Review
Chitosan nanoparticle based delivery systems for sustainable agriculture

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Abstract

Development of technologies that improve food productivity without any adverse impact on the ecosystem is the need of hour. In this context, development of controlled delivery systems for slow and sustained release of agrochemicals or genetic materials is crucial. Chitosan has emerged as a valuable carrier for controlled delivery of agrochemicals and genetic materials because of its proven biocompatibility, biodegradability, non-toxicity, and adsorption abilities. The major advantages of encapsulating agrochemicals and genetic material in a chitosan matrix include its ability to function as a protective reservoir for the active ingredients, protecting the ingredients from the surrounding environment while they are in the chitosan domain, and then controlling their release, allowing them to serve as efficient gene delivery systems for plant transformation or controlled release of pesticides. Despite the great progress in the use of chitosan in the area of medical and pharmaceutical sciences, there is still a wide knowledge gap regarding the potential application of chitosan for encapsulation of active ingredients in agriculture. Hence, the present article describes the current status of chitosan nanoparticle-based delivery systems in agriculture, and to highlight challenges that need to be overcome.

Introduction

The biggest challenge faced by agricultural researchers is to produce sufficient quantity and quality of food to feed the ever increasing global population without degrading the soil health and agro-ecosystem. It has been estimated that global food production must increase by 70–100% by 2050 to meet the demand of the growing population explosion [1]. Agricultural production continues to be challenged by a large number of insect pests, diseases, and weeds accounting for 40% losses to the tune of US $2000 billion per year [2]. To manage these losses and enhance productivity, farmers are making excessive and indiscriminate use of agrochemicals which leads to deterioration of soil health, degradation of agro-ecosystems, residue problems, environmental pollution and pesticide resistance in insects and pathogens. Hence, there is an urgent need to change the manner in which we use agrochemicals. Changes can include (i) judicious deployment of pesticide and fertilizer, (ii) rapid and precise detection of pathogens and pests, as well as pesticides and nutrient levels, and (iii) promoting soil health by agrochemical degradation. In this context, nanotechnology has emerged as a technological advancement that can transform agriculture and allied sectors by providing with novel tools for the molecular management of biotic and abiotic stresses, rapid disease detection and enhancing the ability of plants to absorb nutrients or pesticides [3], [4], [5]. Besides this, nanobiotechnology can also improve our understanding of crop biology and thus can potentially enhance crop yields or their nutritional values. Nanosensors and nano-based smart delivery systems are some of the nanotechnology applications that are currently employed in the agricultural industry to aid with combating crop pathogens, minimizing nutrient losses in fertilization, improving crop productivity through optimized water and nutrient management as well as to enhance the efficiency of pesticides at lower dosage rates [6], [7]. Nanotechnology derived devices are also being explored in the field of plant breeding and genetic transformation [8], [9]. Table 1 describes some of the advancements made in the field of agricultural nanotechnology. Among all these advancements, encapsulating active ingredients, such as fertilizers, herbicides, fungicides, insecticides, and micronutrients in controlled release matrices is one of the most promising and viable options for tackling current challenges in the area of agricultural sustainability and food security in the face of climate change. It has been shown that encapsulation of active ingredients in nanoparticles enhances the efficacy of chemical ingredients, reducing their volatilization, and decreasing toxicity and environmental contamination [40].

