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2403Review Received:20November2018 Revised:17January2019 Acceptedarticlepublished:22January2019 Publishedonlinein Wiley Online Library:19Februar y2019 (wileyonlinelibrary. com)DOI10. 1002/ps. 5348 Safenanotechnologiesforincreasingthe effectivenessofenvironmentallyfriendlynaturalagrochemicals Maurizio Vurro,a*Cristina Miguel-Rojasband Alejandro Pérez-de-Luquec ABSTRACT Naturalcompoundsandlivingorganismscontinuetoplayalimitedroleincropprotection,andfewofthemhavereachedthe market,despitetheirattractivenessandtheeffortsmadeinresearch. Veryoftentheseproductshavenegativecharacteristicscompared to synthetic compounds, e. g., higher costs of production, lower effectiveness, lack of persistence, and inability toreach and penetrate the target plant. Conversely, nanotechnologies are having an enormous impact on all human activities,including agriculture, even if the production of some nanomaterials is not environmentally friendly or could have adverseeffectsonagricultureandtheenvironment. Thus,certainnanomaterialscouldfacilitatethedevelopmentofformulatednaturalpesticides, making them more effective and more environmentally friendly. Nanoformulations can improve efficacy, reduceeffectivedoses,andincreaseshelf-lifeandpersistence. Suchcontrolled-releaseproductscanimprovedeliverytothetargetpest. Thisreviewconsiderscertainavailablenanomaterialsandnanotechnologiesforuseinagriculture,discussingtheirpropertiesandthefeasibilityoftheiruseinsustainablecropprotection,inparticular,inimprovingtheeffectivenessofnaturalbio-basedagrochemicals. ©2019Societyof Chemical Industry Keywords: naturalagrochemicals;nanotechnologies;biologicalcontrol;naturalpolymers;nanomaterials;nanopesticides 1 INTRODUCTION Biological constraints, for example, fungal and bacterial pathogens, viruses, arthropods, and weeds, are responsiblefor major losses in quality and in yield of crops and grasslands. Effective pest management is a major challenge in modern agri-culture,withaneedtoconsidercontrolefficacy,costaffordability,environmental safety, toxicity towards non-target organisms, and sustainability of the production system. Despite progress in many technological fields, most management of these con-straints is still based on the use of synthetic chemicals. However, a large number of pesticides have already been withdrawn for regulatory reasons, because of their hazardous effects on theecosystem or on the food chain, or because they have become ineffective as the result of increasing pesticide resistance. 1These compounds are not being effectively replaced, causing seriousdifficulties for farmers in managing pests. As a consequence, there is a renewed interest in the development of alternatives to syntheticpesticides. 2 POTENTIALANDLIMITSOFNATURAL AGROCHEMICALS-TWOFACESOFTHESAMECOIN Organisms interact with each other, protecting themselves from attack by the others or combating the others' defencebarriers, by producing an enormous number of mostly still unex-plored secondary metabolites; e. g., allelopathic compounds, phytoalexins, antibiotics, repellents, fungal toxins, antifeedants,and insecticides. These chemicals are the result of co-evolutionoftheproducingorganismanditsbioticenvironment,andcould represent an extraordinary source of new biologically activecompounds, with novel chemical structures and mechanismsof action, to be used in crop protection. Isolating and identifying thesecompoundshasbeenanarduoustaskinthepast,butmod-ern instrumentation (e. g., high throughput screening systems oradvanced analytical equipment) and sophisticated approaches(e. g., 'omics' tools) have simplified this process and reduced its cost. 2Although many natural compounds have been described, many have yet to be discovered. There are good examples ofnatural products used as herbicides (e. g., bialophos produced by Streptomyceshigroscopic ), insecticides (e. g., spinosyns, a family of macrocyclic lactones derived from species of the actinomycete bacterium Saccharopolyspora spinosa ), and fungicides (e. g., stro-bilurins, with its name from Strobilurustenacellus, a wood-rotting fungusfromwhichthefirstcompoundinthisgroupwasisolated),andmanyreviewsareavailableonthissubject. 3,4 ∗Correspondence to: M Vurro, Institute of Sciences of Food Production (ISPA), National Research Council (CNR), Via Amendola 122/O-70126 Bari, Italy. E-mail:maurizio. vurro@ispa. cnr. it a Instituteof Sciencesof Food Production,National Research Council(CNR),Bari, Italy b Departmentof Scienceand High Technology,Universityof Insubriaand Total Scattering Laboratory,Como,Italy c Genomicand Biotechnology,Centre Alamedadel Obispo,Andalusian Institute of Agriculturaland Fisheries Researchand Training(IFAPA),Cordoba,Spain Pest Manag Sci 2019;75:2403-2412 www. soci. org ©2019Societyof Chemical Industry | Pest Management Science - 2019 - Vurro - Safe nanotechnologies for increasing the effectiveness of environmentally friendly.pdf |
