 245 nm. Percentage oil load, oil content and encapsulation efficiency was calculated using the following equation.15,16 All the experiments were carried out in triplicate.
image file: c5ra05508c-t1.tif

image file: c5ra05508c-t2.tif

image file: c5ra05508c-t3.tif
where, W is the weight of the nanospheres; W1 is the actual amount of lemongrass oil encapsulated in a known amount of nanospheres; W2 is the amount of lemongrass oil introduced in the same amount of nanospheres; W3 is the total amount of polymer used including the cross linker.
Surface tension and interfacial tension measurements using tensiometry. Surface tension and interfacial tension measurements for the oil and product were performed on a GBX 3S tensiometer from France, employing a platinum Du Nuoy ring probe with an accuracy of 0.1 mN m−1 and standardised with Milli-Q water. All measurements were performed at 25 ± 0.1 °C in triplicate. Surface tension measurements and the interfacial tension measurements at the hexane/water interface were performed on lemongrass oil and also on the formulated emulsion.
Dynamic light scattering (DLS) measurements. The hydrodynamic size and charge of the emulsion and nanospheres were determined using dynamic light scattering (Zetasizer nano, Malvern instruments U.K.) at 25 °C. The prepared encapsulated products (in solution form) were diluted in Milli-Q water before the experiments. All the experiments were performed in triplicate and an average was taken.
FT-IR spectral analysis. Fourier transform infrared (FT-IR) spectra of the lyophilized encapsulated lemongrass oil in nanospheres were recorded using an ABB MB 3000 FT-IR spectrometer at room temperature. The spectra were taken at 4 cm−1 resolution, and averaged over 31 scans in the range of 4000–650 cm−1. Before taking the spectrum, the samples were mixed with potassium bromide (IR grade KBr was used as scanning matrix) to make nearly transparent and homogeneous pellets. The final spectra were collected after subtracting background spectra.
Optical, scanning (SEM) and transmission electron microscopic (TEM) analysis. The emulsion and encapsulated product were mounted on a glass slide and viewed under an optical microscope. The optical micrographs of the emulsified and encapsulated lemongrass oil were obtained with the help of a camera attached to the microscope. The surface morphology of microencapsulated product was investigated by scanning electron microscopy (Quanta series 200). The microencapsulated product was coated onto a glass plate for the formation of film under controlled drying. The sample coated glass plate was then mounted on the metal stubs using an adhesive. After being vacuum-coated with a thin layer (100–150 Å) of gold, the formulated microemulsion products were analysed by SEM at 12 kV. Encapsulation of lemongrass oil in chitosan and acrylic acid wall material has been confirmed using TEM. A sample was placed on a carbon coated copper grid and allowed to dry at room temperature for two hours prior to imaging. TEM images of encapsulated product were obtained using a Hitachi H-7650 at an acceleration voltage of 80 kV at room temperature.
Application of encapsulated lemongrass oil into leather. Wet blue sheep skins of 4 sq. ft. with 1 mm thickness were used for application studies. The product was applied into leather based on the shaved weight. The leathers were shaved, washed and neutralised to a pH of 5.5–6. In order to optimize the offer stage in leather processing, the encapsulated product was applied during various post-tanning operations such as after neutralization (in fresh bath), re-tanning, fat-liquoring and before fixing. In order to optimize the percentage offer of the encapsulated product in post-tanning operations after neutralization, various percentages from 1% to 6% were used (Table S2 ESI†). The control (without scent infusion) and experimental (lemongrass oil infused leathers) garment leathers were produced. The leather was rinsed, and piled overnight. The next day it was set, hooked for drying, staked, buffed and trimmed, and subsequently the final leathers were subject to evaluation. The detailed post-tanning process recipes for the control and experimental leathers were tabulated in Tables S3 and S4 (ESI†). The final leathers were evaluated for their colour, organoleptic and strength properties.
Characterization of encapsulated lemongrass oil infused leather.
Perception evaluation. The ultimate test for the product is performance in the hands of the user. It is necessary, often by utilizing a panel of people, to take a properly dried fragranced sample and let the users test it. Testing the intensity of scent was carried out through a perception method using evaluators, where the evaluation was carried out according to the following: the evaluators were asked to sense the smell and to give a rating based on the scale presented in Table 1. The intensity of scent has been indicated with the “degree of intensity” number. Since the degrees have been denoted with a numerical value, responses that varied from the sentences provided were allocated to the closest degree value.
Table 1 Perception evaluation for lemongrass oil infused leather
Intensity of fragrance emission by leather Degree of intensity
Unperceived 1
Perceived 2
Clear perception with no feeling of discomfort 3
Perception with feeling of discomfort 4
Perception with a strong feeling of discomfort 5
Unbearable 6