Chitosan has emerged as one of the most promising polymers for the efficient delivery of agrochemicals and micronutrients in nanoparticles (Fig. 1; Table 2). The enhanced efficiency and efficacy of nanoformulations are due to higher surface area, induction of systemic activity due to smaller particle size and higher mobility, and lower toxicity due to elimination of organic solvents in comparison to conventionally used pesticides and their formulations [62], [63]. Chitosan nanoparticles have been investigated as a carrier for active ingredient delivery for various applications (Fig. 1) owing to their biocompatibility, biodegradability, high permeability, cost-effectiveness, non-toxicity and excellent film forming ability [64]. Over the past three decades, various procedures like cross-linking, emulsion formation, coacervation, precipitation and self-assembly, etc. have been employed to synthesize chitosan nanoparticles [65], [66]. Chitosan has also known for its broad spectrum antimicrobial and insecticidal activities [67], [68]. Further, it is biodegradable giving non-toxic residues with its rate of degradation corresponding to molecular mass and degree of deacetylation [69], [70]. However, the low solubility of bulk chitosan in aqueous media limits its wide spectrum activity as an antimicrobial agent. Therefore, various strategies have been employed to enhance its antifungal potential [41]. Chitosan is able to chelate various organic and inorganic compounds, making it well-suited for improving the stability, solubility and biocidal activity of chelated fungicides or other pesticides [64]. For example, copper (Cu) compounds are well known for their antifungal nature and have been used with chitosan for antibacterial and antifungal activities. The majority of the research on chitosan nanoparticles in agricultural research studied their biocidal and antagonistic effects on bacteria and fungi, and gave encouraging results [71], [72], [73]. Chitosan-based nanocomposite films, especially silver-containing ones, showed antimicrobial activity against several pathogens [74], but some effect was also observed with chitosan films alone [75]. Other studies investigated the use of chitosan–PVA hydrogels for antimicrobial and food packaging applications [76], [77], [78]. The combination of silver nanoparticles within a chitosan–PVA polymeric material also emerged as one of the most promising candidates for new antimicrobial materials [44]. Recently, application of chitosan particles loaded with copper has been reported in waste water treatment [79], [80]. Considering the growing interest, and recent advances, in chitosan-based nanomaterials in medical and pharmacological applications, the purpose of this article is to review the current and ongoing research and developmental efforts into chitosan nanoparticles as a delivery system, with particular focus on describing methods that would be suitable for promoting crop productivity.

There have been several reports describing the use of chitosan for biotic and abiotic stress management in agriculture [73], [81], [82], [83], [84], [85]. Table 3 lists some of the applications of chitosan in crop production and protection. For the first time, Allan and Hadwiger [130] described the application of chitosan as an antimicrobial agent. This has led to the exploitation of its antimicrobial potential in various sectors of agriculture. Since the 1980s, the study of chitosan has been shift from a general sewage treatment agent to plant growth regulator, soil conditioner, vegetables and fruits antistaling agent, and seed coating agent, especially in the crop disease management. Several studies showed that chitosan is not only an antimicrobial agent but also an effective elicitor of plant systemic acquired resistance to pathogens [73], [82], [84], [131]. This polymer has been reported to be the enhancer and regulator of plant growth, development and yield [85], [132], [133]. Chitosan has been demonstrated to induce plant defences in tomato [87], [89], cucumber [97], chilli seeds [102], strawberry fruits [88] and rose shrubs [99]. Chitosan can activate innate immunity by stimulating hydrogen peroxide (H2O2) production in rice [134], [135], induce a defense response by nitric oxide (NO) pathways in tobacco [136], [137], promote the development and drought resistance of coffee [138], support the synthesis of phytoalexin [139], impact the jasmonic acid–ethylene (JA/ET) signaling marker in oilseed rape [140], cause changes in protein phosphorylation [141], activate mitogen-activated protein kinases (MAPKs) [142] and trigger defense-related gene expression [143]. Even applied on plants together with biological control agents, chitosan enhanced the efficacy in the control of pathogens [144], [145]. Soil amendment with chitosan has frequently been shown to control Fusarium wilts [90], [146], [147] and gray molds [97], [103] in a number of crops. It is interesting to note that these studies show chitosan to be fungistatic against both biotrophic and necrotrophic pathogens. Besides this, another one of the most important bioactivity of chitosan on plants is stimulation of seed germination in response to abiotic stress. In peanut, seed coated with chitosan enhance the energy of germination and germination percentage [95]. Dzung and Thang [96] suggested that chitosan could enhance growth and yield in soybean. Seed soaked with chitosan increased germination rate, length and weight of hypocotyls and radicle in rapeseed [148]. Chandrkrachang [94] also found that the application of chitosan could increase the germination rate of cucumber, chilli, pumpkin and cabbage. Manjunatha et al. [107] reported that seed priming with chitosan enhances seed germination and seedling vigor in pearl millet. Further, it is also noticed that seed priming with acidic chitosan solutions improved the maize vigor [101]. Similarly, rice seedlings treated with chitosan induced defense responses against the rice blast pathogen, Magnaporthe grisea by inducing the production of the phytoalexins (sakuranetin and monilactone A) in leaves [104]. Moreover, chitosan also stimulated the growth and yield of rice along with reinforcing the defense response [149]. In addition, other studies also supported a role of chitosan in modulating the plant response to several abiotic stresses including salt and water stress [121], [138], [150]. For instance, Boonlertnirun et al. [151] found that chitosan treatments had a significant effect on the growth or yield of drought-stressed rice plants compared to control plants. It is interesting to note that the effect was greatest when chitosan was applied before the onset of stressful conditions. Bittelli et al. [91] also noticed that the water use of pepper plants treated with chitosan reduced by 26–43%, with no significant change in biomass production or yield. These findings indicate that chitosan has potential to be developed as an antitranspirant in agricultural situations where excessive water loss is undesirable.

