Optical Brighteners (OBs) are fluorescent dyes that are added in tiny quantities to most laundry products. They fluoresce under UV light which makes whites and colours look brighter. They are made to be residual, meaning they stay on the fabrics.
OBs are broken down by UV light or washed away in subsequent washes. They are not detrimental to fabrics or human health and are in nearly every laundry product on the market, including those marketed for babies such as Purity/Amolin.
In the scientific literature, there is no evidence that OBs cause skin reactions. Studies of people with allergic contact dermatitis show no reaction to laundry products in nearly 100% of cases. 
In studies that have been conducted in waterways, the level of OB present is well below the level that could be described to be harmful to the aquatic life. Of the studies that have been conducted at wastewater treatment plants (end point of waste water from households connected to mains plumbing), up to 98% of the OBs that arrive at the treatment plant are removed during the water treatment process.
There is nothing about the chemistry of OBs that can cause ‘build up’ or repelling in your nappies. These dyes are also used in personal care products such as toothpaste, makeup and shampoo.
For more information on what is in detergents and the function of the raw materials see The Wash Wise Detergent Ingredients Info Sheet
 https://www.ncbi.nlm.nih.gov/pubmed/11807430 [Allergic contact dermatitis to detergents: a multicenter study to assess prevalence]