Highly Sensitive Near-Infrared SERS Nanoprobes for In Vivo Imaging Using Gold-Assembled Silica Nanoparticles with Controllable Nanogaps

In vivo imaging using NIR light has attracted considerable attention owing to good penetration ability of NIR raditation through tissues. However, previous NIR imaging probes still possess issues such as lack of multiplex capacity, due to spectral overlap. In vivo imaging using surface-enhanced Raman scattering (SERS) has emerged as effective solution, owing to its high sensitivity, multiplexing ability, and non-photobleaching feature. To take such advantages, development of highly enhanced SERS nanoprobes in near-infrared (NIR) region is needed. A well-controlled morphology and biocompatibility are essential features of NIR SERS nanoprobes. Among several types of nanoparticle(NP)s, Gold(Au) NPs has been widely used under biological conditions owing to its good biocompatibility, but nanoprobes based on Au NP still suffer from weak SERS signal enhancement ability. Herein, we suggest Au-assembled nanostructures with controllable nanogaps with highly enhanced SERS signals within multiple hotspots as an breakthrough.<br/>Au-assembled silica (SiO₂) nanoparticles (NPs) (SiO₂@Au@Au NPs) as NIR SERS nanoprobes are synthesized using the seed-mediated growth method. SiO₂@Au@Au NPs with six different sizes of Au NPs (SiO₂@Au@Au₅₀–SiO₂@Au@Au₅₀₀) were prepared by controlling the concentration of Au precursor in the growth step. The nanogaps between Au NPs on the SiO₂ surface could be controlled from 4.16 to 0.98 nm by adjusting the concentration of Au precursor (hence increasing Au NP sizes), which resulted in the formation of effective SERS hotspots. SiO₂@Au@Au₅₀₀ NPs showed effective absorption of NIR light, with a 0.98-nm gap between Au NPs. SiO₂@Au@Au₅₀₀ NPs showed high SERS enhancement factor of approximately 3.8 × 10⁶ under 785-nm photoexcitation with good uniformity. SiO₂@Au@Au₅₀₀ nanoprobes showed detectable <i>in vivo</i> SERS signals at a concentration of 16 μg/mL in animal tissue. SiO₂@Au@Au₅₀₀ nanoprobes showed good penetration ability at animal tissue, detectable until it reaches 7 mm deep inside the porcine tissue. SiO₂@Au@Au₅₀₀ NPs with 14 different Raman label compounds exhibited distinct SERS signals upon subcutaneous injection into nude mice. SiO₂@Au@Au NPs showed high potential for <i>in vivo</i> applications as multiplex nanoprobes with high SERS sensitivity in the NIR region.