Unexpected Chirality of Nanoparticle Dimers and Ultrasensitive Chiroplasmonic Bioanalysis

Abstract

Chiral assemblies of nanoparticles (NPs) are typically constructed with helical or tetrahedral geometries. Simple pairs of NPs are not expected to display chirality due to basic symmetry considerations made under the assumption of their spherical geometry. In this study we demonstrate that assemblies consisting of two metallic NPs do possess chirality and strongly rotate polarization of light. Their chiroplasmonic properties are attributed to the prolate geometry of individual colloidal particles. When bridged by biomolecules, the NP pairs acquire scissor-like geometry, with the long axes of NPs forming an angle of ~9°. This small dihedral angle results in chirality of the NP pair, while the consistency of its sign due to the specific conformation of the bridging biomacromolecules breaks the enantiomeric equivalence of the NP pairs. Strong polarization rotation in these nanoassemblies makes possible their utilization in biological analysis. Heterodimers of gold and silver NPs were made using antibody-antigen bridges. Taking advantage of their chiroplasmonic properties, we investigated their bioanalitical potential for detection of an environmental toxin, microcystin-LR, and a cancer biomarker, prostate-specific antigen. The order-of-magnitude improvements in limits of detection compared to all other analytical techniques are attributed to plasmonic enhancement of intrinsic chirality of biological compounds, strong optical coupling of photons with NP assemblies with twisted geometries, and signal amplification due to the bisignate nature of circular dichroism bands.

Nanoparticle Superlattices: The Roles of Soft Ligands

“…Attaching biomolecules such as DNA,11, 46, 47, 135 proteins136, 137, 138, 139, 140 and antibodies141, 142 to nanoparticle surfaces offer a unique specific route to control their assembly. Such assemblies are attractive because they can be programmed into complex structures, such as chiral architectures 141.…”

SERS encoded nanoparticle heterodimers for the ultrasensitive detection of folic acid

“…Under the slight stirring, the monoclonal antibody against FA (1.6 mL, 5 mg/mL) and antigen (0.8 mL, 4 mg/mL) were added into the AuNPs and AgNPs, respectively (Wu et al, 2013). Next, 2 mL of Raman reporter molecule, 4-aminothiophenol Scheme 1.…”

“…The modified AuNPs solution and AgNPs solution were then centrifuged for 10 min at the speed of 6500 rpm and 13,000 rpm, respectively. The supernatants were removed, and the remainder was resuspended in 0.01 M phosphate buffer (pH¼7.2), and mixed together to assemble the AuNP-AgNP heterodimers (Wu et al, 2013). The samples were analyzed by TEM, DLS, UV-vis and Raman spectra.…”

“…And the molar ratio of the AuNPs to monoclonal antibody and Ag NPs to antigen was 1:1.75 and 1:1.6, respectively. Following the conjugation of biomacromolecule to NPs, the functionalized NPs assembled to heterodimers according to the classic antibody-antigen immunoreaction (Scheme 1, Wu et al, 2013). TEM and UV-vis spectrum were used to characterize the AuNPs and AgNPs (Figs.…”

“…Our recent study demonstrated the strong plasmonic coupling of the NPs assemblies, especially for the AgNPs nanostructures, which could realize the ultrasensitive biosensing (Wu et al, 2013;Xu et al, 2015). In the present study, SERS encoded AuNP-AgNP heterodimers were controllably assembled with high yield, taking the advantage of strong optical coupling and intense hot spots of NPs heterodimers (Campion and Kambhampati, 1998;Kleinman et al, 2013;Nie and Emory, 1997;Xu et al, 2015), as well as the high affinity of antigen-antibody immune recognition (Wu et al, 2013). This developed SERS sensor based on the AuNPAgNP heterodimers substrate has achieved the the analysis of FA for the first time, with the lowest LOD of 0.86 pg/mL reported so far.…”