Chiral Plasmonic Fields Probe Structural Order of Biointerfaces.

Christopher Kelly,Laurence D Barron,Graeme Cooke,Adrian J Lapthorn,Malcolm Kadodwala,Affar S Karimullah,Ryan Tullius,Vincent M Rotello,Nikolaj Gadegaard

doi:10.1021/jacs.8b03634

Abstract

The structural order of biopolymers, such as proteins, at interfaces defines the physical and chemical interactions of biological systems with their surroundings and is hence a critical parameter in a range of biological problems. Known spectroscopic methods for routine rapid monitoring of structural order in biolayers are generally only applied to model single-component systems that possess a spectral fingerprint which is highly sensitive to orientation. This spectroscopic behavior is not a generic property and may require the addition of a label. Importantly, such techniques cannot readily be applied to real multicomponent biolayers, have ill-defined or unknown compositions, and have complex spectroscopic signatures with many overlapping bands. Here, we demonstrate the sensitivity of plasmonic fields with enhanced chirality, a property referred to as superchirality, to global orientational order within both simple model and “real” complex protein layers. The sensitivity to structural order is derived from the capability of superchiral fields to detect the anisotropic nature of electric dipole–magnetic dipole response of the layer; this is validated by numerical simulations. As a model study, the evolution of orientational order with increasing surface density in layers of the antibody immunoglobulin G was monitored. As an exemplar of greater complexity, superchiral fields are demonstrated, without knowledge of exact composition, to be able to monitor how qualitative changes in composition alter the structural order of protein layers formed from blood serum, thereby establishing the efficacy of the phenomenon as a tool for studying complex biological interfaces.

Highlights

Biointerfaces play a key role in biology and biotechnology, providing the contact point between biological systems and their environment
There are some examples of the use of sum frequency generation (SFG)[1] and linear dichroism (LD)[2,3] to monitor orientational order in single layers of biomolecules on inorganic surfaces
We present a new concept for probing orientational order within protein layers

Summary

■ INTRODUCTION

Biointerfaces play a key role in biology and biotechnology, providing the contact point between biological systems and their environment. There are some examples of the use of sum frequency generation (SFG)[1] and linear dichroism (LD)[2,3] to monitor orientational order in single layers of biomolecules on inorganic surfaces. These examples use systems which contain a chromophore, such as a heme group or dye, which can provide an effective “tag” of orientation. We present a new concept for probing orientational order within protein layers This phenomenon is not reliant on an optical spectroscopic fingerprint (i.e., UV/vis or IR absorption bands) of the constituent material. The ability of chiral fields to probe pathologically relevant protein layers points toward the utilization of the phenomenon in biosensing applications

■ RESULTS AND DISCUSSION

■ ACKNOWLEDGMENTS

■ REFERENCES