Controlled biocide release from hierarchically-structured biogenic silica: surface chemistry to tune release rate and responsiveness

Biocides are essential for crop protection, packaging and several other biosystem applications. Therein, properties such as tailored and controlled release are paramount in the development of sustainable biocide delivery systems. We explore the self-similar nano-organized architecture of biogenic silica particles to achieve high biocide payload. The high surface area accessibility of the carrier allowed us to develop an efficient, low energy loading strategy, reaching significant dynamic loadings of up to 100 mg·g−1. The release rate and responsiveness were tuned by manipulating the interfaces, using either the native hydroxyl surfaces of the carrier or systems modified with amines or carboxylic acids in high density. We thoroughly evaluated the impact of the carrier-biocide interactions on the release rate as a function of pH, ionic strength and temperature. The amine and carboxyl functionalization strategy led to three-fold decrease in the release rate, while higher responsiveness against important agro-industrial variables. Key to our discoveries, nanostructuring thymol in the biogenic silica endowed systems with controlled, responsive release promoting remarkable, high and localized biocidal activity. The interfacial factors affecting related delivery were elucidated for an increased and localized biocidal activity, bringing a new light for the development of controlled release systems from porous materials.


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Detailed information on the thin layer chromatograph (TLC) experiments. The thin layer chromatography (TLC) plates were prepared using the unmodified and surface modified biogenic silica nanoparticles as stationary phase. For this, 1.5 g silica was suspended into 30 ml of destilled water and ultrasonicated for 15 minutes, then 225 mg calcium sulfate hemidrate (15 wt% related to mass of silica) were added to the suspension and quickly homogenized. The suspension was dispensed on cleaned glass slides, which were allowed to air-dry overnight.
Before use it, the plates were activated at 110 o C for 30 minutes. Hexane (HEX), dichloromethane (DCM), ethyl acetate (EtAc), ethyl alcohol (EtOH), acetic acid (AcOH) and deionized water (dH 2 O) were used as solvents to calculate the retention factor (Rf) of the thymol on silica.
Image processing for quantitative evaluation of the agar diffusion tests. The software ImageJ was used to create grayscale profiles of neighboring area of the discs observed in the biocidal assay (Fig. S1). To avoid external influence from the light reflection of the images, we have presented the results as the ratio between the intensity of region where bacteria colonies were unaffected by the BDS and the region next to the BDS pellets. Also, we measured the diameter of the pellets in pixels as an internal comparison for bactericidal assessment of the BDS. Three-line measurements were made spaced by 120°. The referred gray value was taken as an average of the entire zone in reference.

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Surface characterization of BSiO 2 nanoparticles through XPS analysis. The XPS analysis stating the surface functionalization is presented in the Fig. S3. The same characteristic carbon peak related to the C-H and C-C chemical structures was observed at 285 eV for the SiOH and SiNH 2 particles (Fig. S3c); however, a new carbon moiety with peak at 288.5 eV was verified after deconvolution of the high-resolution carbon spectra of the SiCOOH particles (Fig. S3d).
The high-resolution XPS spectra of N 1s for the SiCOOH (Fig. S3g) particles shows notable differences when compared to the SiNH 2 (Fig. S3f). Free amino groups appear at 399.1 eV for both SiNH 2 and SiCOOH particles; however, the contribution of these groups to the total nitrogen response decreased from 63.7 to 32.6% after carboxyl-functionalization because part was converted into amide groups via ring opening linker elongation reaction with maleic anhydride. The interactions between neighboring amino groups undergo H-bonding or between amino groups and unreacted silica hydroxyl groups appeared at around 401.5 eV. Again, this peak had lower contribution (20.0%) in the N 1s main peak for the SiCOOH particles when compared to the SiNH 2 (36.3%). A small shift was observed for this component, and it may be attributed to the interaction of the amino groups with OH moieties from carboxyl termination instead with those from silanol groups. After deconvolution, a new nitrogen component related to the amide group appeared at 399.9 eV and it contributed to 47.5% of the total N 1s spectrum of the SiCOOH particles. From these results, it is possible to say that the free amino groups were partially converted into amide groups via chemical reaction with maleic anhydride resulting in the carboxyl termination.
S7 Thermal decomposition patterns of biogenic silica. Thymol release rate from SiOH@Thy prepared with the different silica particle sizes showing that there is no influence of the particles sizes (in the studied range) on the delivery (b). S11 Fig. S8. Theoretical thymol profiles with tethered release rate through fraction compositions between the BDS with the fastest and slowest release rate (a). Elovich linearization of the thymol release out profiles at different temperatures for all obtained BDS (b) in order to calculate the activation energy of their release using the Arrhenius plot (c). An additional release profile at 15oC was carried out in order to precisely obtain the activation energy. S12 Control plates of the agar diffusion assay.