**3. Developing new nanopesticides**

Many attempts have been made to manage plague insects, for example, using biological control, which is very time consuming. Controlled release systems dawn in this scenario as a very attractive alternative in this battle field.

Controlled release formulations (CRFs) associate the active compound with inert materials. The last ones are responsible for protecting and managing the rate of compound release into the target site in a defined period of time. The main purpose of controlled release systems is ruling the (bio) availability of the active compound after the application [48]. They find the greatest applicabilities in two major agricultural fields: nutrition and protection. In the first one, CRFs are employed in the delivery of fertilizers [49-51]. In the second one, CRFs are mostly used to target plague insects in a sustainable way [52,53], but they can also be ap‐ plied to block the growth of weeds [54]. Tomioka *et al.*, 2010. Controlled release formula‐ tions become especially interesting in cases of antagonist activity of biocides, what can naturally leads to a lower in effectiveness of one or both compounds. In this case the formu‐ lation should be "programmed" to release each one at different times [55,56]. Furthermore, still talking about protection, the application of CRFs in wood surfaces, like furniture or floor covering, helps to prevent the deterioration. Van Voris *et al.* [57] patented a formula‐ tion in which an insecticide is continually released in a minimum level for a long period of time and is absorbed by the wood. It thus creates a "chemical barrier", blocking the insect attacks.

cy (> 95%) for this compound and a UV stability at least of 30 times more when compared

The interesting results obtained in academic researches over the last few decades have been closely followed by several companies. Nevertheless, R&D in nano-based agrochemicals is led mainly by world's largest agroscience companies, further enhancing their market share and consolidating the market structure based on oligopoly that have been seen in late 20th century

Some companies over the last decade, such as Syngenta, Bayer, Monsanto, Sumitomo, BASF, and Dow Agrosciences have already deposited several different patents comprising a wide range of protocols for production and application of encapsulated formulations, which can be used to produce nanoinsecticides [37-46]. Despite the hard work and heavy investment, no commercial nano-insecticide formulation has been extensively commercialized up to 2012.

Along with those big industries, several other companies, as well as individual researches have been actively depositing patents in the area, thus promoting even more the research and investments in this new field of applied technology. However, as strongly reinforced throughout the world by dozens of organizations such as the ETC Group, the impact of nanotechnology is still unclear, and care should be taken to assure that its use will not bring

Many attempts have been made to manage plague insects, for example, using biological control, which is very time consuming. Controlled release systems dawn in this scenario as a

Controlled release formulations (CRFs) associate the active compound with inert materials. The last ones are responsible for protecting and managing the rate of compound release into the target site in a defined period of time. The main purpose of controlled release systems is

and early 21th century, when the 10 biggest companies hold around 80% of market [36].

**Figure 1.** Scanning electron microscopy images of nanoparticles containing extracts of Neem.

with commercial products.

528 Insecticides - Development of Safer and More Effective Technologies

**2.5. Commercial products**

more problems than solutions [47].

**3. Developing new nanopesticides**

very attractive alternative in this battle field.

Most of those controlled release biocides applications were and still are successfully made due to the advances in nanotechnology area.

Micro- and nanomaterials-based formulations are known for some decades. The first micro‐ capsule-based formulation became commercially available in the 1970s [58]. Nanocapsules have been widely used in medicinal area as drug carrier in treatment of diverse diseases [59], from tropical ones [60] up to cancer [61].

