A wetness sensor for an absorbent article that is formed from an ink is provided. The ink includes a proton-accepting chromogen and a proton-donating agent (or color developer). Prior to use, the ink is generally dry and in a protonated form so that it has a visible color. However, upon contact with bodily fluids (e.g., urine, fecal matter, mucus, menses, vaginal fluid, etc.), water in the fluid can lead to deprotonation of the chromogen, thereby resulting in a shift of the absorption maxima of the chromogen towards either the red (“bathochromic shift”) or blue end of the spectrum (“hypsochromic shift”). To increase the rate of the color change during use, the proton-donating agent is an aliphatic carboxylic acid that is highly soluble in the bodily fluid (e.g., urine), and therefore results in a color change that is very rapid and may be detected within a relatively short period of time.
