Carrageenan oligosaccharide (CAO) derived from marine red algae was used as a reducing and capping agent to obtain AuNPs by an eco-friendly, efficient, and simple synthetic route for the first time.

Gold nanoparticles (AuNPs) are commonly used nanoparticles which have been widely employed in photoacoustic imaging, catalysis, photothermal therapy and targeted drug delivery.

Some marine carbohydrates, such as fucoidan, agarose, and chitosan oligosaccharide have been reported to prepare AuNPs for biodegradability, abundance, and non-toxicity.

Carrageenan oligosaccharide

Carrageenan is a sulfated linear polysaccharide extracted from marine red algae and composed of d-galactose residues linked alternately by (1→3)-linked β-d-galactopyranose (unit G) and (1→4)-linked α-d-galactopyranose (unit D).

Carrageenan oligosaccharide (CAO) possesses various pharmacological activities, such as anti-tumor and anti-viral activity, for its small molecular weight, good water solubility, and a certain degree of sulphation.

The synthetic conditions of AuNPs were optimized by response surface methodology (RSM), and the CAO-AuNPs obtained were demonstrated to be ellipsoidal, stable and crystalline by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD).

The CAO-AuNPs showed localized surface plasmon resonance (LSPR) oscillation at about 530 nm with a mean diameter of 35 ± 8 nm. The zeta potential of CAO-AuNPs was around −20 mV, which was related to the negatively charged CAO around AuNPs.

The CAO-AuNPs exhibited significant cytotoxic activities in HCT-116 and MDA-MB-231 cells, which could be a promising nanomaterial for drug delivery. CAO derived from red algae served a dual role as both the reducing and stabilizing agents to synthesize AuNPs for the first time.

This synthetic method is green, efficient and eco-friendly, and RSM optimized the reaction conditions. The biocompatible and stable CAO-AuNPs obtained were proved to be crystal, monodispersed and mostly ellipsoidal using XRD, TEM, SEM and a particle analyzer.

Furthermore, the CAO-AuNPs exhibited significant activities against HCT-116 and MDA-MB-231 cells. This study provides references for the green preparation of AuNPs by marine carbohydrates in the field of nanomaterials.

Furthermore, the CAO-AuNPs nanosystem shows a promising prospect of application in nanomaterial-based drug delivery, and it could be worthy of further investigation of CAO-AuNPs as a nanocarrier for anti-cancer drugs.