Microencapsulation has been used as a versatile tool for hydrophobic pesticides, enhancing their dispersion in aqueous media and allowing a controlled release of the active compound. The use of nanotechnology is a recent approach, and has been a growing subject on several different areas of the science, with an overwhelming perspective. In general, materials that are assembled in nanometric scales (<1000nm) have distinct and almost always better char‐ acteristics when compared to the same material built in a conventional manner [62]. One nanometer is a billionth of a meter (1nm = 109 m). In general, the chemical properties of mate‐ rials in nanometric scale may be controlled to promote an efficient assemble of a structure which could present several advantages, such as the possibility to better interaction and mode of action at a target site of the plant or in a desired pest due to its tunable controlled release system and larger superficial area, acting as an artificial immune system for plants [34,63]. As smart delivery systems, they confer more selectivity, without hindering in the bi‐ oactive compounds towards the target pathogen [65]. Other advantages of the use of nano‐ particle insecticides are the possibility of preparing formulations which contain insoluble compounds that can be more readily dispersed in solution. It reduces the problems associat‐ ed with drifting and leaching, due to its solid nature, and leads to a more effective interac‐ tion with the target insect. These features enable the use of smaller amount of active compound per area, as long as the formulation may provide an optimal concentration deliv‐ ery for the target insecticide for longer times. Since there is no need for re-applications, they also decrease the costs), reduce the irritation of the human mucous-membrane, the phyto‐ toxicity, and the environmental damage to other untargeted organisms and even the crops themselves [65,66]. In a few words, nanotechnology can be applied in several ways in order to enhance efficacy of insecticides in crops.

#### **3.1. Biopolymers**

When a commercial formulation for a practical field application is desired, it is very impor‐ tant to employ materials that are compatible with the proposed applications: environmentfriendly, readily biodegradable, not generating toxic degradation by-products and low-cost. The use of several biopolymers, i.e., polymers that are produced by natural sources, which at the same time have good physical and chemical properties and still present mild biode‐ gradation conditions, are an interesting approach to avoid the use of petrochemical deriva‐ tives that might be another source of environmental contamination. The common polymers (synthetic and natural ones) used in CRFs for insecticides application are listed in Table 1.

**Polymer Active compound Nanomaterial Ref.**

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Deltamethrin Capsule [70]

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Bifenthrin Capsule [71]

Cypermethrin Gel [78]

Novaluron Powder [86]

Polyethylene glycol Β-Cyfluthrin Capsule [68] Chitosan Etofenprox Capsule [69]

Polyethylene glycol Garlic Essential Oil Capsule [32]

Acrylic acid-Bu acrylate Itraconazole Capsule [72] Carboxymethylcellulose Carbaryl Capsule [73] Alginate-glutaraldehyde Neen Seed Oil Capsule [74] Alginate-bentonite Imidacloprid or Cyromazine Clay [75] Polyamide Pheromones Fiber [76] Starch-based polyethylene Endosulfan Film [77]

> Lignin Aldicarb Gel [79] Lignin Imidacloprid Or Cyromazine Granules [75]

octadecanol glycidyl ether Rotenone Micelle [33] Polyethyleneglycol-dimethyl esters Carbofuran Micelle [80] Carboxymethyl chitosan-ricinoleic acid Azadirachtin Particle*<sup>a</sup>* [34] Chitosan-poly(lactide) Imidacloprid Particle*<sup>a</sup>* [81] polyvinylchloride Chlorpyrifos Particle*<sup>a</sup>* [82] Cashew gum *Moringa Oleifera* Extract Particle*<sup>a</sup>* [83] Chitosan-angico gum *Lippia Sidoides* Essentioan Oil Particle*<sup>a</sup>* [84] Polyvinylpyrrolidone Triclosan Particle*<sup>a</sup>* [85]

Vinylethylene and vinylacetate Pheromones Resin [87] Glyceryl ester of fatty acids Carbaryl Spheres [15] Poly(ε-caprolactone) Active Ingredients*<sup>b</sup>* Spheres [88]

Polyvinylpyrrolidone Carbofuran Suspension [89]

The authors do not mention which active compounds they encapsulated in the nanospheres; b The authors do not

Lignin-polyethylene glycol-ethylcellulose Imidacloprid Capsule [67]

Polyethylene Piperonyl Butoxide And

Poly(acrylic acid)-b-poly(butyl acrylate) Polyvinyl alcohol Polyvinylpyrrolidone

Methyl methacrylate and methacrylic acid with and without 2-hydroxy ethyl methacrylate crosslinkage