Physical testing and evaluation of organoleptic properties. This test was carried out using INSTRON Series IX to check whether the microencapsulated lemongrass oil induced any changes to the physical properties like tensile strength and stitch tear strength in the leather. Samples for various physical tests from experimental and control crust leathers were obtained as per the IUP method.17 Specimens were conditioned at 20 ± 2 °C and 65 ± 2% relative humidity (R.H.) over a period of 48 hours. Physical properties such as tensile strength and tear strength were examined as per the standard procedures.18,19 Each value reported is an average of four (2 along and 2 across the backbone) measurements. Further, the leathers were assessed for fullness, softness, grain tightness, grain smoothness, and colour uniformity, by standard hand and visual evaluation techniques and ratings were given on a scale of 1–10.
Softness. The softness of the control and experimental leather was measured using a MSA ST 300 digital leather softness tester supplied by MSA Engineering system limited. The method permits measurements of softness of leather without defacing the sample. The method of measurements were made as per IUP 36.20 Prior to softness measurements the leathers were conditioned. Measurements were performed on locations specified under IUP 2.17 The measurements were performed on locations using a 35 mm ring at 20 ± 2 °C, with a relative humidity of 65 ± 2% and with a thickness of leather of 0.8 mm. Higher values indicate higher softness.
Washability test in water and solvent. A known weight of leather is taken and put in a beaker containing 10 mL of water and dry cleaning agent (perchloroethylene). It is allowed to remain in water for a period of 5 minutes. It is then checked for the retention of fragrance after drying. This is repeated about 3 times.
Determination of contact angle and water diffusion coefficient. To evaluate the surface properties of the control and experimental leather samples, contact angle measurements were carried out using home built contact angle equipment. In this experiment, 5 μL of water and hexadecane were placed on the surfaces of samples, measurements were made and contact angle and surface energies were calculated.

Here, the assumption is that a decrease in the diffusion coefficient would indicate the coverage of pores by the microencapsulated particles. Based on this, a sample that has maximum penetration and blockage of pores was determined. Samples of leather with similar length, thickness and shape, including a control sample, were taken. Their dry weights were recorded accurately. Then they were placed in beakers containing water. The wet samples of leather were weighed at periodic intervals. Each time before measuring the weights, the surface water was gently removed by using tissue paper. From these measurements, the water uptake (%) can be calculated using the following formula:

image file: c5ra05508c-t4.tif
where, Wt is the weight of the sample at any time; Wo is the dry weight of the sample; W∞ is the weight of the sample at infinity. A time versus water uptake (%) graph was plotted for the samples.

Diffusion coefficients for each sample were calculated using the formula,

image file: c5ra05508c-t5.tif
where, ρ is the density.


Scanning electron microscopy. Samples of uniform thickness from the control (C) and experimental (E) leathers were cut from the official sampling position,17 and taken directly, without any pre-treatment, for analysis using a Quanta 200 series. Scanning electron microscopy of the grain surfaces and cross sections were obtained by operating at low vacuum and an accelerating voltage of 12 kV with different magnification levels.
Antimicrobial activity of the encapsulated lemongrass oil.
Antibacterial activity. The antibacterial activity was measured by a standard well disc diffusion method on Mueller-Hinton agar (MHA). 100 μL of the bacterial culture was inoculated into MHA and wells were created using sterile cork borer. Different concentrations of the encapsulated oils were loaded into the wells and the plates were incubated at 37 °C for 24 h. Diameters of the inhibition zones were measured in mm.
Antifungal activity. Three mycelial fungi Rhizoctonia solani, Macrophomina phaseolina, and Aspergillus fumigates were used to assess in vitro antifungal activity. Each agar plug loaded with fungus was placed onto potato dextrose agar. The wells were made using a sterile cork borer (5 mm). The wells were then loaded with encapsulated oil samples of concentrations 25, 50, 75 or 100 mg mL¬1. The plates were incubated at 35 °C and observed after 48 and 72 h. The inhibition zones were read at the point of complete inhibition of growth (ZOI).

