The freshly prepared cit-AuNPs were functionalized with the HS-PEG-COOH. Carbodiimide coupling chemistry was then employed to covalently conjugate Cyt It was important to begin with to physically characterise the particles to allow later quantification. We initially perform light scattering techniques to assess the change in hydrodynamic diameter (hd) and zeta potential of cit-AuNPs, AuNPs-PEG, and AuNP-PEG-Cyt C/Zn Porph samples. The length of the fully extended HS-PEG-COOH (2000 Da) is ca. 7.9 nm, while the diameter of Cyt C is ca. 3.4 nm and the diameter of of Zn Porph is around ca. 1.6 nm (calculated using ChemDraw3D). The hydrodynamic diameters (hd) of cit-AuNPs, AuNP-PEG and AuNP-PEG-Cyt C/Zn Porph were measured using DLS (Figure ). This analysis revealed a difference in the hd of the Cit-AuNPs from 19.6±1.0 nm to 33.54±0.50 nm in AuNP-PEG and to 38.9±1.1 nm for AuNP-PEG-Cyt C/Zn Porph, suggesting successful conjugation. Zeta potential measurements are useful in providing an insight into the change in surface chemistry by measuring the surface charge. Therefore, the zeta potential was used to confirm the successful conjugation of molecules to AuNPs. The zeta potential measurement of cit-AuNPs was recorded to be -36.77±2.33 mV which a slightly lower value of to -30.77 ±1.4 mV for AuNP-PEG and +14.7 ±0.9 mV for AuNP-PEG-C (Figure ). TEM was carried out to analyse the morphology, aggregation, and size of surface functionalized AuNPs. TEM analysis revealed that the AuNP-PEG-Cyt C /Zn Porph particles are monodisperse and the functionalization process does not cause any aggregation (Figure ). Furthermore, the cumulative frequency graph (inset of Figure ) reveals a mean average diameter of AuNP-PEG-Cyt C /Zn Porph samples to be around 19.15±2.12 nm (Gaussian fit). The size discrepancy between TEM and DLS is because TEM. DLS measures the hydrodynamic diameter of particles in suspension and therefore can differentiate between the Cit-AuNPs and the multifunctionalized AuNPs.