We investigated the conjugation of various sequences of amyloid beta (AB) protein with gold colloidal suspension of 20 nm size. Absorption spectroscopy was utilized to identify changes in the optical property of gold colloid for pHs, ranging from pH 2 to pH 10. Color changes were seen for all tested proteins in this study at a higher pH than where bare gold colloid exhibits its color change at pH =3.1. All tested AB sequences exhibited color changes around pI of AB(1-40), c.a. pH 5.0, except for AB(1-42)which constantly exhibited clear precipitants in all pH lower than pH 7. The color change confirmed at a pH lower than 5 is attributed to the unfolded AB monomer units around the gold colloidal surface. The bonding between AB and gold colloidal surface is speculated to be due to ionic interactions between positively charged amino acid side chains and the negatively charged colloid surface or hydrophobic attraction between nonpolar amino acids and the negatively charged lyophobic sol surface. The segments containing a positive net charge at pH 7 tend to exhibit color change at a relatively higher pH value compared to those sequences containing only neutral or net negative charges. The hydrophobic AB(1-42) had signs clearly indicating precipitation as a mixture was made and the spectral feature indicated a deviation from the rest of the sequences. Interestingly, only AB(1-40) – coated gold colloidal nanoparticles exhibited a reversible color change as the pH was externally altered between pH 4 and 10.