2404www. soci. org MVurro,CMiguel-Rojas,APérez-de-Luque Table1. Useofnaturalcompoundsasagrochemicals:thetwosides ofthesamecoin(inspiredby Table1in Dayanetal. 20125) Expectedadvantages Possibleconstraints Newandunusual structures Chemicalstructurestoo complex Newsitesand mechanismsofaction Unwantedactivitiesagainst non-targetorganisms Structuresoptimizedfor bioactivity Physic-chemicalproperties notsuitedforthecommercial/applicative needs Eco-friendlyproducts Half-lifetooshort Expectationtobeused atlowdoses Loweffectiveness Obtainablefromliving organisms Notsuitableforindustrial scaling-up Extractablefrom renewableresources Lowyields/highcostof extraction Fasterandsimpler screeninganddiscoveryprocedures Re-evaluationand productionofalreadyknowncompoundstoo expensiveornot marketable Higheracceptabilityby publicopinion Registrationprocedures similartochemicals Possiblelower registrationcosts Limitedintellectual protection Despite the potential of natural metabolites to be used as safe and environmentally friendly agrochemicals, some of their char-acteristics often concurrently represent possible constraints andlimiting factors for practical application (Table1). 5For example, natural agrochemicals could offer novel chemical structures withnewmodesofaction,butthesecompoundsareoftentoocomplextobeobtainedviaaffordablesynthesis. Theyareobtainedfromliv-ingorganismsbut,inmanycases,inverymodestamounts,orthepurification procedures are too expensive and/or not sufficientlyenvironmentallyfriendly. Theyarebelievedtohaveminimalenvi-ronmentalimpact,butthischaracteristicisusuallyassociatedwitha short half-life due to instability or excessive biodegradability,making them commercially unattractive. Natural agrochemicalscan be too specific or slow acting, or may not reach the in vivo target. Sometimes they must be applied at very high rates, mak-ingthemtooexpensiveordifficulttoapply. Thus,althoughthelistofpromisingorproposednaturalagrochemicalsisverylong,theirsuccessonthemarketisstillquitelimited,beinglesscompetitiveandsatisfactorythansyntheticagrochemicals. 3 NANOMATERIALSINAGRICULTURE Nanotechnology could help change this scenario by developingnew tools to improve the effectiveness of natural bioproductsand by overcoming the weaknesses and other factors limitingtheir use (Table2). Nanoscale-based delivery systems usuallyrange in particle size from 1 to 100nm, although, in pharma-ceutical science, nanoparticles can be up to 1000nm. Differentdefinitions have been proposed for nanomaterials. In 2011 the European Commissiondefinedananomaterialinamore-technical,but wider-ranging, way as “a natural, incidental, or manufacturedmaterial containing particles, in an unbound state or as anaggregate or as an agglomerate and where, for 50% or moreof the particles in the number size distribution, one or more Table2. Somepossibleadvantagesofnanotechnologyusetoover-come natural agrochemical weaknesses (examples in the review andintheprovidedreferences) Naturalagrochemical weakness Nanotechnologyimprovement Solubility Favoursthesolubilityoflow-soluble naturalcompounds Ecologicalfriendliness Reducesoravoidstheuseof organicsolventsforagrochemicaldelivery Bioavailability Modulate/slowthereleaseofthe compoundagainstthetarget/in theenvironment Dose Minimise/optimiseeffectivedoses Mobility Reducetheriskofleachingor volatilization Targetselection Helpthecompoundtoselectively recognize/attackthetarget Shelf-life Preservefromdegradationdueto bioticandabioticagents Adhesion/penetration Favoursthestick-on,orthe penetration-throughplantortargetsurface Non-targeteffects Reducesthetoxicitytonon-target organisms external dimensions is in the size range of 1-100nm” (https:// eur-lex. europa. eu/legal-content/EN/TXT/HTML/?uri=CELEX:32011H0696&from=EN). According to the International Organization for Standardization(ISO),ananoobjectis“adiscretepieceofmate-rialwithone,twoorthreeexternaldimensionsonthenanoscale,i. e. ranging from approximately 1 to 100nm. Nanoparticles are nanoobjects with all external dimensions on the nanoscale, where the lengths of the longest and shortest axes do not dif-fer significantly. If the dimensions differ significantly, typicallyby more than a factor of three, other terms, such as 'nanofibre'(two external dimensions in the nanoscale) or 'nanoplate' (oneexternal dimension on the nanoscale) may be preferred to the term nanoparticle” (https://www. iso. org/standard/54440. html). However, an 'agreed' standard definition of nanomaterial is stillan open question, and will certainly have a strong influence onregulatory development and industrial/research interests (see above). Inthisregard,severalyearsago,the European Food Safety Authority (EFSA) commissioned a study aimed to prepare aninventoryofnanotechnologyapplicationsintheagricultural,feedandfoodsector. 6Asreported,nanomaterialscanhaveanorganic or inorganic nature, or can be derived by a combination of the two. Themainpossibleapproachestousingnanomaterialsincrop protection are: (i) the direct use of inorganic nanomaterials asnanopesticides(NPs),suchasnoblemetalsorsilicon-basedmate-rials; (ii) the use of natural/synthetic nanoscale delivery systems,suchasnaturalpolymers,tobetterdeliveractiveingredients(AIs); and(iii)theformulationofthecurrentlyavailableagrochemicalsat ananoscaledimension,bypreparingimprovednanoformulationsandnanodispersions. 7 Although the use of nanomaterials can bring significant bene-fits to the agro-food sector, some health and safety issues must be considered. The risk of using these technologies is relatedmainlytothesmallsizeofthenanoparticlesandthelargesurfacearea-to-volume ratio, which increases their reactivity and couldresultineasydispersion,crossingofanatomicalbarriers,reaching wileyonlinelibrary. com/journal/ps ©2019Societyof Chemical Industry Pest Manag Sci 2019;75:2403-2412 15264998, 2019, 9, Downloaded from https://scijournals. onlinelibrary. wiley. com/doi/10. 1002/ps. 5348 by <Shibboleth>-member@gla. ac. uk, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary. wiley. com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License | Pest Management Science - 2019 - Vurro - Safe nanotechnologies for increasing the effectiveness of environmentally friendly.pdf |
