Role of Synthetic Plant Extracts on the Production of Silver-Derived Nanoparticles

Abstract

The main antioxidants present in plant extracts-quercetin, β-carotene, gallic acid, ascorbic acid, hydroxybenzoic acid, caffeic acid, catechin and scopoletin-are able to synthesize silver nanoparticles when reacting with a Ag NO₃ solution. The UV-visible absorption spectrum recorded with most of the antioxidants shows the characteristic surface plasmon resonance band of silver nanoparticles. Nanoparticles synthesised with ascorbic, hydroxybenzoic, caffeic, and gallic acids and scopoletin are spherical. Nanoparticles synthesised with quercetin are grouped together to form micellar structures. Nanoparticles synthesised by β-carotene, were triangular and polyhedral forms with truncated corners. Pentagonal nanoparticles were synthesized with catechin. We used Fourier-transform infrared spectroscopy to check that the biomolecules coat the synthesised silver nanoparticles. X-ray powder diffractograms showed the presence of silver, AgO, Ag₂O, Ag₃O₄ and Ag₂O₃. Rod-like structures were obtained with quercetin and gallic acid and cookie-like structures in the nanoparticles obtained with scopoletin, as a consequence of their reactivity with cyanide. This analysis explained the role played by the various agents responsible for the bio-reduction triggered by nanoparticle synthesis in their shape, size and activity. This will facilitate targeted synthesis and the application of biotechnological techniques to optimise the green synthesis of nanoparticles.

Keywords: cyanide; micellar structures; plant non-enzymatic antioxidants; silver nanoparticles.

Figure 1

Natural non-enzymatic antioxidants present in plants and used in this work.

Figure 2

Colour changes after the addition of different antioxidants of plant origin to a 2 M silver nitrate solution. (A) inception (time 0); (B) after 5 h of incubation at 40 °C in the dark. The numbers identify the different antioxidants used: 1 = quercetin; 2 = β-carotene; 3 = gallic acid; 4 = ascorbic acid; 5 = hydroxybenzoic acid; 6 = caffeic acid; 7 = catechin; 8 = scopoletin; and 9 = control (no antioxidant added).

Figure 3

Analysis of quercetin-synthesised silver nanoparticles (AgNP). (A–C) images of AgNPs obtained by transmission electron microscopy (TEM). (A) Micelle-like structures. (B) Micellar structure detail. (C) Detail of vesicles located between the silver nanoparticles. (D–F) TEM images of AgNPs after cyanide addition. (D) rod-like structure. (E) Detail of filament showing the aggregation of the nanoparticles. (F) Detail of rod-like structures with their size diameter and comparison with the size of the nanoparticles. (G) Fourier transform infrared (FTIR) spectrum of quercetin-derived nanoparticles. (H) FTIR spectrum of quercetin. (I) Ultraviolet (UV)-visible AgNP spectra prepared with quercetin and AgNO₃ prior (blue line) and after (orange line) cyanide addition. (J) X-ray diffraction (XRD) profile of AgNPs obtained by quercetin reaction with AgNO₃ (an identification table has been inserted). Peaks identifying the presence of silver oxides have been indicated in the XRD diffractogram.

Figure 4

Analysis of the silver nanoparticles synthesised with β-carotene. (A–D) images of AgNPs obtained by TEM. (A,B) A triangular nanoparticle under two illumination angles, which allows observing its volume. (C) Polyhedral shapes (indicated by the hollow arrow) and triangular shapes with truncated vertexes (indicated by the black arrow). (D) Polyhedral Amorphous AgNPs after treatment with cyanide. (E) Fourier transform infrared (FTIR) spectrum of β-carotene-derived nanoparticles. (F) FTIR spectrum of β-carotene. (G) Ultraviolet (UV)-visible AgNP spectra prepared with β-carotene and AgNO₃ prior (black line) and after (gray line) cyanide addition. (H) X-ray diffraction (XRD) profiles of the AgNPs obtained by the reaction of β-carotene with Ag NO₃; in the insert, the table with the identifications. Peaks identifying the presence of silver oxides have been indicated in the XRD diffractogram.

Figure 5

Analysis of silver nanoparticles synthesised with gallic acid. (A) TEM image of AgNPs. (B,C) TEM images of AgNPs after cyanide treatment. (B) Aggregation of nanoparticles prior to Figure 3. concentrations with gallic acid for 5 h (at one-hour intervals). (D) Synthesis of silver nanoparticles by the reaction of different AgNO₃ concentrations with gallic acid for 5 h (in one-hour intervals). (E,F) FTIR spectra of gallic acid-derived nanoparticles (E) and gallic acid (F). (G) Relative increase of OD after cyanide addition to silver nanoparticles. (H) XRD diffractogram of the AgNPs obtained by the reaction of gallic acid with Ag NO₃; in the insert, the table with the identifications. Peaks identifying the presence of silver oxides have been indicated in the XRD diffractogram.