N-(octadecanol-1-glycidyl ether)-O-sulfate chitosan-

Anionic surfactants (sodium linear alkyl benzene sulfonate, naphthalene sulfonate condensate sodium salt and sodium dodecyl sulfate)

Poly(methyl methacrylate)-poly(ethylene glycol)

**Table 1.** Several examples of polymers often used in the nanoparticle production.

mention if the particles are spheres or capsules

a

#### **3.2. The nanoparticles used in biocides controlled release formulations**

The most popular shape of nanomaterials (Figure 2) that have been using in CRFs for bio‐ cides delivery are:


**Figure 2.** Morphological representation of different nanoparticles.


**3.1. Biopolymers**

cides delivery are:

umes of water

into the polymeric matrix;

and hydrophobic moieties.

core, lined by the matrix polymer;

530 Insecticides - Development of Safer and More Effective Technologies

**Figure 2.** Morphological representation of different nanoparticles.

When a commercial formulation for a practical field application is desired, it is very impor‐ tant to employ materials that are compatible with the proposed applications: environmentfriendly, readily biodegradable, not generating toxic degradation by-products and low-cost. The use of several biopolymers, i.e., polymers that are produced by natural sources, which at the same time have good physical and chemical properties and still present mild biode‐ gradation conditions, are an interesting approach to avoid the use of petrochemical deriva‐ tives that might be another source of environmental contamination. The common polymers (synthetic and natural ones) used in CRFs for insecticides application are listed in Table 1.

The most popular shape of nanomaterials (Figure 2) that have been using in CRFs for bio‐

**a.** Nanospheres: aggregate in which the active compound is homogeneously distributed

**b.** Nanocapsules: aggregate in which the active compound is concentrated near the center

**c.** Nanogels: hydrophilic (generally cross-linked) polymers which can absorb high vol‐

**d.** Micelles: aggregate formed in aqueous solutions by molecules containing hydrophilic

**3.2. The nanoparticles used in biocides controlled release formulations**

a The authors do not mention which active compounds they encapsulated in the nanospheres; b The authors do not mention if the particles are spheres or capsules

**Table 1.** Several examples of polymers often used in the nanoparticle production.

Dendrimers, nanoclays, nanopowders and nanofibers are other possible formulations which might be used during nano or microparticle production[75, 76, 86, 90]. On the other hand, nanotubes are mostly applied in plants improvement. The polymeric nanoparticles and gels are by far the mostly used for insecticides application, because they have an extra advantage of being biodegradable.

#### **3.3. Methods for preparation of nanomaterials based controlled-release formulations for biocides application**

According to Wilkins [48], the methods for CRF preparation can be separated in chemical or physical ones (Figures 3 and 4, respectively).

The chemical methods are based on a chemical bond (usually a covalent one) formed be‐ tween the active compound and the coating matrix, such as a polymer. This bound can be placed in two different sites: in the main polymeric chain or in a side chain. In the first one, the new "macromolecule" is also called a pro-biocide, because the compound will get its properties in fact when it is released. In the second one, the insecticide molecule can bind initially to the side-chain of one monomer and then the polymerization reaction takes place or the polymerization occurs first and only after that, the biocide binds to the side chain. There is still a third way, based on the intermolecular interactions. In this case, the biocide is "immobilized" in the net produced by the cross-linkages in the polymer.

**Figure 4.** Physical methods for CRF preparation

based. Some are described below.

*3.4.1. The physicochemical-based techniques*

**3.4. Micro and nanoencapsulation techniques**

encapsulation process.

