To protect a DIY Hyaluronic Acid serum from microbial growth a preservative must be used (alternatively, you have to store the serum in the fridge and use within several days).
Has to be protected against gram positive bacteria, gram negative bacteria, and fungi. For this reason, a broad spectrum preservative is required. The preservatives that act against all three are Benzostat, Euxyl K940 (Benzyl Alcohol), Liquid germall plus, Optiphen and Optiphen plus. Optiphen is the only natural preservative on this list that provides protection against all three (a lower level of protection against Fungi)
https://makingskincare.com/preservatives/
Has to be protected against gram positive bacteria, gram negative bacteria, and fungi. For this reason, a broad spectrum preservative is required. The preservatives that act against all three are Benzostat, Euxyl K940 (Benzyl Alcohol), Liquid germall plus, Optiphen and Optiphen plus. Optiphen is the only natural preservative on this list that provides protection against all three (a lower level of protection against Fungi)
https://makingskincare.com/preservatives/