Fig. 2

FTIR spectroscopy analysis. FTIR spectra of cysteine shows the characteristics peaks of cysteine at position 752.5 cm⁻¹ CH₂ rock, at 805.3 cm⁻¹ COO wagg, at 866.4 cm⁻¹ N–O stretch, at 941.9 cm⁻¹ SH bend, at 1062.5 cm⁻¹ NH₃ rock, at 1139.2 cm⁻¹ SO₂ stretch, at 1196.2 cm⁻¹ CH₂ twist, at 1295.7 cm⁻¹ CH₂ wagg, at 1345.6 cm⁻¹ NH₃ bend, at 1418.7 cm⁻¹ CH2 bend, at 1520.5 cm⁻¹ N=O stretch, 1575.5 cm⁻¹ NH₃ bend, at 2549.7 cm⁻¹ SH stretch and at 2952.9 cm⁻¹ CH2 stretch. FTIR spectra of free doxorubicin hydrochloride shows the characteristics peaks of doxorubicin including strong stretch of primary alcohol at 989.8 cm⁻¹ and other stretching peaks of alcoholic groups at 1000–1150 cm⁻¹, amine stretching at 3299.9 cm⁻¹, alkane stretch at 2942.7 cm⁻¹, aromatic group stretch peaks in area 1412–1616 cm⁻¹, C–O–C stretch of ether at position 1283.1 cm⁻¹ and C=O stretch of ketone at 1730.2 cm⁻¹. FTIR spectroscopic analysis of GNP shows the spinal structure of GNP. The peak at 1634.55 cm⁻¹ represents the ketonic carbon oxygen double bond. The peak at 1541.46 cm⁻¹ represents the R-CO₂ stretching, 1029.92 cm⁻¹ is the C–O stretching and the peak at 3252.916 cm⁻¹ represents the O–H group stretching. FTIR spectroscopic analysis of GNP–cysteine shows the successful incorporation of cysteine on GNP. The disappearance of S–H peak at 2549.7 cm⁻¹ and the appearance of cysteine functional group amine N–H stretch at 3447.36 cm⁻¹, N–H bend at area 1580–1650 cm⁻¹, carboxylic group C=O stretch at 1650.76 cm⁻¹ and C–O stretch at 1005.19 cm⁻¹ confirm the conjugation of cysteine with GNP. FTIR spectroscopic analysis of GNP–cysteine–doxorubicin shows the successful conjugation of doxorubicin with cysteine carboxylic group on GNP. The disappearance of C=O stretch at 1650.761 and C–O stretch at 1005.194 of GNP–Cys spinal structure and appearance of amide bond (C–N) stretching peak at 1278.702 and doxorubicin-specific peaks confirm the conjugation of doxorubicin with GNP–Cys nanoparticles. FTIR spectroscopic analysis of GNP–cysteine–doxorubicin–transferrin shows the successful conjugation of transferrin with cysteine amine group on GNP. The disappearance of N–H stretching peak at position 3270.710 and N–H bent at 1614.247 of GNP–Cys–Dox spinal structure confirm the conjugation of doxorubicin with GNP–Cys–Dox nanoparticles