2405Nanotechnologiesfornaturalagrochemicals www. soci. org more distal regions of the animal or human body, and displaying potentialtoxicity. Intheagriculturesector,handlingofnanofertil-izers and pesticides, which can be easily dispersed into the soil,water, or atmosphere, may increase the health risk of applica-torsandmayincreaseenvironmentalrisksaswell. Thus,designing low-toxic,biodegradableandeco-friendlynanoparticleswouldbenecessary. 3. 1 Solidnanoparticlesasnanopesticidesandpesticide carriers Many different nanoparticles have intrinsic pesticide properties and,thus,havebeenconsideredbothaspotentialactiveingredi-ents(nanopesticides)andasnanocarriersforthedeliveryof AIs. 3. 1. 1 “Inertnanomaterials” Thisgroupincludesanumberofmaterials(amorphousnanosilica, nanoclays, nanohydroxyapatite), of natural or synthetic origin,that have been considered eco-friendly pesticides because they act mainly via physical mechanisms, being physio-sorbed by the cuticular lipids and disrupting the protective epidermis layer. 8 For example, different amorphous silica nanoparticles (SNPs)provedtobemoreeffectivethanbulksilicaagainstthericeweevil Sitophilus oryzae. 9Surface-charged, modified hydrophobic silica NPs were successfully used to control some agricultural insects andectoparasitesofveterinaryimportance. 10Theyweresuccess-fully applied as a thin film on seeds to decrease fungal growth andboostcerealgermination. Applicationofsilica NPsontheleaf andstemsurfacedidnotaltereitherphotosynthesisorrespirationin several groups of horticultural and crop plants. They did not cause alteration of gene expression in insect trachea and were, thus,qualifiedforapprovalasnanobiopesticides, 11althoughtheir toxicity remains to be understood. 12A novel formulation based on silica NPs has also been proposed for improving the effective-ness and slowing the release of the pro-insecticide chlorfenapyr,with promising results. Field tests showed that the insecticidal activityassociatedwithsilica NPswasatleasttwicethatofchlorfe-napyr,associatedwithmicroparticlesorwithoutparticles,andtheinsecticide release was slowed down to over 20weeks, providing high-localizedconcentrationoveralongperiod. 13 Mesoporous silica nanoparticles (MSNs) are synthetic parti-cles possessing well defined and tuneable pore sizes (2-50nm), large pore volumes, high surface areas with easily-modifiable surface properties, chemical stability, resistance to microbialattack, tailorable nanostructures, biocompatibility, and aqueous degradability. 14,15Moreover, MSN protect loaded active ingredi-ents(AIs)againstenzymaticdegradationasnoswellingorchangesin porosity occur in response to external stimuli, such as p H and temperature. 16MSNs are excellent pesticide delivery carriers, as their structural properties can be modified to either enhance orslowreleasekinetics. 17,18MSNswerestudiedforstorageandcon-trolled release of the fungicide metalaxyl. The fungicide, loaded into MSNporesfromanaqueoussolutionbyarotaryevaporationmethod,wasreleasedinsoilandwaterveryslowlyover30days. 19 In other studies, MSNs proved to be effective carriers for thedeliveryofthenaturalpesticideavermectin,beingabletoprotectthe AIagainst UVdegradationandtoslowitsrelease,dependent ontheporediameterandshellthickness. 20 Diatomaceousearth(DE)isalmostpureamorphoussilicondiox-ide,madeupoffossilisedphytoplankton. Bymilling,afine,talc-like powderordustisobtained;itisextremelystableandnotreactive and is considered non-toxic to mammals. It acts as an insecti-cide by absorption of epicuticular lipids and fatty acids, leadingt od e s i c c a t i o ni na r t h r o p o d s. 21Recent research has focused on enhanced DEswithotherinsecticidestoallowcontrolatlowdoses. For example, a mixture of DE with the plant ( Celastrusangulatus ) extract bitterbarkomycin (sesquiterpenes) was evaluated againstthe grain pest, Rhyzoperthadominica, and was found to be effec-tiveatdoseratesaslowas150mgkg-1ofwheat. 22 Nanostructuredalumina(NSA)wasdiscoveredandproposedas an effective insecticide against two grain pests, Sitophilus oryzae and R. dominica. Itwasmoreeffectivethanacommercialdiatoma-ceousearthproduct. 23More recent research on NSA, synthesized using a modified glycine-nitrate combustion process, revealedthatthemechanismofinsecticideactionisbasedonphysicalphe-nomena rather than biochemical mechanisms. Moreover, parti-cle size, surface area, and morphology are key factors that deter-mine insecticidal efficacy. Modifications of the synthesis routecould allow the achievement of better results for the targetedspecies. 24,25 Nanoclays are thin sheets of organic silicate material ( ∼1nm thickand70-150nmwide)thatisproducedfrommontmorilloniteclays commonly found in volcanic ash. Their size is reduced andtheirsurfacemodifiedtoformbio-compatibleandlow-toxicnan-oclays. They have been successfully used as carriers for the plant growth regulator, 𝛼-naphthalene acetate and for the controlled releaseoftheherbicide,2,4-dichlorophenoxyacetate. 