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Gold nanoparticles (AuNPs) possess unique physical and chemical properties such as electronic, photonic, and magnetic properties. These properties depend on the functionality of the stabilizing ligand and the size of the gold core. Incorporating azo compounds onto the surface of AuNPs can provide a light responsive platform where its physical and chemical properties can be changed via isomerization of the azo moiety. In this dissertation the synthesis and characterization of an azopyridine modified gold nanoparticle (AzoPy-AuNP) is introduced. 1H NMR spectroscopy, TEM imaging, UV-vis spectroscopy and thermal gravimetric analysis have been used to characterize the AzoPy-AuNP. The trans/cis photoisomerization of the azo moiety and the coordination of pyridine moiety to a bulky molecule (zinc tetraphenylporphyrin, ZnTPP) anchored to the gold core is studied. A very important size dependent property of AuNPs is their optical property. AuNPs larger than 3 nm have a strong absorption, so-called plasmon band, in the visible region. Facile synthesis of AuNPs of different sizes is still of great interest. Herein, photoinduced size evolution of 1-dodecanethiolate stabilized gold nanoparticles (C12-AUNP) in chlorinated solvents will be introduced. Irradiation of C12-AUNP in oxygen free chlorinated solvents resulted in the liberation of the thiolate stabilizing ligand and coalescence of the gold core to larger particles. This is due to the photodegradation of the solvent and creation of radicals, chloride anion, and hydrochloric acid which all lead to the liberation of the stabilizing ligand and coalescence of the AuNPs. The size evolution was tracked by both UV-vis spectroscopy and TEM imaging.