Results and discussion

Transmittance measurements to standardise the proportions of oil[thin space (1/6-em)]:[thin space (1/6-em)]surfactant

Oil and water are immiscible due to high interfacial energy, which results in phase separation. Hence, it becomes essential to reduce the interfacial energy between water and oil to form a dispersion. Coverage of oil by non-ionic surfactant can decrease the interfacial energy to as low as 5 mN m−1. The particles could be stopped from coagulating by surrounding them with surfactant. The charge on the surfactant repels other particles electrostatically. The main function of the surfactant in microemulsion polymerization was the isolation of nano droplets of oil by micelle formation, thus preventing phase separation. Micelles only form when the concentration of surfactant is greater than the critical micelle concentration (CMC). Hence, it is essential to optimize the amount of surfactant used. Different ratios of surfactant[thin space (1/6-em)]:[thin space (1/6-em)]oil ranging from 0.2[thin space (1/6-em)]:[thin space (1/6-em)]1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]1 with increasing order of 0.2, and the higher ratio 2[thin space (1/6-em)]:[thin space (1/6-em)]1 were tested to identify the critical concentration required to produce a homogenized dispersion (i.e. complete coverage of droplets lowering the interfacial energy). Transmittance measurements were taken for the series of solutions containing various proportions of non-ionic surfactant at ratios 0.2, 0.4, 0.6, 0.8 and 1[thin space (1/6-em)]:[thin space (1/6-em)]1. The results are tabulated in Table 2. The higher the transmittance percentage, the greater the transparency of the emulsion, which implies that the particle sizes are smaller. From the table it is observed that a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio has a higher transmittance value compared to the other compositions indicating that the solution is less cloudy than the other ratios and provides better dispersion. Also, concentrations of non-ionic surfactants greater than 1[thin space (1/6-em)]:[thin space (1/6-em)]1 gave good dispersion. 1[thin space (1/6-em)]:[thin space (1/6-em)]1 has been taken as the optimized concentration because excess surfactant can result in foaming, and additionally, greater concentrations of surfactant cannot be used in the formulation required here (lemongrass oil + water + surfactant), because it will result in the migration of oil without encapsulation.
Table 2 Transmittance values of various ratios of oil to surfactant
Oil[thin space (1/6-em)]:[thin space (1/6-em)]non-ionic surfactant Transmittance (%)
1[thin space (1/6-em)]:[thin space (1/6-em)]0.2 0.02
1[thin space (1/6-em)]:[thin space (1/6-em)]0.4 0.02
1[thin space (1/6-em)]:[thin space (1/6-em)]0.6 0.02
1[thin space (1/6-em)]:[thin space (1/6-em)]0.8 0.03
1[thin space (1/6-em)]:[thin space (1/6-em)]1.0 71.93

Encapsulation of lemongrass oil through emulsion polymerization

An emulsion polymerization technique has been adopted to encapsulate the emulsified lemongrass oil because it is one of the fastest methods and is readily scalable. Chitosan and acrylic acid act as a wall material for the emulsified oil droplets and the reaction completion has been confirmed by the formation of a clear solution (from an emulsion) in water. Encapsulation using chitosan provides a shell which reduces the volatility of oil and may help in sustained release. The oil loading (%), oil content (%), and encapsulation efficiency (%) of lemongrass oil in nanospheres are 299.39 ± 2.31%, 91.77 ± 1.16% and 32.66 ± 1.13%, respectively.

Determination of interfacial energy of lemongrass oil

The density of oils is almost equal to 1, which indicates that the fluidity of oils is similar to water (ρ = 1). Miscibility of oils in water is not influenced by density. Interfacial energy is an indication of the hydrophobic or hydrophilic nature of any material. If the material has low interfacial energy it indicates the presence of hydrophilic groups. The interfacial energy for the lemongrass oil has been measured and it was observed that lemongrass oil has an interfacial energy of 11.15 mN m−1. The interfacial energy is reduced by the wall material, which indicates that lemongrass oil may have high hydrophobic interaction with chitosan and stability of the encapsulation increases with an increase in the interaction between the oil and wall material.