26Theywere further studied as a carrier for the natural antibiotic cinnamate,27 whichisaproblematicagrochemicalbecauseofitsrapiddegrada-tioninsoilandthehighdosagesnecessaryforeffectiveness. When loaded in nanoclays, it proved to be released more slowly and to beretainedinthesoilforalongerperiod. Thisindicatesexcellentpromisefortheuseofnanoclaysforslowandtargeteddeliveryofpesticidesandfungicides,aswellas DNA(seeabove). 28 3. 1. 2 Metalnanoparticles Differentmetallicandmetaloxidenanoparticles,MNPs(copper,sil-ver,titaniumoxide),havebeenconsideredasantimicrobialagents. Becauseoftheirlargersurface-to-volumeratioandtheircrystalline structure,theymoreeffectivelytriggerbiologicalresponsescom-paredtothetraditionalionicformofthemetals. 29-31Although,in some studies, MNPs proved to be less toxic to mammalian cellsthan their corresponding ionic forms and to have a prolonged effect as a source of elements in an organism, alongside reduced risk for the environment and non-target organisms, their possi-ble use in large amounts for agricultural purposes is still an opendebate. Production costs and regulatory obstacles are also issuesthat must be resolved. Improvements in the process of MNP syn-thesis have been obtained by including organisms in their pro-duction. This green synthesis has some advantages comparedto other methods, as it is less costly, is scalable for large pro-duction, and it avoids waste of energy and the use of harmfultoxic substances. 32Hence, new strategies using bioactive materi-als from various biological sources are of special interest. Groups of different microorganisms, including fungi, bacteria and yeasts,and plant extracts, are being used to produce NPs. 33,34Using this approach, Trichoderma-mediated Seleniumnanoparticles(Se NPs) weresynthesizedrecentlyandusedtocontroldownymildewdis-ease in pearl millet. 35The antimicrobial activity of those MNPs seemstooccurvia:(i)photocatalysis-absorbedphotonsleadingtothereleaseofsuperoxideradicals,whichcausethedeathofbacte-rial, fungal, and viral organisms by oxidation of critical molecularstructures;(ii)accumulationanddissipationinthecellmembrane, leading to membrane damage and release of cell contents; and Pest Manag Sci 2019;75:2403-2412 ©2019Societyof Chemical Industry wileyonlinelibrary. com/journal/ps 15264998, 2019, 9, Downloaded from https://scijournals. onlinelibrary. wiley. com/doi/10. 1002/ps. 5348 by <Shibboleth>-member@gla. ac. uk, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary. wiley. com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License | Pest Management Science - 2019 - Vurro - Safe nanotechnologies for increasing the effectiveness of environmentally friendly.pdf |
2406www. soci. org MVurro,CMiguel-Rojas,APérez-de-Luque (iii)uptakeofmetallicionsintocellsfollowedbydisruptionof DNA replication. 36 Silver nanoparticles (Ag NPs) have been studied for a relatively long time. They are the most active MNPs, with both bactericidal and fungicidal efficacy. For instance, they were tested in lab conditions against Raffeleasp., the fungal causal agent of oak wilt,37andagainstanumberofotherplantpathogens. 38However, the potential side effects and the environmental impact of these MNPsremaintobedetermined. Anumberofmicroorganisms,e. g., plant growth-promoting rhizobacteria (PGPR), were also used for thebiosynthesisof Ag NPs. Forexample,astrainofthebacterium Serratiasp. isolatedfromagriculturalsoilshowedthepotentialto synthesize Ag NPs with strong antifungal activity against Bipolaris sorokiniana,thespotblotchpathogenofwheat. 39 Theantibacterialpotentialofphotocatalyticnanoscaletitanium dioxide (Ti O2), by itself or augmented with other metals, was evaluated against Xanthomonas perforans, the causal agent for bacterial spot disease of tomato. The absorption of photons by Ti O2resulted in the creation of free electrons interacting with watermoleculestocreatehighlychemicallyreactivehydroxyland superoxidefreeradicals. 29Theextentofmicrobialkillingvariesas afunctionofthetargetorganism,theintensityofillumination,the efficiency of photo-catalysis, and the duration of exposure. Both Ti O2and Zn have been reported to have lower ecological and toxicological risks at the application rates investigated than the copper-basedbactericides in normaluse. Ti O2occurs naturally in soilsandinhighlypurifiedforminmanycommercialproductsover decades,andisclassifiedasnontoxic. Inotherstudies,40coppernanoparticleswereencapsulatedand stabilizedwithhighlybiocompatiblegelatinthatisexpectedtobe advantageousforinteractionoftheparticleswithcellmembranes and their subsequent entry into the cell cytosol. Those NPs per-formedbetterthantheequivalentamountoftheirprecursor Cu Cl2 against Gram-positiveand Gram-negativebacteria. Morerecently,the same authors 36showed, in E. coli,that the effect of NPs was due to multiple toxic effects such as generation of reactive oxy-genspecies,lipidperoxidation,proteinoxidationand DNAdegra-dation. Copper NPs also have in vitrosynergistic effects against somefungalphytopathogenswhencombinedwithcopper-based fungicides. 