Biocompatible surface functionalization architecture for a diamond quantum sensor

Diamond with nitrogen-vacancy (NV) centers is a quintessential material used in quantum sensing and other high precision measurements. One promising application of such novel NV sensors is to study single-molecule biophysics, which generally require chemical modification of the substrate to immobilize target molecules of interest and prevent nonspecific adsorption of unwanted molecular species. However, diamond surfaces are known to be difficult to chemically modify, bottlenecking the realization of single-molecule experiments on target molecules under their biologically relevant conditions.

Here, we introduce a general strategy to tackle this challenge. A thin layer of alumina is uniformly laid on diamond surface by atomic layer deposition, which is then passivated with functional polyethylene glycol (PEG) molecules. The functional PEG layer recruits target molecules via highly specific interactions, such as biotin-streptavidin interaction and strain-promoted azide-alkyne cycloaddition “click chemistry”, while greatly reducing nonspecific binding. This allows us to position target molecules within 5 nm distance from the diamond surfaces. The grafting density can be precisely controlled by tuning the composition of PEG molecules, and the functionalization is stable over days under physiological conditions. The impact of this functional layers on the spin coherence properties of NV centers is found to be minimal, with only about 15% reduction in T2 and negligible change in T1. This method should lay a solid foundation for NV-based single-molecule electron paramagnetic resonance (EPR) or nuclear magnetic resonance (NMR) experiments on a variety of biomolecules, which will deepen our mechanistic understanding of their biological functions.