Figure 6

Analysis of silver nanoparticles synthesized with ascorbic acid. (A) TEM image of AgNPs. (B) image of AgNPs after treatment with cyanide. (C–E) Synthesis of silver nanoparticles by the reaction of different AgNO₃ concentrations with ascorbic acid for 5 h (in one-hour intervals). (C) maximum at wavelength 402 nm (20 nm-diameter nanoparticles). (D) maximum at wavelength 421 nm (50 nm-diameter nanoparticles). (E) maximum at wavelength 467 nm (80 nm-diameter nanoparticles). (F) decrease of the OD after cyanide addition to silver nanoparticles. (G) Fourier transform infrared (FTIR) spectrum of ascorbic acid-derived nanoparticles. (H) FTIR spectrum of ascorbic acid. (I) Redox reaction of ascorbic acid to dehydroascorbic acid and effect of cyanide addition. (J) FTIR Scheme 3. in the insert, the table with the identifications. Peaks identifying the presence of silver oxides have been indicated in the XRD diffractogram.

Figure 7

Analysis of silver nanoparticles synthesised with hydroxybenzoic acid. (A) TEM image of AgNPs prior to treatment with cyanide. (B) image of AgNPs after treatment with cyanide. (C,D) Synthesis of silver nanoparticles by the reaction of different AgNO₃ concentrations with hydroxybenzoic acid for 5 h (in one-hour intervals). (C) maximum at wavelength 421 nm (50 nm diameter nanoparticles). (D) maximum at wavelength 467 nm (80 nm diameter nanoparticles). (E) FTIR spectrum of hydroxybenzoic acid-derived nanoparticles. (F) FTIR spectrum of hydroxybenzoic acid. (G) Relative reduction of OD after cyanide addition to silver nanoparticles. (H): XRD profile of the AgNPs obtained by the reaction of hydroxybenzoic acid with AgNO₃; in the insert, the table with the identifications. Peaks identifying the presence of silver oxides have been indicated in the XRD diffractogram.

Figure 8

Analysis of silver nanoparticles synthesised with caffeic acid. (A) TEM image of AgNPs prior treatment with cyanide. (B) TEM image of AgNPs after treatment with cyanide. (C,D) Scheme 3. concentrations with caffeic acid for 5 h (in one-hour intervals). C: maximum at wavelength 421 nm (50 nm diameter nanoparticles). (D) maximum at wavelength 467 nm (80 nm diameter nanoparticles). (E,F) FTIR spectra of caffeic acid-derived nanoparticles (E) and caffeic acid (F). (G) decrease in OD after cyanide addition to silver nanoparticles. (H) XRD profile of the AgNPs obtained by the reaction of caffeic acid with AgNO₃; in the insert, the table with the identifications. Peaks identifying the presence of silver oxides have been indicated in the XRD diffractogram.

Figure 9

Analysis of catechin-synthesised silver nanoparticles. (A) TEM image of pentagonal (solid arrow) and polyhedral (hollow arrow) AgNPs prior to cyanide treatment. (B) TEM image of AgNPs after treatment with cyanide. (C–E) Synthesis of silver nanoparticles by the reaction of different AgNO₃ concentrations with catechin for 5 h (in one-hour intervals). (C) Maximum at 402 nm wavelength (20 nm diameter nanoparticles); (D) 421 nm (50 nm diameter nanoparticles) and (E) maximum at wavelength 467 nm (80 nm diameter nanoparticles). (F) Relative increase of OD after cyanide addition to silver nanoparticles. (G,H) FTIR spectra of catechin-derived nanoparticles (G) and catechin (H). (I) Synthesis reaction of silver nanoparticles with catechin mediated by semiquinone and quinone formation. (J) XRD profile of the AgNPs obtained by the reaction of the catechin with AgNO₃; in the insert, the table with the identifications. Peaks identifying the presence of silver oxides have been indicated in the XRD diffractogram.

Figure 10

Analysis of the silver nanoparticles synthesised with scopoletin. (A) TEM image of AgNPs prior to cyanide treatment. (B) TEM image of AgNPs after cyanide treatment. (C) Synthesis of silver nanoparticles by the reaction of different AgNO₃ concentrations with scopoletin for 5 h (in one-hour intervals). Maximum at 440 nm wavelength (70 nm diameter nanoparticles). (D) Detail of nanoparticle aggregation, with cookie-like form. (E,F) FTIR spectra of scopoletin-derived nanoparticles (E) and scopoletin (F). (G) decrease in OD after cyanide addition to silver nanoparticles. (H) XRD profile of AgNPs obtained by reaction of scopoletin with AgNO₃; in the insert, the table with the identifications. Peaks identifying the presence of silver oxides have been indicated in the XRD diffractogram.