41However, evaluation of the environmental fate and the possible adverse effects on non-target organisms under field conditionsshouldbedetermined. 3. 2 Nanoemulsionsandnanodispersions One of the main limitations with the use of synthetic or natural pesticides is that they are generally poorly soluble in water; thus, they must be dispersed in a liquid phase for application, making necessarytheuseoflargeamountsoforganicsolventstodissolve them. One way by which the problem is solved in commercial formulations is to combine the pesticide with a surfactant, thus increasing solubility for suitable efficacy and uniform application inthefield. Inthecaseofliquidpesticides,theseformulationsare calledemulsifiableconcentrates,whilesolidpesticidesarereferred to as wettable powders. Typically, particle sizes for these formu-lations are in the micron range (1-20 μm in diameter). However, this approach has some disadvantages, such as increased costs, increasedenvironmentalpollutants,andincreaseduserrisks. Nan-otechnologyoffersmoreenvironmentallyfriendlyandsustainable alternatives,suchasdispersingthe AIinaliquidasacolloidinthe form of nano-sized droplets or solid particles, stabilized with the aid of surfactants. 42Nanoemulsions may be of the oil-in-water (O/W)orwater-in-oil(W/O)types,dependingonwhethertheoilisdispersedasdropletsinwater,or viceversa. 43Solidlipidnanoparti-cles(SLNs)areotherpromisingcarriersystemsthatcanbeusedto transportnonpolarsubstances,themobilityofwhichisrestricted by interaction with the lipids. 44Although the main objective is to improve water solubility, recent research has shown improve-mentsinotherimportantpropertiesofthepesticide,forexample,(i) increased bioavailability, due to a combination of greater sur-faceareaforexposureandenhancedpenetrationintothetarget; 45 (ii) enhanced stability, e. g., via protection from UV lights46or from hydrolysis;47and (iii) controlled release, to slow the release process, resulting in more sustained exposure and longer termefficacy. 48,49Numerousreviewshavecompared AInanoemulsions and nanodispersions to the respective traditional commercial micro-formulations,42,50and many others have considered their technological properties or fabrication methods. Many studieshavealsobeencarriedouttoimprovetheeffectivenessofnaturalagrochemicalsbyusingtheseapproaches. 51-53Thisbeinganenor-mousfieldofresearch,onlyseveralexampleshavebeendiscussed. 3. 3 Polymer-basednanopesticides Polymer nanoparticles and nanocapsules are composed of natu-ral or synthetic polymeric materials, some of which have a desir-abletrait:theyarebiodegradable. Thesubstancesthatcanbeusedfor synthesis of these nanodevices include starch, polypeptides,albumin, sodium alginate, chitin, gelatin and cellulose, amongstothers (Fig. 1). The first work in this area was undertaken approx-imately50yearsagobya Germangroup 54lookingforpharmaco-logicalapplications. Chitosan is a polymer that can be obtained by treating chitin from shrimp and other crustaceans with a base (alkaline sub-stance), producing a polymeric 𝛽-glucan. It is well known as an elicitor of defence responses in plants and it possesses antifun-galpropertiesthatmakeitveryattractiveforapplicationsinplantprotection. 55However, it can be used as a carrier for pesticides when synthesized in the form of nanoparticles, either alone56 or in combination with other polymers. 57This double function as nanocarrier and active substance itself, in addition to its ori-gin from a waste by-product from the fishing industry, turnschitosan into a promising material for nanoformulating naturalcompounds. 58 Alginateisanotherlinear 𝛽-linkedpolysaccharideisolatedform thebrownalgaegroup,Phaeophyceae,commonlyknownassea-weed. It can be treated in several ways to produce hydrogels,microspheres,nanoparticlesandnanocapsules, 59andcanbecom-binedwithotherpolymerssuchaschitosan,57producingahighly versatile system for nanoformulation of agrochemicals. Experi-mentsunderfieldconditionswithinsecticides(imidacloprid)haveshownthatareductioninthedoseoftheactiveingredientcanbeachieved while the effectiveness of the treatment is not compro-misedandisevenimprovedovertime. 60 Following with carbohydrate polymers, cyclodextrins are cyclic oligosaccharidescomposedofbetween6and8glucosesubunits(𝛼,𝛽and 𝛾cyclodextrins, respectively). They have a hydrophobic core and a hydrophilic shell, and they self-assemble in aque-ous solutions with other components to form nanoparticles and aggregates. In addition, cyclodextrins can be conjugated with other nanomaterials, enhancing their characteristics asnanocarriers. 58,61They have been successfully loaded with fungi-cides62and tested for treatment of fungal plant diseases in the field,63orhavebeencombinedwithfungicidalnaturalcompounds suchasgeraniol. 64 wileyonlinelibrary. com/journal/ps ©2019Societyof Chemical Industry Pest Manag Sci 2019;75:2403-2412 15264998, 2019, 9, Downloaded from https://scijournals. onlinelibrary. wiley. com/doi/10. 1002/ps. 5348 by <Shibboleth>-member@gla. ac. uk, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary. wiley. com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License | Pest Management Science - 2019 - Vurro - Safe nanotechnologies for increasing the effectiveness of environmentally friendly.pdf |