Recently, chitosan coatings have emerged as an ideal alternative to chemically synthesized pesticides. It has been reported to reduce the growth of decay and induced resistance in the host tissue [152]. Chitosan can also help to protect the safety of edible products. The protection of fresh cut broccoli with chitosan against E. coli and Listeria monocytogenes was assisted with bioactive components such as bee pollen and extracts from propolis and pomegranate [153]. Chitosan protection by exclusion occurs with soybean seed treatments. In this case the major advantage was protection from insects such as agarotis, ypsilon, soybean pod borer, and soybean aphids. Additionally, the treatment was also accompanied by increases in seed germination, plant growth and soybean yield. From the above points, it is clear that the chitosan products are more effective and can be used in a numbers of ways to reduce disease levels and enhance crop productivity in a eco-friendly and sustainable manner.

Chitosan is one of the most widely used polymers in the field of drug delivery. Its attractiveness relies on its useful structural and biological properties [154], [155], which include a cationic character, solubility in aqueous acidic media, and biodegradability. Chitosan has a low solubility at physiological pH of 7.4 as it is a weak base (pKa 6.2–7). Chitosan is synthesized by removing the acetate moiety from chitin through amide hydrolysis under alkaline conditions (concentrated NaOH) or through enzymatic hydrolysis in the presence of chitin deacetylase [156]. Chitosan's amine groups readily complex with a variety of oppositely charged polymers such as poly(acrylic acid), sodium salt of poly(acrylic acid), carboxymethyl cellulose, xanthan, carrageenan, alginate and pectin, etc. [157]. Chitosan also provides considerable flexibility for development of formulation, as it is available in wide range of molecular weights (500–1400 kDa) and degrees of acetylation. Chitosan's amine group also readily lends itself to other chemical modifications. Chitosan easily absorbs to plant surfaces (e.g. leaf and stems), which helps to prolong the contact time between agrochemicals and the target absorptive surface. Chitosan nanoparticles are known to facilitate active molecule or compound uptake through the cell membrane. The absorption enhancing effect of chitosan nanoparticles improves the molecular bioavailability of the active ingredients contained within the nanoparticles [158]. Taken together, these advantages indicate that chitosan has a bright future as a drug delivery system in the field of sustainable agriculture.

Chitosan nanoparicles can be synthesized by various techniques viz., emulsion cross-linking, emulsion-droplet coalescence, precipitation, ionotropic gelation, reverse micelles and sieving through nano-scaled controlled release devices. A comparison of these techniques, their merits and demerits are summarized in Table 4. The selection of methods for chitosan nanoparticles synthesis depends on requirements such as the particle size and shape, thermal stability, release time of the active ingredients, and residual toxicity of the final product.

