Highly Tunable Hollow Gold Nanospheres: Gaining Size Control and Uniform Galvanic Exchange of Sacrificial Cobalt Boride Scaffolds

Abstract

In principle, the diameter and surface plasmon resonance (SPR) frequency of hollow metal nanostructures can be independently adjusted, allowing the formation of targeted photoactivated structures of specific size and optical functionality. Although tunable SPRs have been reported for various systems, the shift in SPR is usually concomitant with a change in particle size. As such, more advanced tunability, including constant diameter with varying SPR or constant SPR with varying diameter, has not been properly achieved experimentally. Herein, we demonstrate this advanced tunability with hollow gold nanospheres (HGNs). HGNs were synthesized through galvanic exchange using cobalt-based nanoparticles (NPs) as sacrificial scaffolds. Co 2 B NP scaffolds were prepared by sodium borohydride nucleation of aqueous cobalt chloride and characterized using UV−vis, dynamic light scattering, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy. Careful control over the size of the Co 2 B scaffold and its galvanic conversion is essential to realize fine control of the resultant HGN diameter and shell thickness. In pursuit of size control, we introduce B(OH) 4 − (the final product of NaBH 4 hydrolysis) as a growth agent to obtain hydrodynamic diameters ranging from ∼17−85 nm with relative standard deviation <3%. The highly monodisperse Co 2 B NPs were then used as scaffolds for the formation of HGNs. In controlling HGN shell thickness and uniformity, environmental oxygen was shown to affect both the structural and optical properties of the resultant gold shells. With careful control of these key factors, we demonstrate an HGN synthesis that enables independent variation of diameter and shell thickness, and thereby SPR, with unprecedented uniformity. The new synthesis method creates a truly tunable plasmonic nanostructure platform highly desirable for a wide range of applications, including sensing, catalysis, and photothermal therapy. KEYWORDS: cobalt boride, size control, sodium borohydride, galvanic exchange, hollow gold nanospheres, surface plasmon resonance ■ INTRODUCTIONPlasmonic metal nanostructures exhibit beneficial optical properties owing to their surface plasmon resonance (SPR), the collective oscillation of conduction band electrons that manifests as strong absorption, and/or scattering at the oscillation frequency.1,2 As oscillation frequency is structure dependent, the SPR may be tuned by changing the size or shape of the nanoparticle (NP). 3−5 This tunability positions plasmonic metal NPs as highly attractive components in nanomedicine, 6−12 optoelectronics, 13−18 sensing, [6][7][8][9]13,18,19 and solar energy conversion. 20−24 In these applications, hollow metal nanostructures have distinct advantages over their solid metal counterparts including lower mass per particle for reduced material costs, higher surface-area-to-volume ratio for increased density of loading or catalytic sites, and enhanced plasmonic performance in applications like surface-enhanced Raman scattering (SERS), drug ...

Miniature Hollow Gold Nanorods with Enhanced Effect for In Vivo Photoacoustic Imaging in the NIR‐II Window

“…Moreover, the shell appears not uniform, which is similar to previously reported hollow Au nanoshells or nanocages. [33][34][35][36] The phenomenon is probably caused by rapid reaction. From amplifying images (Figure 2c,d) we can see that both ends of the hollow nanorods are closed.…”

“…[33] Furthermore, as the hollow core exist, hollow AuNPs have one more parameter to adjust plasmonic absorption bands, which is in favor of acquiring smaller NIR-II-windowresponsive AuNPs. [34,35] However, as far as we know, no hollow AuNPs with a size less than 50 nm have been reported for PAI in NIR-II window until now. The existence of some nonnegligible deficiencies in previous hollow AuNPs is probably the dominated reason.…”

Hollow Au Nanosphere-Cu2O Core–Shell Nanostructures with Controllable Core Surface Morphology

“…HGNs were prepared through galvanic exchange with sacrificial Co x B y NP templates, according to a previous report. 8 First, a 100 mL solution of 0.40 mM was stirred for two minutes and then left to sit under nitrogen protection for an hour to allow full hydrolysis of NaBH 4 . For galvanic exchange, 15.0 mL of the Co x B y NP solution was transferred to aqueous HAuCl 4 (15.0 mL, 53.3 μM) which was deaerated by bubbling with nitrogen for 1 hour in advance.…”

“…To control surface morphology, the galvanic exchange step was altered with respect to dissolved oxygen content and pH, according to a previous report. 8…”

“…Broadening of the SPR for bumpy particles is expected due to the roughening of the surface and formation of nanoscaled protrusions. 7,8…”