Determination of surface tension and particle size

Surface tension analysis was done to analyse whether the oil has been encapsulated or not and measurements are tabulated in Table 3. The surface tension value of the product is found to be less than that of the lemongrass oil. The decrease in the surface tension is due to the presence of surfactant. The surfactant, Triton X-100 reduces the surface tension between the water and oil. Therefore, molecules of water are no longer in the state of cohesion and this leads to the formation of tiny droplets.
Table 3 Characteristics of lemongrass oil and encapsulated product
S. no. Characteristic study Lemongrass oil Encapsulated product
1 Density (g mL−1) 0.898 —
2 Interfacial energy (mN m−1) 11.15 —
3 Surface tension (mN m−1) 29.5 28.3
4 Average particle size (nm) — 102.1
5 Concentration of essential oil (μM L−1) 21.79 6.06

Particle size measurement was carried out to analyse the size of the particles and also to evaluate the homogeneity of the emulsion product. The mean particle size of nanospheres was measured and the results showed that the diameter is in the range of 100–270 nm. The average diameter (intensity based harmonic mean) of the nanospheres is 117 ± 11 nm. These nanospheres may block the pores of the fibril level, which has a pore size of 50–500 nm. Interaction of these nanospheres with the leather matrix could be through hydrogen bonding with hydroxyl groups of chitosan. Particles are distributed between 5–10 nm and 100–500 nm (figure given in ESI, Fig. S1 †). This indicates that nanosphere formation is not in a homogenized manner, but small spheres are preferred because they can fit into the interspaces present in the leather matrix and can be easily adsorbed on the leather matrix. Variation in particle size can accommodate various levels of pore size presented in collagen matrix/skin. Nanospheres with diameters between 5 and 10 nm can penetrate up to micro fibril level but intensity of the particle presence is comparatively low. Particles with diameters between 100 and 500 nm can penetrate up to sub fibril and facile level pores. The concentrations of essential oil in the lemongrass oil and formulated product have been listed in Table 3. This shows the high essential oil content of the lemongrass oil.

Oxidation index measurements

Oxidation of oils is expected to be accelerated through a free radical mechanism.22 However, in the product, oil was encapsulated within the solid matrix and the oxygen could only reach the oil by permeating through the solid wall matrix. Oxygen permeability depends both on solubility and diffusion. Surfactant content is an important factor affecting both solubility and diffusion and therefore oxygen permeability.22

Optical, scanning and transmission electron microscopy of product

The optical micrographs of product show that the encapsulated products are polydispersed and uniformly distributed as the tiny droplets surrounded by the wall material; this can be distinctly observed in Fig. 1.
image file: c5ra05508c-f1.tif
Fig. 1 Optical microscopy images of the (a) encapsulated and (b) emulsion of lemongrass oil.

Scanning electron micrographs were taken to analyze the surface morphology of the sample and to analyse the size and shape of the microspheres. Micrographs were taken at different magnifications of 500×, 1000× and 4000×. The scanning electron micrographs of encapsulated lemongrass oil at different magnifications have been presented in Fig. 2. The micrographs of lemongrass oil encapsulated product show that the surface morphology is smooth and most of the spheres are in circular shape with some of them irregular in shape with less than 5 μm size.


image file: c5ra05508c-f2.tif
Fig. 2 Scanning electron micrographs of encapsulated product with different magnifications of (a) 4000×, (b) 1000× and (c) 500×.

Fig. 3 consists of the TEM pictures of nanospheres. These show that the wall material and oil can be differentiated visually by lighter and darker colour, with the nanospheres exhibiting a definite shell (darker colour) around the oil. Particle sizes of the nanospheres are less than 50 nm and are aggregated on the surface.


image file: c5ra05508c-f3.tif
Fig. 3 TEM images of the encapsulated product.

FT-IR analysis

An FT-IR spectrum of chitosan–acrylic acid encapsulate containing lemongrass oil is given in Fig. 4. Lemongrass oil contains the usual active components such as citral (geranial and neral) and essential oil (1–2% on a dry basis) as well as some additional unusual active components namely limonene, citronellal, β-myrcene and geraniol. There are 65 compounds found in the chemical composition of the essential oil of lemongrass.23 Lemongrass oil showed a strong absorption peak at a wave number of 1694 cm−1, which is the carbonyl stretching band of lemongrass oil; this is shifted to 1726 cm−1 in the case of the encapsulated product (Fig. 4). The peak at 1511 cm−1 and broad band at 3415 cm−1 can be assigned to NH bending of the NH3+ functional group present in the chitosan. The 1512 and 1457 cm−1 peaks can be assigned to the asymmetric and symmetric stretching vibration peaks of the carboxyl groups of poly acrylic acid.24 These results reveal that the encapsulated product contains lemongrass oil and the carboxylic groups of the acrylic acid and amino group of the chitosan which interact electrostatically during the polymerization. Furthermore, there is a shift in the characteristic peak of lemongrass oil, which indicates that there is an interaction between lemongrass oil and the wall material; this may help in retaining the fragrance in the encapsulated product.
image file: c5ra05508c-f4.tif
Fig. 4 FT-IR spectrum of the encapsulated product.