2407Nanotechnologiesfornaturalagrochemicals www. soci. org Figure1. Someofthemostimportantnaturalpolymersusedforthesynthesisofnanocarriersandtheirsourceoforigin. Plants store energy in the form of starch and, starch being one of the most abundant biomass materials in nature, it has mul-tiple applications in industry. Nanostructures from starch have been developed, leading to different results depending on theprotocols, but mostly producing nanocrystals and amorphous nanoparticles. 65Theyhavebeenusedfordeliveringnucleicacids inside plant cells66or producing slow release of insecticides67 and fertilizers through nanocomposites. 68This role of protection and slow release of the active components makes starch-based nanodevices quite attractive for combination with natural com-pounds. Lignins are cross-linked polymers of phenolic compounds that are constituents of the plant cell walls. Nanoprecipitation methods with lignin produce nanoparticles that protect the coated materials against corrosive agents 69and degradation by UV and oxidants. 70They can increase the efficacy of herbicide application71andtheyopeninterestingpossibilitiesforprotecting naturalcompoundsagainstdegradationbyexternalagents. Addi-tionally, many plant pathogens, such as fungi, produce specific enzymes for degrading cell wall constituents (e. g., lignins); thus, these nanocarriers could be targeted and degraded specificallyat the place where the fungal pathogen is acting, releasing the activeingredients. Viruseshavelongbeenpostulatedasameansofpestcontrol, 72 but more recently they have emerged as polyvalent nanocap-sules capable of auto-assembly and of carrying different substances ranging from drugs to nucleic acids. 73Specifically, plant viral particles are gaining importance in this role, and some formulations have been developed and tested for deliv-ering pesticides 74and natural compounds75against parasitic nematodes. Many other biodegradable polymeric nanocarriers are under development; these show promising results for use innanoformulations for pesticides or natural compounds, such asthose based on zein, 76cellulose,77lipid/protein nanodisks,78or synteticpolymers(e. g.,poly-𝜀-caprolactone). 79Moreover,particu-larlyintriguingforthepractical“consequences”arethestudiesonnanopolymersthataretriggeredunderspecificenvironmentalorbioticconditions. 80 4 TOXICITYANDENVIRONMENTALIMPACT Theideaofusingnanomaterialsforfieldapplicationsinagriculturemustbeaddressedcarefullyinordertoavoidthecreationofnewproblemswhilesolvingotherproblems. Forthatreason,determi-nation of the toxicity and the potential negative environmentalimpacts of nanodevices is necessary before approval. One of the bestwaystodealwiththisobstacleistoturntonanomaterialsthat have proven to be innocuous and safe for human consumption. However, this in not always a guarantee that massive applicationin the field of a product already in used in the food industry willnothavenegativeenvironmentaleffects,asisthecasewithsilvernanoparticles. 81,82Toxicity is a relevant factor to be tested before use of a nanodevice for agricultural applications. The direct toxiceffectsofnanoparticlesareusuallyassociatedwiththeirchemical composition and their high specific surface area (high reactivity), whichmakesthembiologicallyreactive. 83However,itisimportant to differentiate between a compound that produces cytotoxicityandthosethataretoxicfortheentireorganism(acuteorchronic). Becauseoftheirhighreactivity,somenanomaterialscanbeocca-sionallycytotoxicandlethalforindividualcells,buttheireffectontheentireorganismisnegligibleandinnocuous. Pest Manag Sci 2019;75:2403-2412 ©2019Societyof Chemical Industry wileyonlinelibrary. com/journal/ps 15264998, 2019, 9, Downloaded from https://scijournals. onlinelibrary. wiley. com/doi/10. 1002/ps. 5348 by <Shibboleth>-member@gla. ac. uk, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary. wiley. com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License | Pest Management Science - 2019 - Vurro - Safe nanotechnologies for increasing the effectiveness of environmentally friendly.pdf |
2408www. soci. org MVurro,CMiguel-Rojas,APérez-de-Luque Nanomaterials can have damaging effects on plants and other organisms, or can affect environmental processes. In the case of plants and algae, the negative consequences can involve alterationsinphotosynthesisduetoseveralfactorssuchasreduc-tion in light availability and gas exchange, leading to decreased CO 2fixation,84or by directly inactivating the plant photosystem and affecting the electron transport chain. 85Additionally, plant growth and physiology can be negatively altered,86and DNA damage (genotoxicity) has been reported. 87Both terrestrial88,89 and aquatic fauna90can be severely affected when exposed to certain nanoparticles at high concentrations. Also, soil microor-ganisms that play important beneficial roles, such as mycor-rhizal fungi 91and bacteria,81can show negative responses to the presence of some nanomaterials in their surroundings. Theseeffects can lead to altered properties in the soil such as micro-bial respiration, transport of liquid and/or gases, and failed sym-biotic relationships. Finally, nanomaterials may directly influenceenvironmental processes, for example, by altering precipitationby acting as nuclei for raindrops, 92by interacting with pollu-tants and, consequently, altering their toxic effects,93by dis-rupting nutrient cycles,94or by detrimentally affecting water purification. 