Although there are some different kinds of nanomaterials that can be used in CR formula‐ tions, the micro- and nanocapsules are by far the most widely used for controlled release of biocides. For this reason, the techniques described here will be restricted to micro and nano‐

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The first formulation containing polymeric-based nanocarriers for controlled release of bio‐ cides dates from the early 1970's [11,92]. Recently, John *et al*. [93] reviewed the most com‐ monly techniques used to prepare micro- and nanocapsules containing microorganisms (for this kind of application, see section 2.3). However, the techniques they commented can be also utilized to prepare nanocapsules for insecticides application in general. Shahidi and Han [94] and Wilkins [48] classified them as physicochemical, chemical or physical process-

**a.** *Emulsion*: This technique is used to produce a system of two immiscible liquid phases (wa‐ ter and oil), where one (the dispersed phase) is dispersed into the other (continuous phase) in a controlled way (usually in a dropwise one). The bioactive compound (usually watersoluble) and the polymer are solubilized each one in a phase (water or oil). One of the solu‐ tions is gradually dripped into the other under vigorous stirring. After the homogenization, the emulsion is formed. If the oil is the dispersed phase, the emulsion is classified as O/W (oil/water). If it is water, the emulsion is called W/O (water/oil) [95].The emulsion itself also

**b.** *Coacervation*: This process is based on the reduction of polymer's solubility. According to Wilkins[48] the encapsulation goes through a separation of phases and can be simple

represents a crucial step for some other more complexes preparation ones.

**Figure 3.** Chemical methods for CRF preparation

The physical methods can also be split in two distinct categories. In the first, a mixture of biocide and polymer is made. As the last has a higher energy density, it moves to a more external layer, forming a kind of monolithic structure. In the other one, the polymeric chain forms a "membrane" isolating the bioactive compound from the external environment. This is the method which will produce the nanocapsules themselves.

Polymeric Nanoparticle-Based Insecticides: A Controlled Release Purpose for Agrochemicals http://dx.doi.org/10.5772/53355 533

**Figure 4.** Physical methods for CRF preparation

Dendrimers, nanoclays, nanopowders and nanofibers are other possible formulations which might be used during nano or microparticle production[75, 76, 86, 90]. On the other hand, nanotubes are mostly applied in plants improvement. The polymeric nanoparticles and gels are by far the mostly used for insecticides application, because they have an extra advantage

**3.3. Methods for preparation of nanomaterials based controlled-release formulations for**

According to Wilkins [48], the methods for CRF preparation can be separated in chemical or

The chemical methods are based on a chemical bond (usually a covalent one) formed be‐ tween the active compound and the coating matrix, such as a polymer. This bound can be placed in two different sites: in the main polymeric chain or in a side chain. In the first one, the new "macromolecule" is also called a pro-biocide, because the compound will get its properties in fact when it is released. In the second one, the insecticide molecule can bind initially to the side-chain of one monomer and then the polymerization reaction takes place or the polymerization occurs first and only after that, the biocide binds to the side chain. There is still a third way, based on the intermolecular interactions. In this case, the biocide is

The physical methods can also be split in two distinct categories. In the first, a mixture of biocide and polymer is made. As the last has a higher energy density, it moves to a more external layer, forming a kind of monolithic structure. In the other one, the polymeric chain forms a "membrane" isolating the bioactive compound from the external environment. This

"immobilized" in the net produced by the cross-linkages in the polymer.

of being biodegradable.

**biocides application**

physical ones (Figures 3 and 4, respectively).

532 Insecticides - Development of Safer and More Effective Technologies

**Figure 3.** Chemical methods for CRF preparation

is the method which will produce the nanocapsules themselves.

Although there are some different kinds of nanomaterials that can be used in CR formula‐ tions, the micro- and nanocapsules are by far the most widely used for controlled release of biocides. For this reason, the techniques described here will be restricted to micro and nano‐ encapsulation process.

#### **3.4. Micro and nanoencapsulation techniques**

The first formulation containing polymeric-based nanocarriers for controlled release of bio‐ cides dates from the early 1970's [11,92]. Recently, John *et al*. [93] reviewed the most com‐ monly techniques used to prepare micro- and nanocapsules containing microorganisms (for this kind of application, see section 2.3). However, the techniques they commented can be also utilized to prepare nanocapsules for insecticides application in general. Shahidi and Han [94] and Wilkins [48] classified them as physicochemical, chemical or physical processbased. Some are described below.