Application studies and leather characterization

These studies were primarily carried out to optimize the % offer and stage of incorporation of the synthesised product. Based on the exhaustion and release, it was found that 5% oil content offer on shaved weight after neutralization resulted in a better fragrance. This might be due to the fact that after neutralization, the leather matrix will have lot of free pore spaces for the encapsulated oils to go into. The control and experimental leathers were produced as stated in the experimental section.

Perception evaluation

The evaluators were asked to sense the smell and give rating based on the statements given in Table 1. A leather sample with rating of 3 is considered to be best as it indicates a nice fragrance without irritation, see Table 1. It has been observed that leathers scented with lemongrass feel wet when touched and that the degree of intensity of smell is 3.8.

Contact angle and diffusion coefficient

Both the control and the experiment leathers showed good hydrophilicity and lipophilicity suggesting that the scented oil has penetrated the leathers and does not reside on the surface. Contact angle measurements did not show difference in the surface properties between the control and experimental samples, though an indirect method to evaluate penetration of microencapsulated oils in to the samples needs to be developed.21 The calculated results of rate of water uptake (%) and diffusion coefficients are presented in Table 4 and its pictorial representation has been shown in Fig. 5. Assuming that the leather samples have similar pore size distribution, their diffusion coefficients can be compared. The results indicate that water diffusion is slower in lemongrass scented leather than in the control. Though absolute values of D do not have much significance, a comparison suggests the diffusion coefficient value of the lemongrass infused leather is better due to blockage of the pores by the microencapsulated particles. For the control leather sample, water takes a shorter time to penetrate, which means either the pores are not completely covered or these microemulsion particles in the pores are not very stable and oils evaporate faster.
Table 4 Diffusion coefficient, % water uptake and perception evaluation of leathers
Sample Rate of % water uptake Diffusion coefficient (m2 min−1) Degree of intensity
T = 10 min T = 25 min T = 45 min
Control 24.952 23.825 25.002 13.93  
Lemongrass 21.642 23.395 24.930 12.4 3.8


image file: c5ra05508c-f5.tif
Fig. 5 Pictorial representation of water diffusion inhibitions in fibrous packed porous matrix.

Washability test in water and solvent

A washability test with water and solvent has been carried out to analyse the perception of fragrance after washing. Three washings were made with intervals of five minutes. The fragrance was perceived in the encapsulated lemongrass oil incorporated leather. The perception of fragrance in the scented leathers after washing in water results are given in Table 5. When pH > 6, chitosan, which is a wall material, is insoluble in water.25 This insoluble nature may help to protect the lemongrass oil during water washing and storage. These results confirmed that the microencapsulated lemongrass oil has been released in a slow and sustained manner. Washability tests with solvent (trichloroethylene) have been carried out to analyse the perception of fragrance after dry cleaning, because trichloroethylene is a commonly used dry cleaning solvent. Three washings were carried out with five minute intervals. The fragrance intensity increased initially. After the second wash the fragrance was perceived in all the leathers just as before washing, except the orange oil infused leather because orange oil intensity is low.
Table 5 Fragrance perception test in water and solvent washing
Experimental leathers Water Solvent
Washing 1 Perceived High intensity perceived
Washing 2 Perceived Perceived
Washing 3 Perceived Perceived

Strength and organoleptic properties

The leather treated with scented encapsulate has been tested for physical strength characteristics to assess whether scent infusion had any effect on the strength characteristics of the leather and the results are tabulated in Table 6. The scent infused leather has been made for the application of leather goods and garments. The strength properties are comparable with, the standard values of the control leather.
Table 6 Organoleptic and physical strength properties of leather
Sample Control Encapsulated product
Tensile strength (kg cm−2) 264.72 ± 2.5 251.52 ± 4
Tear strength (N mm−1) 121.40 ± 3 115.80 ± 4
Fullness 7 8
Softness 9 8
Grain tightness 8 7.5
Grain smoothness 9 8
Colour uniformity 8 7

