Gold-nanosphere formation using food sample endogenous polyphenols for in-vitro assessment of antioxidant capacity

Abstract

In this work it was demonstrated that sample endogenous polyphenols are selectively driving the gold-nanoparticle (AuNPs)-formation process when representative food samples were used as natural sources of reducing compounds. The process of AuNPs formation was characterized by UV–visible spectroscopy and was described by a sigmoidal curve (R ² ≥ 0.990) which gave information about the polyphenol concentration at which the localized surface plasmon resonance (LSPR) absorption reached its half-value, X ⁵⁰_c , and about AuNPs production per polyphenol concentration unit, K _AuNPs. The behavior of phenolic acids was different, with lower X ⁵⁰_c and higher K _AuNPs values than flavonoids. For the food samples tea, apple, pear, wine, and honey X ⁵⁰_c values were 0.22, 7.3, 11.5, 20.4, 30.3, and 53.5 (mg mL⁻¹) and K _AuNPs values were 28.7, 0.70, 0.60, 0.20, 0.14, and 0.10 (mg⁻¹ mL), respectively. Excellent correlation between K _AuNPs and total phenolics (TP) was obtained (r = 0.98, p-value < 0.05), implying K _AuNPs is a novel marker for evaluation of food sample antioxidant capacity in vitro. The K _AuNPs values of samples indicated their antioxidant capacity was in the order: tea > apple > pear > wine > honey. The reproducibility of the AuNPs formation approach was excellent, not only for polyphenol standards (RSD < 6 % for X ⁵⁰_c and RSD < 11 % for K _AuNPs) but also for food samples (RSD < 9 % for X ⁵⁰_c and RSD < 15 % for K _AuNPs). Transmission electronic microscopy (TEM) enabled confirmation of the formation of stabilized Au-nanospheres from endogenous polyphenols with very well-defined sizes under 20 nm diameter for all the food samples investigated.