#### *3.4.1. The physicochemical-based techniques*


or complex. In a simple coacervation, the addition of an external agent, like a salt or wa‐ ter-miscible solvent, to an aqueous solution containing a hydrophilic polymer-insecti‐ cide complex causes its precipitation. Complex coacervation involves opposite charges and electrostatic attraction. A solution containing different ionizable polymers is sub‐ mitted to a pH change. The polymers turn positively or negatively charged. The electro‐ static attractive forces between the opposite charges become much stronger than the particle-solvent intermolecular ones, leading to the copolymer precipitation.

*3.4.2. The chemical techniques*

interactions.

*3.4.3. The physical techniques*

cording to the viscosity of colloids.

**a.** *Interfacial polymerization*: As the name says, this technique is based in a polymerization reaction which occurs in an interface of two immiscible liquids. According to Wilkins [48], polymerization can occur through an addition or condensation reaction. In the mostly addition-governed process, the polymerization starts in the oil phase, where the monomers and insecticide are dispersed. However, the reaction only takes place when it is catalyzed by free radicals, which are dissolved in the aqueous phase. In condensa‐ tion-governed process (the most suitable route for biocides nanoencapsulation), the re‐ active monomers are dissolved each one in a different phase. As the dispersed phase is dripped into the continuous phase, the reaction occurs in the droplet interface, produc‐ ing the polymer. When a solvent with a low boiling temperature is used as the oil phase (either in dispersed or continuous one) and contains the monomers dissolved, the proc‐ ess is a little different. After the dripping, the system is heated. The solvent thus evapo‐ rates, leaving the particules that, due to the water insolubility, precipitates. This particular technique variation can also be called interfacial polymer deposition [100].

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**b.** *Molecular inclusion*: This technique is used to increase the solubility of water-insoluble compounds in aqueous solution. Macromolecules like cyclodextrins [101] have an inner hydrophobic face and an outer hydrophilic face. An oil phase containing the biocide is dripped, under continuous stirring, into the aqueous macromolecule solution. During the dripping, the macromolecule "traps" the insecticide molecules via intermolecular

**a.** *Extrusion:* The bioactive compound is mixed with hydrocolloids and then, the colloid is squeezed out under pressure. The pressure during the process should be adjusted ac‐

**b.** *Spray drying*: This technique is based in solvent evaporation at high temperatures. The spray drying process has already been described in details by Ré [102]. The following text is only a brief resume. Initially, the active compound and the polymeric matrix are solubi‐ lized in their respective solvents, which should not be miscible. Then, they are mixed un‐ der vigorous stirring to form an emulsion (or dispersion whether one of the components is in the solid state). The emulsion undergoes an atomization to produce droplets. In the next step, the droplets are submitted to a hot air flow that forces the solvent (generally water) evaporation, leaving only a dry powder. The greatest advantage of this technique is that it

**c.** *Freeze drying*: This technique is also known as liophilization. It is the opposite of the spray drying, because it uses a low temperature system. A suspension or emulsion is prepared to enable the polymer-insecticide formation. For emulsions, an additional step is required before the execution of the technique: the removal of the oil or organic sol‐ vent under reduced pressure. For both (emulsion and suspension), the aqueous phase is

can be easily scaled up for a large scale nanocapsules production.


In the first step, an organic solution (chloroform or methanol ones) containing hydrophobic molecules such phospholipids and cholesterol is prepared. The solvent then is evaporated forming a thin film. Next, an aqueous solution containing the bioactive compound is spread over this film. Some mechanical or thermal perturbation like ultrasound or heating is ap‐ plied to the system to promote the formation of single or double layer sheet. The sheet will detach from the support, closing itself, forming the liposomes. During this closing process, the sheet traps the biocides molecules.