Emulsions are a standard process leading to nanoparticulate phases, while cross-linking is a common way to stabilize a particle structure and to manipulate the controlled-release properties of that particle. Altering the cross-linking degree of a particle modifies an agrochemical's permeability through it. Cross-linking enhances the mechanical strength of the final particle by introducing a three-dimensional network structure into the nano-emulsion. The process begins when a chitosan solution is emulsified in an oil phase (water-in-oil emulsion). The chitosan phase is first stabilized by a suitable surfactant, and is then reacted with an appropriate cross linking agent (e.g. formaldehyde, glutaraldehyde, genipin, glyoxal, etc.). This is followed by washing and drying of the resulting nanoparticles [159]. This method is schematically represented in Fig. 2(A). The particle size is mainly determined by the size of the emulsion droplet, which in turn is dependent on the type of surfactant, degree of crosslinking and the stirring speed [166]. The molecular weight and concentration of chitosan also affect the preparation and performance of the nanoparticles [40], [167]. The major drawback of this method is that it is somewhat tedious and the use of harsh, and often expensive, cross-linking agents can induce undesirable chemical reactions with the active agent. Recently, Fan et al. [47] studied the synthesis and controlled release characteristics of auxin-loaded chitosan microspheres using a cross-linker. They found that the cumulative release of the auxins from the particles reached a maximum (60%) after about 120 h. They also observed that maximum encapsulation efficiency was significantly influenced by the type of cross-linker, cross-linking time and the oil/water phase ratio. Based on these results this procedure is suitable to prepare chitosan nanoparticles for prolonged controlled release of compounds, possibly spanning weeks or months, and do so with greater safety to non-target organisms.

This method follows the principles of emulsion by cross-linking but uses precipitation techniques [168], [169]. An emulsion is first prepared by dispersing chitosan solution and liquid paraffin oil. The active ingredient and a sodium hydroxide solution are combined and added to the first emulsion to produce additional droplets. High-speed mixing is then used to generate collisions between the different droplets, randomly combining them and precipitating particles of small size [169]. The particle size depends primarily on the degree of deacetylation of chitosan. Generally, at lower degree of deacetylation, large size particles with less ability to retain the active ingredients are obtained [170]. The pictorial representation of the method is shown in Fig. 2B. Using this procedure, Tokumistu et al. [164] synthesized gadopentetic acid loaded chitosan nanoparticles (452 nm) with 45% drug loading efficiency. A similar methodology has been adopted by Anto et al. [168] to encapsulate 5-fluorouracil. Interestingly, when two emulsions with equal outer phase are mixed together, droplets of each collide randomly and coalesce, resulting in final droplets with uniform content.

