No-Stirring Synthesis of sub-50 nm Hollow Silver Nanoshells with Dimethylglyoxime-Induced Plasmons in Visible and Second NIR Windows for Biomedical Applications

A unique no-stirring synthesis has been developed to obtain highly monodisperse hollow silver nanoshells (HAgNSs) with plasmons in the second near-IR (NIR-II) window. The method also introduces dimethylglyoxime (DMG) as a quadrupole-supporting agent. The quadrupole surface plasmon resonance (Q-SPR) was found to be highly intense and tunable from 450 to 558 nm. Two types of dipolar resonances, symmetric dipole surface plasmon resonance (SD-SPR) and antisymmetric dipole surface plasmon resonance (AD-SPR), are also observed. The AD-SPR peaks remain constant at 333 nm while the SD-SPR peaks are tuned gradually from 780 to 850 -> 920 -> 1000 -> 1150 nm. They were accordingly named HAgNS-780, HAgNS-850, HAgNS-920, HAgNS-1000, and HAgNS-1150, and their outer diameters were found to be 53 +/- 4, 49 +/- 3, 54 +/- 3, 62 +/- 5, and 39 +/- 3 nm, respectively. The corresponding aspect ratios (outer diameter/shell thickness) were 3.31, 3.37, 3.48, 4.13, and 5.2, respectively. A correlation between the tunability of SD-SPR, AD-SPR, and Q-SPR and aspect ratio has been established. The shape and size parameters were utilized for the simulation of the extinction spectra by the discrete dipole approximation (DDA) method. Second derivative FTIR analysis reveals the peculiar binding mode of DMG to the HAgNS which is the genesis of Q-SPR in such smaller-sized nanoshells. The further red shift of Q-SPR and SD-SPR was observed with the addition of folic acid (FA). It also imparts greater aqueous solubility, colloidal stability, and biocompatibility, making them suitable for biomedical applications.