#### *3.4.2. The chemical techniques*

or complex. In a simple coacervation, the addition of an external agent, like a salt or wa‐ ter-miscible solvent, to an aqueous solution containing a hydrophilic polymer-insecti‐ cide complex causes its precipitation. Complex coacervation involves opposite charges and electrostatic attraction. A solution containing different ionizable polymers is sub‐ mitted to a pH change. The polymers turn positively or negatively charged. The electro‐ static attractive forces between the opposite charges become much stronger than the

particle-solvent intermolecular ones, leading to the copolymer precipitation.

ticles are ready for use.

the particles which are collected at the top.

534 Insecticides - Development of Safer and More Effective Technologies

*al*. [99]. The standard one is resumed here.

the sheet traps the biocides molecules.

emulsion at 60o

**c.** *Emulsion-solvent evaporation*: According to Iwata and McGinity [96] this technique com‐ prises two or three steps. In the first one, an O/W (or W/O) emulsification must be ini‐ tially formed. The polymer is usually solubilized in the dispersed phase. If the emulsion has only two components like this one, it is called a single emulsion. For this type, the whole process has only two steps and the first one ends here. However, there is also other type, called double emulsion, represented as W/O/W', where the emulsion al‐ ready prepared in the first step is dispersed into an organic solvent, like acetonitrile. In this case, the aqueous solution containing the active compound is dripped in an oil phase (usually a vegetable oil), under stirring. This emulsion is then dispersed, under stirring, in an organic solvent solution containing the polymer. The last step, common for single and double emulsion, is the evaporation of the solvent, what can be per‐ formed at room temperature or under reduced pressure. After solvent removal, the par‐

**d.** *Emulsion crystallization/ solidification*: According to the procedure published by Iqbal *et al*. [97], an emulsion is initially prepared as already described in this section. The only difference remains in the temperature in which it is made. The authors prepared the

pumped through a capillary partially immersed in a coolant liquid (temperature: 10o

**e.** *Diffusion-controlled emulsion*: In this process, a monomer rich phase is laid over the aque‐ ous solution containing the insecticide, under a smooth stir. The monomers then diffuse into the aqueous fase, "trapping" the bioactive molecules in a micellar structure [98].

**f.** *Liposome entrapment*: Some protocols to prepare liposomes are described by Mozafari *et*

In the first step, an organic solution (chloroform or methanol ones) containing hydrophobic molecules such phospholipids and cholesterol is prepared. The solvent then is evaporated forming a thin film. Next, an aqueous solution containing the bioactive compound is spread over this film. Some mechanical or thermal perturbation like ultrasound or heating is ap‐ plied to the system to promote the formation of single or double layer sheet. The sheet will detach from the support, closing itself, forming the liposomes. During this closing process,

At the capillary exit, the emulsion forms spherical drips which move to raise the cooling liquid's surface. The drop is cooled down during the course, solidifying and forming

C. The next step is crucial for technique success. The warm emulsion is

C).


#### *3.4.3. The physical techniques*


frozen and submitted to a low pressure system. When the pressure is drastically re‐ duced, the water sublimes (goes from solid to vapor state), leaving only the particles.

biocompound solubility in water, faster the reaction occurs. Concerning the chemical proper‐ ties of the polymer, Allan *et al.* [11] studied the differences in the release kinetics when 2-meth‐ yl-4-chlorophenoxy acetic is chemically bound to polyvinylalcohol (a water-soluble polymer) or when it is bound to cellulose or lignin (water-insoluble polymers). In the first situation, the

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will always exist. I the last situation, as the "free polymer" is water–insoluble, the equilibri‐ um moves towards the right side and the level of the applied herbicide tends to go up (Fig‐

Whatever the mode of liberation, it should be kept in mind that controlled release formula‐ tions have a limited maximum amount for the release of the biocide[116]. This means that the total amount of released product may not be necessarily equal to the amount of chemicals in‐ corporated to the formulation neither to the amount of free applied product[117]. This is the reason why the concentration of the active compound in a CRF is usually higher than in a con‐ ventional one. However it does not contradict what was said earlier about advantageous re‐