The chitosan nanoparticles produced through this method are stable, non-toxic and organic solvent free [41], [48], [169], [170], [171], [172]. It is very simple, and employs the use of oppositely charged complexes (polyanions) to bond to the oppositely charged amino groups of chitosan (NH3+). Tripolyphosphate (TPP) is the most commonly used ionic cross-linker, and relies on electrostatic interaction instead of chemical cross-linking, avoiding the possible toxicity of reagents and other adverse reactions. However, the cross-linking is pH-dependent. In this procedure, chitosan is dissolved in a weak acidic medium and added drop wise under constant stirring to an aqueous solution containing the other reagents (Fig. 2C). Due to the complexation between oppositely charged species, chitosan undergoes ionic gelation and the spherical nanoparticles precipitate [173]. The chitosan/TPP molar ratio largely controls the mean diameter of the nanoparticles, which can also affect the drug release characteristics. Interestingly, the mechanism of nanoparticle formation through ionic gelation is well described by several workers [174], [175]. It has been suggested that all ionic groups of TPP participated in interactions with chitosan amine groups. The ion pairs, formed through the negatively charged TPP with the protonated amine functionality of chitosan in ionotropic gelation provided chitosan with an amphoteric character, which enhanced the protein adhesion and subsequently accelerated the attachment of anchorage dependant cells. Recently, Koukaras et al. [174] provided insights into the intermolecular interactions responsible for the ionic cross-linking during ionotropic gelation by means of all electron density functional theory. They reported that the maximum-interaction relative configurations of TPP and chitosan oligomers depended on the primary ionic cross-linking types (H-, M- and T-links). In all three of the linking types, there is a high degree of correspondence between chitosan monomers and TPP polyanions, and thus, these correspond to low β (and α) ratios. As a result, at low β ratios, the high concentration of TPP permits the formation of dense H-links. At high β (and α) ratios, the dihedral bias deters the formation of parallel CS chains and impels the formation of irregular and smaller size nanoparticle cores. At even higher β ratios, the very low concentration of TPP results in low nanoparticle core densities because of the increased distance between successive H-links, ultimately leading to an increased nanoparticle size. Besides this, a recent work on chitosan/TPP nanoparticles has also established that the concentration of acetic acid used to dissolve chitosan and the temperature at which the cross-linking process occurs, strongly affect the size distribution of the obtained nanoparticles [176]. Fàbregasa et al. [177] found that the stirring speed during ionic gelation significantly affect reaction yield. Therefore, manipulation of this parameter can be used to give some control over size range that is obtained to favor the maximum yield of nanoparticles of desired size.

This method is quite simple. Chitosan nanoparticles are produced by blowing a chitosan solution into an alkaline solution [e.g. NaOH(aq)] or methanol. The blowing is accomplished with a compressed air nozzle, thereby forming the coacervate particles. These are separated and purified by filtration and followed by washing with hot and cold water [178]. The method is schematically represented in Fig. 2D. Generally, various parameters viz., compressed air pressure, spray nozzle diameter and chitosan concentration affects the particle shape and size. Although this method is simple, cross-linking is required to enhance the particle stability, and even then particles have weak mechanical strength and irregular morphology.

This method uses a thermodynamically stable mixture of water, oil and lipophilic surfactant. Using this method, it is possible to obtain very small polymeric nanoparticles (≤10 nm) with a uniform distribution compared with other methods. The size, polydispersity and thermodynamic stability of the particles are maintained in a dynamic equilibrium system. Briefly, the method consists of preparing a surfactant solution (e.g. sodium bis(ethyl hexyl) sulfosuccinate or cetyl trimethylammonium bromide) in an organic solvent (e.g. n-hexane), to which a chitosan solution and the active ingredient are added under constant stirring, forming a transparent mini- or micro-emulsion. Subsequently, a cross-linking agent (e.g. glutaraldehyde) is added and the system is maintained under constant agitation. The organic solvent is then evaporated, producing a dry and transparent mass that is dispersed in water. A salt is then added to this system, which precipitates the surfactant. The resulting mixture is centrifuged and the supernatant, containing nanoparticles loaded with the active substance, is collected. The nanoparticles are separated by dialysis and lyophilized to obtain a dry powder [165]. The method is schematically represented in Fig. 2E. Brunel et al. [179] used a reverse micellar method to prepare chitosan nanoparticles. They emphasized that chitosan of low molecular weight is preferable to achieve better control over particle size and distribution. This may be due to a reduction in the viscosity of the internal aqueous phase or entanglement of the polymer chains during the process. In recent times, this method has been use for enzyme immobilization [180] and to encapsulate oligonucleotides [181]. To date, despite some of the advantages, the use of this method to produce chitosan nanoparticles is limited, due to the difficulty of isolating the nanoparticles and need to use a large quantity of organic solvent.

