Fluorescence Modulation of Nanodiamond NV- Centers for Neurotransmitter Detection

Conductive diamond films emerged twenty-five years ago as an advantageous electrode material for longer-term electrochemical sensing in variety of chemical arenas including in neurochemical detection, with promise of higher chemical sensitivities and less fouling and interferences. More recently reported, nanodiamond (ND) particle sensors, based on modulation of their lattice defect centers, represent a new direction in sensing, while maintaining many inherent advantages of the diamond material. In this work, red fluorescent, carboxylated ND particles with negatively charged nitrogen vacancy (NV⁻) centers are investigated for quantitative detection of dopamine, caffeine and ascorbic acid, with initial comparison of selectivity with electrochemical measurements of these species with diamond electrodes. These molecules are not fluorescent but modulate the ND fluorescence at 685 nm when mixed in a neutral phosphate buffer media. Initial mixing of ND with 25 nM dopamine or caffeine quenched the fluorescence by 10.5% and 7% respectively, with additional quenching of 18.5 and 10.9% respectively for a concentration increase of 75 nM. The quenching was linearly dependent on concentration in the nanomolar range, and was higher for dopamine than caffeine. Ascorbic acid (AA) enhanced the fluorescence by 0.51% when mixing 1 nM AA with ND; its nanomolar sensitivity was less (2.8 % enhancement for a 100 nM change) than for the other species. For micromolar concentrations, the sensitivities for all species was lower.<br/>Fluorescence selectivity for caffeine-dopamine and dopamine-ascorbic acid mixtures were examined and the level of interference was expressed as percentage changes in quenching or enhancement. For caffeine-dopamine mixtures, the interference was less than 1% increase in quenching in the nanomolar or micromolar ranges. For dopamine-ascorbic acid mixtures, the interference was a nominal 3% increase in enhancement in the nanomolar range, and less than 0.5% in the micromolar range. For initial comparison, electrochemical selectivity for dopamine-ascorbic acid mixtures was compared using a carboxyphenyl-modified diamond electrode; dopamine showed higher sensitivity and the AA current was significantly reduced. Caffeine detection was inherently selective since its oxidation potential is readily distinguishable from the potentials for DA and AA. These initial studies demonstrate that for similar surface chemistries, the fluorescence modulation of ND may provide a new sensor modality for comparable detection of important bioanalytes, and may represent an appealing complimentary method to electrochemical detection due to its inherent, high sensitivity, simplicity, speed and low cost.