Studies recently published suggested that the encapsulation of biocides reduces their toxici‐ ty [117,118]. However, many issues regarding the toxicity of the nanomaterial themselves to‐

The increasing worldwide demand for foods requires modern techniques of agricultural production minimizing losses in the crops, transportation and storage. Among the main causes of agricultural losses there are the plague insects. Insecticides are an important con‐ trol tool. However, some collateral effects may be credited to their indiscriminate use such

duced amount if biocide applied, since the number of applications should be smaller.

wards the environmental and even the worker's health remains unclear [119].

level of the applied herbicide tends to go down, because the equilibrium

*Polymer* −*insecticide* ⇌ *Polymer* + *Insecticide*

**Figure 5.** Trend in active compound's application rate (*Adapted* from [11]).

ure 5).

**4. Conclusion**

#### **3.5. Mechanism of biocide release**

In the paper published by Kratz *et al.* [103] the text begins with the statement: "Nanoparti‐ cles only start working after they are placed in a desired location". In other words, an effi‐ cient CR formulation must remain inactive until the active compound is released.

The way how an inert material, such the nanopolymers, controlss the amount and rate a chemical is released is object of study since the late 1960's [104] and early 1970's [105].

How the release of the bioactive compound occurs depends basically on the chemical nature of the formulation. In various polymeric nanomaterials, the controlled release proceeds via diffusion. It does not matter if the bioactive compound is dissolved (micro- or nanospheres) or if it is encapsulated (micro or nanocapsules). The process does not depend on the chemi‐ cal structure of the formulation constituents [11] neither on the intermolecular interactions. The rate control is made based on the interactions between the carrier and the biocide. The stronger the interaction will be slower the release rate. In the 1990's, the release dynamics was investigated via the use of 14C-labelled molecules of herbicides [106,107]. Qi *et al.* [107] studied the dynamic of controlled release for herbicides. They used 14C-labelled molecules of benthiocarb and butachlor and observed that the release is made by a diffusive process. Some years later and without any radiolabeled molecules, Fernandez-Perez et al. [108] found the same results. They prepared a granule-based CRF constituted by lignin and imi‐ dacloprid. They measured the amount of compound released in water under a dynamic flow condition during a defined period of time. The data fitted a diffusion curve based on the model proposed by Ritger and Papas [109,110]. Since then, other similar studies have been published [111-114].

Some other polymeric nanomatrixes, especially those formed by a carboxylic acid and a met‐ allic cation, can be disassembled when in contact with water, releasing the bioactive com‐ pound [92]. The release rates depend on the physicochemical characteristic of both molecules. The more hydrophobic the polymer slower will be the bioactive compound re‐ lease. The same applies to the last one: the higher water-solubility, faster it will be released. The formulation itself also affects directly the release rate. In water-based one, the rate con‐ trol tends to disappear, due to the matrix (or support) degradation. If the particles are solu‐ bilized in an organic solvent, like acetone, the formulation becomes sticky and the release rate slows down. A granule-based formulation sounds more efficient. It can be applied di‐ rect to the soil and the bioactive compound will be released according to the soil moisturize (water content), leading to a long lasting control.

In other formulations, the bioactive compound is covalently bound to the polymeric matrix [115]. To the release takes place, a chemical interaction must be broken. It usually occurs via a hydrolysis reaction, what affects many polymer-insecticide bounds in a chain reaction. The re‐ lease control depends on the strength of those chemical bounds, the chemical properties of both molecules and on the size and structure of the macromolecule formed [11]. The higher the biocompound solubility in water, faster the reaction occurs. Concerning the chemical proper‐ ties of the polymer, Allan *et al.* [11] studied the differences in the release kinetics when 2-meth‐ yl-4-chlorophenoxy acetic is chemically bound to polyvinylalcohol (a water-soluble polymer) or when it is bound to cellulose or lignin (water-insoluble polymers). In the first situation, the level of the applied herbicide tends to go down, because the equilibrium

#### *Polymer* −*insecticide* ⇌ *Polymer* + *Insecticide*

frozen and submitted to a low pressure system. When the pressure is drastically re‐ duced, the water sublimes (goes from solid to vapor state), leaving only the particles.