Leathers were also assessed by three experienced tanners for organoleptic properties through standard hand and visual evaluation techniques, and ratings were given on a scale of 1–10 for each property. Higher points indicate better properties, the results have been presented in Table 6. The organoleptic properties are comparable to the control leathers. The softness of the leather measured as described in the experimental section shows that the experimental leather has a slightly lower softness values with wet feel when compared with the control. From this it can be inferred that lemongrass oil infusion has a significant role in altering the softness properties of leather.

Scanning electron microscopic analysis of leathers

The scanning electron micrographs of scent infused leather show that the surface of the leather is clean and smooth even at high magnification (Fig. 6). The cross sectional view indicates that the microspheres are blocked in between the pores. The SEM micrographs agree with the product characterization results with respect to the compatibility of the product and also with the water diffusion coefficient measurements (Table 4) of the leather characterization.
image file: c5ra05508c-f6.tif
Fig. 6 Scanning electron micrographs of (a) control and (b) lemongrass oil infused (experimental) leather, with different magnification for surface and cross section.

In vitro antibacterial activity of product

The chitosan encapsulated lemongrass oil exhibited antimicrobial activity against bacteria and fungi (Table 7). The mechanism of inhibition may be due to the individual effect of chitosan and lemongrass oil or a combined effect. Both chitosan and lemongrass oil possess excellent antimicrobial activity as chitosan blocks the ion channel mechanism of the microbes forming an impermeable thick layer around the cells, and lemongrass oil has the ability to interact with microbial cells resulting in poly cation–anion interactions thereby leading to disruption of the cell wall and leakage of cellular constituents.26–28 One possible mechanism is that the low molecular chitosan invades the cell and inhibits the m-RNA synthesis thereby lysing the microbes.28,29 Chitosan has the ability to inhibit the growth of microorganisms such as bacteria and fungi. Chitosan dissolved in acidic media and acts as an excellent antimicrobial agent as it forms a thick layer by precipitation around the cell whose charge is neutral. As a consequence of this, the ion channel, which is considered to be essential for the living cell, is blocked by the thick layer of chitosan.30 Another possible mechanism is that the charge interaction between the low molecular weight chitosan and the microbial cells results in the inhibition of the DNA synthesis and leads to disruption of the cell wall and leakage of the intercellular matter.28 The lemongrass oil, which contains terpenes, has a potent effect on mitochondria and inhibits the respiration of the microbes. The essential oil predominantly consists of polyphenols, which have the ability to alter metabolism, affecting normal cell growth, and leading to death of the microorganisms.31 Hence, the combined actions of chitosan and lemongrass oil could exhibit great antimicrobial activity, which would be an added advantage to the end product.
Table 7 Antimicrobial activity of encapsulated product
Sample name Lemongrass oil product
Bacillus cereus Bacillus subtilis Aspergillus fumigatus
Concentration (mg mL−1) 25 50 75 100 25 50 75 100 25 50 75 100
Zone of inhibition (mm) 4 5 7 9 11 13 13 15.5 7 9 11 13

Conclusion

The present study attempts to produce scented leather using encapsulation as a technique to arrest the volatility of scent oils. Lemongrass oil was encapsulated in chitosan, and acrylic acid was used for the formation of nanospheres resulting in the smaller spheres with the diameter of 117 nm, which may have high preferential application in leather and textiles. This study demonstrated that the proper combination of surfactants and wall material for the encapsulation of lemongrass oil produces nanospheres. These nanospheres were diffused well in the leather matrix and deposited on the collagen fibres. Triton X-100 was used as emulsifier, which produces a stable emulsion with the ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v). The encapsulation has been made from stable emulsion formulations using an emulsion polymerization technique. The lemongrass oil in the nanospheres were well retained in the leather matrix after washing with water and solvent. The encapsulation process and scented products are low in cost, nontoxic, biocompatible, and biodegradable.

Acknowledgements

The authors thank CSIR for funding under the Supra institutional Project–S&T Revolution in Leather with a Green Touch (CSC0201). CSIR-CLRI Communication Code-1173.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05508c
This journal is © The Royal Society of Chemistry 2015
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