95 An importantconsideration in the context of nanotoxicology is theexperimentaldesign. Itisnoteasytodevelopsetsofassaysthatprovidereliableinformationconcerningrealisticconditions. Secur-ingtherightdosage,theproperwayofapplication,theadequateexposure time, and the parameters that affect the performanceofthenanomaterialsindifferentmedia(e. g.,size,agglomeration,mobility,precipitation,etc. )arekeyfactorsthatmightcompromisethevalidityofresults. 96 In general terms, an ideal nanodevice for use in agricultural shouldcomprisethefollowingtraits:(i)itshouldbenon-toxicand environmentally safe, to avoid further contamination problemsandanegativeperceptiononthepartofconsumers;(ii)itssynthe-sisandproductionmustbeeasilyup-scaled;(iii)itshouldinvolvelow-cost materials, so that the cost of the new nanoformulatedproducts does not exceed that of the current agrochemicals andtheyareaffordabletofarmers. 97 Inordertoclarifytheneedsandachieveconformityamongpro-cedures, the EFSA has very recently prepared a guidance state-ment concerning risk assessment of nanoscience, nanoobjects and nanotechnology applications in the food and feed chain for humans and animals. The document provides insights intothe physicochemical properties and into exposure assessmentandhazardcharacterisationofnanomaterials,suggestingwaystoestablishwhetheramaterialisindeedananomaterial,andestab-lishingthekeyparameterstobemeasuredandthetechniquesforcharacterisationofnanomaterials. 98 5 NANO-ANDBIO-TECHNOLOGICAL APPROACHESFORDEVELOPINGANEWGENERATIONOFAGROCHEMICALS In the last two decades, RNA interference (RNAi)-based technol-ogyhasresultedinapowerfultoolforengineeringpest-resistantcrops, 99openingthedoorfornewagrochemicaldesign. Theappli-cationof RNAitechnologyisbasedondeliveryofdouble-stranded RNA (ds RNA) or small interfering RNA (si RNA) to gene silencing. Thus,RNAicanbeconsideredasanaturalgene-basedtechnologyfor highly specific pest control. The use of RNAi in pest manage-ment has been widely studied in different organisms, showingthe potential utility of this technology in both basic and applied science. For instance, expression of transgenes in wheat plantsfor production of ds RNA targeting fungal genes that code for MAP-Kinase and cyclophilin caused a pronounced reduction in leaf rust infection by Puccinia triticina. 100Immunity to Fusarium graminearum was observed in transgenic barley, targeting the sterol 14 𝛼-demethylase (CYP51) genes of the fungus. 101In addi-tion, when using this RNAi technology to silence a gene critical for survival of an insect pest, resistance of the transgenic plants was observed. 102-104Indeed, a new genetically engineered corn based on RNAi technology was developed and has already been approved by the US-EPA. It will reach the market soon and willhelp US farmers to control the corn rootworm (CRW). 105Finally, this RNAitechnologyhasalsobeenusedagainstviruses,bacteria, and nematodes. 106-108However, because of political, regulatory, or technical difficulties, transgenic crops are not always a viable solution. Hence, topical application of ds RNA for pest controlis emerging as an appealing alternative to genetically modified crops. Oneofthemostlarge-scalefieldstudieswasconductedby Hunter etal., 109to test the ability of topical delivery of a ds RNA product to protect honeybees from infection by the Israeli Acute Paralysis Virus (IAPV). The product was used as a food additive for wintering bees, with outstanding results regarding mortality and overall health. Topical spray delivery of ds RNA in planta has been reported to successfully target insect pests feeding on theplant. 110,111Soil applications for root absorption or trunk injec-tions have also been addressed with positive results on gene silencing, confirming that plant root can take up ds RNA and that trunk injections facilitate the delivery of ds RNA through xylem andphloem. 110-112 The rapid degradation of naked ds RNA has been a major chal-lenge in practical application. In general, ds RNA is much more stable than single-stranded RNA, but it must be rapidly taken up in the cells and digested into si RNA. Therefore, the use of nanomaterials as carriers, to reduce ds RNA degradation and toincrease cellular uptake of intact ds RNA, has gained relevance lately. Recently, Mitter etal. 28demonstrated that ds RNA can be loaded on non-toxic, degradable, layered double hydroxide (LDH) clay nanosheets, known as “Bio Clay”. Once loaded on LDH, ds RNA does not wash off, shows sustainable release, andcan be detected on sprayed leaves 30days after application. Moreover, this study confirms that ds RNA could be translo-cated to untreated parts of the plant, affording virus protection even after a single spray. Another type of non-toxic and eas-ily biodegradable nanocarrier is the polymer chitosan. Zhangetal. 113loaded RNAiintothemosquito, Anophelesgambiae using chitosan/ds RNA nanoparticles through larval feeding. Addi-tionally, a cationic core-shell fluorescent nanoparticle (FNP)has been successfully utilized as an efficient ds RNA carrier to knock down key developmental gene expression and kill insect pests. 114Other types of materials that have been used for ds RNA protection with positive results are liposomes,115,116 guanylated polimers,117carbon quantum dot, and silica nanoparticles. 