Fig. 1: Early detection of SARS-CoV-2 antigen by spin-enhanced LFT and FND characterization.

a SARS-CoV-2 infection dynamics of RNA and infectious virus by viral load with the LoD of RT-qPCR at ~10⁰-10² copies/mL, detecting active virus 1–3 days earlier than current antigen-detecting LFTs. Call-out box shows the 1–3 days window for early detection with improved sensitivity of Ag-LFTs. b Schematic of FNDs immobilised at the LFT test line via antibody sandwich complex. The FND is excited at 550 nm and emits from the visible to the NIR. A small omega-shaped resonator is driven at the NV^- zero-field splitting frequency (2.87 GHz) to induce spin population transfer between the m_s = 0 and m_s = ±1 states. The amplitude of this driving field is modulated to provide a time varying fluorescence signal. c Energy level diagram of NV^- centre, showing the zero-field splitting of the spin triplet optical ground and excited states, the spin conserving optical transitions, and the non-radiative decay pathway from the optically excited m_s = ±1 state via the metastable singlet states. Under continuous optical illumination, this non-spin-conserving decay pathway results in an increase in population in the m_s = 0 state in a process known as optical initialisation. Once a population difference between the m_s = 0 or m_s = ±1 is established, if energy resonant with the spin transitions is provided (∆E* = 1.43 GHz and ∆E = 2.87 GHz), population is transferred from the m_s = 0 to the m_s = ±1 states. Due to the singlet decay pathway not resulting in a visible photon, this reduces the detected photoluminescence. This effect is demonstrated by the results of the continuous wave optically detected magnetic resonance (CW-ODMR) experiment shown in the inset with reductions in fluorescence centred at ∆E*and ∆E. d NV centre excitation and emission spectra showing an excitation peak at 550 nm and emission peak at 675 nm. Zero phonon lines for NV⁰ and NV^- centres are labelled. Source data are provided as Source Data file.