Label-Free and Highly Sensitive Detection of Native Proteins by Ag IANPs via Surface-Enhanced Raman Spectroscopy

The use of silver nanoparticles (Ag NPs) as substrates to obtain satisfactory Raman spectra of native proteins is a simple and valuable but challenging process. Herein, the Ag NPs modified with aluminum and iodide ions (Ag IANPs) were introduced for Raman detection of proteins, including acidic BSA (PI 4.7), catalase (PI 5.4), β-casein (PI 4.5), α-casein (PI 4.0), insulin (PI 5.35), basic myoglobin (PI 6.99), and lysozyme (PI 11.2). The Raman signals of all the detected proteins were significantly improved in comparison with the reported spectra obtained by using Ag NPs containing Na₂SO₄, I^-, and Mg²⁺. Specifically, detection sensitivities of the acidic proteins were drastically increased. The limit of detection (LOD) of bovine serum albumin (BSA), α-casein, and β-casein was 0.03 ng/mL. The LOD of insulin and catalase were 0.3 and 3 ng/mL, respectively. As the bands corresponding to disulfide bonds, α-helices, residues of Phe, Trp, and Tyr, and carboxyl groups were also greatly enhanced, it was easy to monitor the folding of native protein and the denaturation of protein under acidic and heated conditions. Thus, Ag IANPs as substrates open a way for surface-enhanced Raman spectroscopy (SERS) detection of proteins. Hence, the method can provide more valuable information about protein and, therefore, has the potential for wide applications.