118 The use of nanomaterials for improved delivery systems will grow in coming years; therefore, we could expect a remarkable increase in the broad range of materials used for ds RNA delivery. This innovative RNAi delivery method was initially developed forhumantherapeutics,andnownanotechnologyisbeingtranslated into crop protection as a sustainable strategy for pest manage-ment, minimizing impact on the environment and reducing the useofchemicalpesticides. wileyonlinelibrary. com/journal/ps ©2019Societyof Chemical Industry Pest Manag Sci 2019;75:2403-2412 15264998, 2019, 9, Downloaded from https://scijournals. onlinelibrary. wiley. com/doi/10. 1002/ps. 5348 by <Shibboleth>-member@gla. ac. uk, Wiley Online Library on [10/09/2024]. See the Terms and Conditions (https://onlinelibrary. wiley. com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License | Pest Management Science - 2019 - Vurro - Safe nanotechnologies for increasing the effectiveness of environmentally friendly.pdf |
2409Nanotechnologiesfornaturalagrochemicals www. soci. org 6 CONCLUDINGREMARKS Regulations for the registration and introduction of nanoagro-chemicals into the market are still missing. Uniform worldwide rules for defining nanoagrochemicals and for harmonising the methods of risk assessment are needed. 119If the rules were based on particle size only, as in EC recommendations,many recent so-called nanoformulations would be excluded. Conversely, many products that have been on the market fordecades without posing particular problems (e. g., microemul-sions,formulantssuchasclaysandpolymers)wouldbe'suddenly' considered nanomaterials. Moreover, regulation of a formulation should rely on a science-based assessment of new risks and thebenefits involved, not only in terms of individual ingredients, butalsobasedonthewaytheentirenanoformulationbehavesintheenvironment. Indeed, such products have the potential to bettermanagetheagricultureinputsand,thus,toreducetheirnegative impact on modern agriculture. Additionally, the potential risks resultingfromexposuretonanomaterialshouldbeassessed,usinganappropriatelytailoredlife-cycleperspective. Thismeanstakingintoaccountallphasesinwhichnanoformulationsmaybefound,including application in the field, potential incorporation intothe food supply, and disposal or re-use of the products, together with the possible influences exerted by peculiar agro-system conditions that may all affect nanomaterial hazardous propertiesand risk characterization. For instance, the improved bioavail-ability of a nanopesticide may affect its environmental fate, aswell as its toxicity or behaviour once absorbed by organisms. Therefore,arobusttoxicologicalassessmentofthepotentialrisks associated with use of nanopesticides, both as nanoformulations of traditional active ingredients or nanomaterials that exhibitpesticideactivity,shouldbeperformed. Thescientificcommunitycan positively or negatively affect public opinion concerningnanoagrochemicals, depending on whether a positive image ofthe technology (green, smart and safe technologies) is providedor,rather,thepotentialrisksarestressed. Thepurposeofachieving sustainableagricultureoverlapswiththeneedfordevelopmentof a 'green nanotechnology', a conceptual approach to balance thebenefits provided by nanoproducts in resolving environmentalchallenges with the assessment and management of environ-mental, health, and safety risks potentially posed by nanoscalematerials. However, to be really 'sustainable', not only the safety and risks of the final product should be taken into consideration, but also the entire production process, for example, the costs,environmental impact, and renewability of all material usedfor synthesis and production. A recent analysis of the literatureshows that the comparison studies between nanoagrochemicalsand conventional agrochemicals is insufficient to assess the true gains in agrochemical efficacy that result from nano-enabled products. 120Comparisons between nanoformulations and AIs can explain changes in AI behaviour. Comparisons with conven-tionalformulationsarenecessarytoshowimprovedperformanceand competitiveness as compared to existing products. Thus,three-way comparisons (nano-formulated and conventionallyformulated products and AIs) would be strongly recommended in future research. The future of nanoagrochemicals may follow twodifferentscenarios. 121Inthefirst,nanoagrochemicalsmaybe considered as emerging contaminants and the development oftechnology will remain limited. In the second, the establishmentof highly collaborative and interdisciplinary research could pro-videafairassessmentofbothrisksandbenefits,allowingfordeep explorationofnanoagrochemicalpotential. 120Focusedstudiesonsafe nanotechnology for improving natural agrochemicals could beanattractivegreenstrategy. ACKNOWLEDGEMENTS Thispaperwaspresentedorallyattheworkshopon Natural Prod-ucts in Pest Management: Innovative approaches for increasing theiruse,whichtookplacein Bellagio,Italyon25-29September 2018, and which was sponsored by the OECD Co-operative Research Programme: Biological Resource Management for Sus-tainable Agricultural Systems whose financial support made it possible for the corresponding author to participate in the workshop. 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