Birefringent Biomineral Microcarriers Stabilise Multimodal Nanodiamond Quantum Sensing in Liquids

Mobile nanodiamond quantum sensors in liquids are limited by Brownian rotation, variable photon collection, and perturbations introduced by optical trapping. Here, we assemble 40-nm nitrogen-vacancy nanodiamonds at the surface of porous, birefringent vaterite microspherulites. This carbonate biomineral provides a polarization-addressable body frame, anisotropic emission redistribution, and a proton-active thermodynamic interface. Under 976-nm optical trapping, the hybrids retain their spin resonance and longitudinal relaxation, with the resonance contrast varying by less than 7 percent and the resonance centre shifting by approximately 1 MHz at 0.8 W. Zeeman-split resonances enable magnetic-field sensing from 0 to 0.8 mT, with a response metric of 78-144 microT per square root Hz. In buffered cell-culture medium, a 10.7 microM nominal proton-equivalent dose shortens T1 from 23.4 +/- 2.3 to 9.0 +/- 1.2 microseconds, corresponding to concentration and pH sensitivities of 6.46 microM per square root Hz and 6.54 mpH per square root Hz, respectively, in DMEM. By contrast, a 500-fold higher proton dose in ethanol produces a substantially weaker spin response, highlighting the role of the carbonate-rich interface. We develop a grand-canonical charge-regulation model coupling proton chemical potential to interfacial switching kinetics within the NV spectral window, capturing medium-dependent dynamic proton transduction. By integrating orientation stabilisation, optical manipulation, magnetic sensing, and interfacial chemical response, this carbonate-rich hybrid platform establishes a versatile approach to multimodal quantum sensing in complex biological liquids.