Colorimetric and SERS-Based Multimode Detection Platform for Cu(II) Ions Using Peptide–Gold Nanoparticles

Excessive copper ions in the human body can cause variety of diseases such as gastrointestinal disorders, cirrhosis, and Alzheimer's diseases. Techniques like Inductively Coupled Plasma Mass, and Atomic Absorption Spectroscopy are available for copper detection, but the associated cost issue for sample preparation and labor limited their application for on-site detection. Herein we are reporting a versatile method for detecting copper ions using a peptide-functionalized gold nanoparticle sensor in combination with various optical spectroscopic techniques. The peptide (CW) showed a selective sensing ability for Cu (II) with visual colorimetric and optical spectroscopic changes as compared with other metal ions tested. CW showed a visual colorimetric response from colorless to light brown color after interaction with Cu (II). Converting CW to gold nanoparticle appended (CW-AuNPs) nanoplatform allowed a multimode detection platform for Cu (II) which utilizes colorimetric, optical spectrum changes, and surface-enhanced Raman spectroscopy (SERS) to enable highly sensitive sensing of Cu (II), even at extremely low concentrations (76 nM). CW-AuNPs exhibit a controlled aggregation property in the presence of Cu (II), resulting in the creation of hot spots for SERS-based detection. Moreover, the peptide unit attached to the gold nanoparticles serves both as a binding motif for Cu (II) and as a Raman reporter for Cu (II) sensing. Our comprehensive analysis, including solution-state and dry-mapping Raman spectroscopic studies, demonstrates remarkable picomolar sensitivity of the peptide-gold nanoparticle system for Cu (II) detection. Moreover, we prepared a paper test strip from CW-AuNPs and used it as a visual colorimetric platform for sensitive detection of copper ions. The CW-AuNPs-Cu (II) nanoaggregate was used as a photodynamic therapy agent (PDT) for invitro studies with a singlet oxygen quantum yield of 0.21%.