This method involves the cross-linking of an aqueous acidic solution of chitosan using glutaraldehyde. The cross-linked chitosan is then passed through a sieve with suitable mesh size to obtain microparticles. The microparticles are then washed with sodium hydroxide (0.1 N) solution to remove the unreacted glutataraldehyde and dried at 40 °C [159]. This method does not seem to be being extensively researched for agricultural use. The method is schematically shown in Fig. 2F.

This method is extensively used for the production of matrices to produce dry powders, granules and pellets from chitosan solutions and suspensions [160]. The technique is quite versatile and can be used for drugs with high or low heat-sensitivity and with high or low water solubility, and hydrophilic or hydrophobic polymers [182]. This procedure is inexpensive and employs a single step to produce small-sized particles that are typically micro-sized, and then these particles are often reformulate into suspensions, capsules or tablets [183]. This technique uses a chitosan solution in acetic acid, to which the active ingredient and the cross-linking agent (glutaraldehyde or sodium tripolyphosphate) are consecutively added (Fig. 2G). The resulting solution is atomized through a hot air stream, causing flash evaporation of the solvent to form the desired particles [184]. The important parameters to modulate particle size in this process are the type of needle, flow speed of the compressed air, air temperature and degree of cross-linking [185]. This method can be used to synthesize particles with or without cross-linking, and has been used to prepare chitosan micro-particles for the delivery of cimetidine, famotidine and nizatidine [186]. Recently, Tokárová et al. [187] used spray-dried chitosan microcarriers for the delivery of silver nanoparticles.

Loading active ingredient into nanoparticulate systems can be done at the time of preparation of particles (incorporation) or after the formation of particles (incubation). In these systems, the active ingredient is physically embedded within the matrix or adsorbed on the surface. Various techniques have been developed to improve the efficiency of loading the active ingredient, but the efficiency largely depends on the method of preparation and the physicochemical properties of the substance. Loading efficiency is generally maximized when the substance is incorporated during the formation of particles, while incubations typically give a much lower degree of incorporation. However, the degree of incorporation is also influenced by the specific process parameters such as exact method of preparation, presence of additives (e.g. cross linking agent, surfactant stabilizers, etc.), and agitation intensity [159]. Both hydrophillic and hydrophobic compounds can be loaded into chitosan-based particulate systems. Water-soluble compounds are mixed with chitosan solution to form a homogeneous mixture, and then, particles can be produced by any of the methods described earlier in the section. Water-insoluble compounds that precipitate in the acidic chitosan solutions can be incorporated after particle preparation by soaking the preformed particles with a saturated solution of the active ingredient. Water-insoluble drugs can also be loaded using a multiple emulsion technique. In this method, compound is dissolved into a suitable solvent and then emulsified in the chitosan solution to form an oil-in-water type emulsion. Sometimes, compounds can be dispersed within a chitosan solution by using a surfactant to form a suspension. The oil in water (o/w) emulsions or suspensions prepared in this manner can be further emulsified into liquid paraffin to get oil-water-oil multiple emulsions. The resulting droplets can be hardened by using a suitable cross-linking agent.

The release of an active ingredient from chitosan-based particles depends upon the morphology, size, density, and extent and rate of cross-linking of the particles, as well as the physicochemical properties of the drug. If any adjuvant is used this can also affect the release rate. Studies showed that under in vitro conditions, the release of an active ingredient is affected by pH, solvent polarity, and the presence of enzymes in the dissolution media [188], [189]. Generally, the release of drug from chitosan particles occurs by one, or a combination of three different mechanisms: (i) an osmotically driven burst mechanism, (ii) a diffusion mechanism, and (iii) erosion or degradation of the polymer. In agricultural systems the release mechanisms are by diffusion release and/or degradation release. The diffusion release mechanism includes several steps viz., (i) penetration of water into particulate system, which causes swelling of the matrix; (ii) conversion of a glassy polymer into a plasticized or rubbery swollen matrix, and (iii) diffusion of compound from the swollen matrix.

