Alginate‑carbon dot nanocomposite: A green approach towards designing turn-on aptasenor for Candida albicans fungus

Candida albicans fungus, an opportunistic microorganism that becomes a pathogen in its host, can often cause superficial and severe infections in human body. In this work, a turn-on fluorescent aptasenor based on Alginate‑carbon dot nanocomposite has been designed through a green approach. Carbon dots (CDs) with average size of 4 ± 1 nm were synthesized hydrothermally using Philodendron bipinnatifidum plant extract as precursor. Then, mixture of carbon dot and alginate was cross-linked by Ca²⁺ ions to make Alginate-CD nanocomposite with negligible effect on fluorescence intensity of CDs (λ_ex = 360 nm, λ_em = 400 nm). The fluorescence quantum yield and lifetime were found as 4.1 % and 3.15 ns, respectively. The nanocomposite was characterize using different spectroscopic (Photoluminescence, UV-visible, XPS, FT-IR) and microscopic (TEM and SEM) analytical techniques. An aptamer was used to selectively target (1 → 3)-β-D-glucan on the surface of the Candida albicans cell wall. It was found that the fluorescence energy resonance transfer between nanocomposite and aptamer quenches the fluorescence intensity of nanocomposite, making it suitable for designing turn-on fluorescent probe. It was found that the fluorescence could be recovered in the presence of Candida albicans fungus (in <30 min), owing to higher affinity between aptamer and (1 → 3)-β-D-glucan on the surface of Candida albicans fungus, where there were two linear ranges: low concentration of 50-200 cells/mL and high concentration of 1000-6000 cells/mL (R² ≥ 0.989 for both linear ranges), with limit of detection of 40 cells/mL. The selectivity of the designed probe was examined against a mixture of microbial agents (Staphylococcus aureus and Escherichia coli) and Curdlan polysaccharide (1→3)-b-D-Glucan (L). The designed biosensor showed an excellent ability to directly detect Candida albicans fungus via a simple and fast (<30 min) approach. The results of this experimental study show that Alginate-CD-aptamer nanocomposite is a bright prospect in clinical applications and a suitable alternative to traditional methods of Candida albicans diagnosis.