In the paper published by Kratz *et al.* [103] the text begins with the statement: "Nanoparti‐ cles only start working after they are placed in a desired location". In other words, an effi‐

The way how an inert material, such the nanopolymers, controlss the amount and rate a chemical is released is object of study since the late 1960's [104] and early 1970's [105].

How the release of the bioactive compound occurs depends basically on the chemical nature of the formulation. In various polymeric nanomaterials, the controlled release proceeds via diffusion. It does not matter if the bioactive compound is dissolved (micro- or nanospheres) or if it is encapsulated (micro or nanocapsules). The process does not depend on the chemi‐ cal structure of the formulation constituents [11] neither on the intermolecular interactions. The rate control is made based on the interactions between the carrier and the biocide. The stronger the interaction will be slower the release rate. In the 1990's, the release dynamics was investigated via the use of 14C-labelled molecules of herbicides [106,107]. Qi *et al.* [107] studied the dynamic of controlled release for herbicides. They used 14C-labelled molecules of benthiocarb and butachlor and observed that the release is made by a diffusive process. Some years later and without any radiolabeled molecules, Fernandez-Perez et al. [108] found the same results. They prepared a granule-based CRF constituted by lignin and imi‐ dacloprid. They measured the amount of compound released in water under a dynamic flow condition during a defined period of time. The data fitted a diffusion curve based on the model proposed by Ritger and Papas [109,110]. Since then, other similar studies have

Some other polymeric nanomatrixes, especially those formed by a carboxylic acid and a met‐ allic cation, can be disassembled when in contact with water, releasing the bioactive com‐ pound [92]. The release rates depend on the physicochemical characteristic of both molecules. The more hydrophobic the polymer slower will be the bioactive compound re‐ lease. The same applies to the last one: the higher water-solubility, faster it will be released. The formulation itself also affects directly the release rate. In water-based one, the rate con‐ trol tends to disappear, due to the matrix (or support) degradation. If the particles are solu‐ bilized in an organic solvent, like acetone, the formulation becomes sticky and the release rate slows down. A granule-based formulation sounds more efficient. It can be applied di‐ rect to the soil and the bioactive compound will be released according to the soil moisturize

In other formulations, the bioactive compound is covalently bound to the polymeric matrix [115]. To the release takes place, a chemical interaction must be broken. It usually occurs via a hydrolysis reaction, what affects many polymer-insecticide bounds in a chain reaction. The re‐ lease control depends on the strength of those chemical bounds, the chemical properties of both molecules and on the size and structure of the macromolecule formed [11]. The higher the

cient CR formulation must remain inactive until the active compound is released.

**3.5. Mechanism of biocide release**

536 Insecticides - Development of Safer and More Effective Technologies

been published [111-114].

(water content), leading to a long lasting control.

will always exist. I the last situation, as the "free polymer" is water–insoluble, the equilibri‐ um moves towards the right side and the level of the applied herbicide tends to go up (Fig‐ ure 5).

**Figure 5.** Trend in active compound's application rate (*Adapted* from [11]).

Whatever the mode of liberation, it should be kept in mind that controlled release formula‐ tions have a limited maximum amount for the release of the biocide[116]. This means that the total amount of released product may not be necessarily equal to the amount of chemicals in‐ corporated to the formulation neither to the amount of free applied product[117]. This is the reason why the concentration of the active compound in a CRF is usually higher than in a con‐ ventional one. However it does not contradict what was said earlier about advantageous re‐ duced amount if biocide applied, since the number of applications should be smaller.

Studies recently published suggested that the encapsulation of biocides reduces their toxici‐ ty [117,118]. However, many issues regarding the toxicity of the nanomaterial themselves to‐ wards the environmental and even the worker's health remains unclear [119].