The original active ingredient content contained in chitosan particles is determined in different ways, but the release from the chitosan particles, is typically measured from particles placed in phosphate buffer saline (PSB; pH 7.4) and kept in a thermostatic incubator at 37 °C. Specified volumes of the buffered medium are removed at regular intervals from the sample being analyzed, and that same amount of fresh buffer is added back into the flask to keep the total solution volume constant throughout the duration of the study. The aliquot of removed sample is then filtered and the transparent filtrate is analyzed. The quantity of active ingredient in the aliquot is typically determined by spectroscopic or chromatographic methods.

Diffusion release of active ingredient is typical for hydrophilic polymers that form hydrogels (e.g. polyvinyl alcohol), while diffusion and degradation release occurs with chitosan. It is not uncommon to observe an initial “burst” release of active ingredient from particles that predominantly release active ingredient by diffusion or degradation. This happens due to the adsorption of active ingredients onto the surface of the particles. Once this burst is exhausted, a slow and steady release is observed that accelerates if and when the particle matrix begins to degrade. Kweon and Kang [190] synthesized chitosan–polyvinylalcohol (PVA) particles to study the compound release mechanism of the active ingredient under various conditions. They calculated the diffusion controlled release by analysis of the linear relationship between the amount of active ingredient released and the square root of the time. Jamnongkan and Kaewpirom [51] demonstrated potassium release kinetics and water retention of controlled-release fertilizers based on chitosan hydrogels is through a quasi-Fickian diffusion mechanism. Similarly, Jameela et al. [191] obtained a good correlation fit for the cumulative drug released vs. square root of time, demonstrating that the drug release from the microsphere matrix is diffusion-controlled and obeys the Higuchi equation [190]. It was demonstrated that the rate of release depends upon the size of microspheres. Orienti et al. [192] studied the correlation between matrix erosion and release kinetics of indomethacin-loaded chitosan microspheres. Release kinetics was correlated with the concentration of chitosan in the microsphere and pH of the release medium. Nam and Park [188] have demonstrated the in vitro release test of drug loaded chitosan microspheres. Agnihotri and Aminabhavi [165] also analyzed the dynamic swelling data of chitosan microparticles and concluded that with increase in cross-linking, swelling of chitosan microparticles decreases. Recently, Khan and Ranjha [193] studied the swelling behavior of chitosan/poly(vinyl alcohol) hydrogels as a function of pH, polymeric compositions and degree of cross-linking. They noticed that swelling increased by increasing poly(vinyl alcohol) contents in the structure of hydrogels at higher pH. They also observed that the cross-linking ratio was inversely related with the swelling of hydrogels. Similar results were also described by Martínez-Ruvalcaba et al. [194], where drug release increased with increasing drug contents in the hydrogels, while release of drug decreased as the ratio of crosslinking agent increased in the hydrogel structure owing to strong physical entanglements between polymers. It is also important to note that the release rate of drugs from hydrophilic matrices based on chitosan is greatly affected by changes in pH. The increase in release rates could be due to an associated increase in the fluid-filled cavities created by dissolution and diffusion of the drug particles near the surface, which in turn results in an increase in the permeability of the drug [195].

Section snippets

Pesticide delivery for crop protection

The difficulties in controlling pests along with concern about the indiscriminate use of pesticides in agriculture have been the subject of intense debate and discussion. The pressure to devise alternative methods of pest control, to reduce the dependency on synthetic pesticides and reduce residue problem, is rising steadily. There are several examples of slow release of encapsulated agrochemicals by polymeric nanoparticles. For example, Liu et al. [196] used polyvinylpyridine and

Conclusions

Application of chitosan based nanoparticles in agriculture is still in a nascent stage. Encouraging and promising results are already being achieved in delivery of agrochemicals and genes for plant transformation using chitosan nanoparticles. The use of such nanomaterials for the delivery of pesticides, micronutrients and fertilizers is expected to reduce the required dosage for efficacy and ensure a controlled delivery. An important advance will have been realized once the application of

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