Stimuli-responsive Ca-alginate-based photothermal system with enhanced foliar adhesion for controlled pesticide release

Highlights

• A Ca-alginate-based multi stimuli-responsive system was fabricated.

• The system displayed a high adhesion capacity on sweet potato leaf surface.

• The system exhibited pH, temperature and light controlled-release properties for imidacloprid.

• The pesticide carrier possessed great stability and biosafety.

Abstract

For minimizing volatilization and leaching of pesticides, and enhancing their residence time on crop surfaces, we synthesized and characterized a novel intelligent pesticide delivery system. Therein, imidacloprid (IMI) was adsorbed by polydopamine modified kaolin (PK) with high adsorption property through hydrogen bonds. Ca-alginate was used as a structural matrix of the system and a protective shell to hinder the pesticide burst release from PK, and endowed the system with unique pH-sensitive property for IMI release. The amino silicone oil (ASO) coating could bind with the waxy layer of crop leaves by the theory of “similarity-intermiscibility”, which increased the adhesion of composite on crop leaves. Moreover, by the excellent light-sensitive property of detonation nanodiamond (DND) and temperature-responsive performance of poly(N-isopropylacrylamide) (PNIPAm), the release of IMI from the functional system could be adjusted by sunlight. Besides, the composite displayed high control efficacy. This novel composite can promote the targeting ability and utilization efficiency of pesticides, thus having a huge potential application prospect in agriculture.

Introduction

Pesticides are highly used to control weeds, insects, and plant diseases; however, as much as 90 % of applied conventional pesticides do not work well due to degradation, volatilization, rainwater washing, and leaching [1,2]. Therefore, how to enhance the efficiency of pesticides, and regulate their loss behaviors has been an important topic in both agricultural and environmental chemistry [3]. In recent years, pesticide carriers are extensively used to decrease the non-wanted transfer of pesticides from crops to soil and water. An ideal pesticide carrier is desired to meet the following conditions: (1) adjustable release behavior according to external environments, (2) high adhesion capacity to leaves, (3) the bound pesticide can be available for the pest.

Intelligent controlled release systems (ICRS) are considered as the effective platform to control the release of pesticides according to external stimuli (like temperature [4], light [5], pH [6], and enzyme [7]). Thus, ICRS could limit the loss of pesticides in transport processes and enhance their utilization efficiency [8]. However, to our best knowledge, sunlight-controlled pesticide release systems in the field of botanical pesticides were scarcely reported. Moreover, when designing an appropriate ICRS in agriculture, interfacial interactions between carriers and crops must be considered to enhance residence times and meet the pesticide demand of crops during the growth period.

Generally, pesticide carrier systems deposited onto crops are easily leached into the soil and water, and volatilized or decomposed upon exposure to light. For solving these problems, many nanocarriers including inorganic adsorbents, organic matrices, and bio-sourced supports have been developed as endowing pesticides or fertilizers with high stabilities, making them long-acting and highly adhesive on crops [9,10]. However, most of them are very expensive, difficult to synthesize, and only provide solutions to individual problems, which limits their applications. The plant epicuticular wax layer consists of long-chain fatty acids and their derivatives such as aldehydes, secondary alcohols, alkyl esters, alkanes, and ketones [11]. The interactions between pesticide-carrier and the epicuticular wax layer were less explored before [12]. The waxy layer and pesticide carrier with similar structures may match better according to the “similarity-intermiscibility” theory. Based on the above principle, the surfaces of pesticide carriers can be easily modified to align with the waxy layer, thus enhancing the adhesion of active compounds on the surface of leaves.

In this article, a new kind of Ca-alginate-based pesticide carrier with high adhesion capacity and controlled release property was prepared. In the system, PDA modified kaolin (PK) with enhanced high adsorption property was used for imidacloprid (IMI) absorption. Sodium alginate (SA) with hydrophilicity, nontoxicity, and biodegradability was usually mixed with minerals to construct agriculture-related delivery systems [13,14]. Thus, alginate was used as the structural matrix of ICRS in this study. Simultaneously, amino silicone oil (ASO) acted as an incomplete coating to increase the adhesion of IMI on crops. The combination of temperature-sensitive polymer poly(N-isopropylacrylamide) (PNIPAm) and photothermal material detonation nanodiamond (DND) facilitated the pesticide delivery through photothermal stimulation. The effect of pH on the release of IMI from IMI/PK/Ca-alginate/PNIPAm/DND@ASO (IPKCPD@ASO) was detected. Additionally, the control efficacy of IPKCPD@ASO on aphids was investigated. Results show that the novel ICRS system exhibited excellent controlled-release behavior as well as good adhesive performance on leaves of sweet potato, which has a potential prospect in sustainable agricultural application.